CVE-2025-49709 Overview
CVE-2025-49709 is a memory corruption vulnerability in Mozilla Firefox triggered by certain canvas operations. Mozilla fixed the issue in Firefox 139.0.4 and documented it in the Mozilla Foundation Security Advisory MFSA-2025-47. The vulnerability is classified as an out-of-bounds write [CWE-787] and is reachable over the network without authentication or user interaction.
A remote attacker can craft a malicious web page that performs specific canvas operations to corrupt memory in the browser process. Successful exploitation may allow arbitrary code execution within the affected Firefox process.
Critical Impact
A network-accessible web page can corrupt browser memory through canvas operations, enabling potential remote code execution with no user interaction beyond visiting the page.
Affected Products
- Mozilla Firefox versions prior to 139.0.4
- Desktop builds of Firefox on Windows, macOS, and Linux
- Any downstream distribution shipping a vulnerable Firefox build
Discovery Timeline
- 2025-06-11 - CVE-2025-49709 published to the National Vulnerability Database (NVD)
- 2025-06-11 - Mozilla published advisory MFSA-2025-47 and fix in Firefox 139.0.4
- 2026-04-13 - Last updated in the NVD database
Technical Details for CVE-2025-49709
Vulnerability Analysis
The vulnerability resides in Firefox's HTML canvas implementation. Certain canvas operations cause the rendering code to write outside the bounds of an allocated buffer, mapping to [CWE-787] Out-of-bounds Write. Memory corruption in this code path executes within the content or compositor process, depending on the operation invoked.
Mozilla's advisory MFSA-2025-47 and the corresponding Mozilla Bug Report #1966083 confirm the bug was reachable from web content. An attacker can stage the necessary canvas state from JavaScript using standard 2D context APIs, then trigger the vulnerable operation to overwrite adjacent memory.
Root Cause
The root cause is improper validation of buffer boundaries during specific canvas drawing or pixel-manipulation operations. When attacker-controlled inputs define the dimensions or coordinates used by the canvas backend, the resulting writes can exceed the allocated region. Mozilla did not publish detailed exploit information, and the technical bug entry remains restricted at the time of writing.
Attack Vector
Exploitation requires only that a user load attacker-controlled content in a vulnerable Firefox build. The attack vector is network-based and requires no privileges. A malicious page, a compromised third-party advertisement, or a cross-site script in trusted content can deliver the canvas operations needed to trigger the bug.
No public proof-of-concept code is available, and no in-the-wild exploitation has been confirmed by CISA. Mozilla's advisory describes the issue in prose without releasing technical exploitation details. See Mozilla Security Advisory MFSA-2025-47 for the vendor's description.
Detection Methods for CVE-2025-49709
Indicators of Compromise
- Firefox process crashes with access violation or segmentation fault signatures correlated with pages that use the <canvas> element
- Unexpected child process creation from firefox.exe or firefox-bin after rendering web content
- Browser telemetry showing repeated content process restarts on specific domains
Detection Strategies
- Inventory Firefox installations and flag any build below 139.0.4 as vulnerable
- Monitor endpoint telemetry for Firefox content process crashes, particularly those involving canvas or graphics modules
- Inspect web proxy logs for repeated visits to low-reputation domains immediately preceding browser instability
Monitoring Recommendations
- Alert on Firefox child processes spawning shells, scripting hosts, or LOLBins such as powershell.exe, cmd.exe, or bash
- Track Firefox version strings reported by EDR or asset management tooling and trigger remediation workflows for outdated builds
- Forward browser crash reports to a centralized data lake for correlation with threat intelligence on canvas-based exploit kits
How to Mitigate CVE-2025-49709
Immediate Actions Required
- Upgrade all Firefox installations to version 139.0.4 or later without delay
- Verify Extended Support Release (ESR) and downstream Linux distribution packages also include the fix
- Restart browsers after patching to ensure the new binaries are loaded into memory
Patch Information
Mozilla addressed CVE-2025-49709 in Firefox 139.0.4. The fix is documented in Mozilla Security Advisory MFSA-2025-47. Administrators managing enterprise deployments should use Mozilla's official channels or their package manager to deliver the update. Confirm the running version through about:support or the firefox --version command on each host.
Workarounds
- If immediate patching is not possible, restrict browsing to trusted internal applications using proxy allowlists
- Deploy enterprise policy to disable JavaScript on untrusted sites until updates are applied
- Apply browser isolation or remote browser solutions to render untrusted web content off-endpoint
# Verify the installed Firefox version after patching
firefox --version
# Expected output: Mozilla Firefox 139.0.4 or later
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
