CVE-2025-0240 Overview
CVE-2025-0240 is a use-after-free vulnerability [CWE-416] affecting Mozilla Firefox and Thunderbird. Parsing a JavaScript module as JSON could, under certain circumstances, trigger cross-compartment access in the JavaScript engine. This condition can result in a use-after-free memory corruption state. Mozilla fixed the issue in Firefox 134, Firefox ESR 128.6, Thunderbird 134, and Thunderbird 128.6. The vulnerability requires local attack conditions and carries limited integrity impact without affecting confidentiality or availability.
Critical Impact
A use-after-free condition in Firefox and Thunderbird module parsing could allow attackers to corrupt memory through cross-compartment access, potentially leading to further exploitation of the browser process.
Affected Products
- Mozilla Firefox prior to version 134
- Mozilla Firefox ESR prior to version 128.6
- Mozilla Thunderbird prior to version 134 and 128.6
Discovery Timeline
- 2025-01-07 - CVE-2025-0240 published to NVD
- 2026-10-05 - Last updated in NVD database
Technical Details for CVE-2025-0240
Vulnerability Analysis
The vulnerability resides in the SpiderMonkey JavaScript engine's module loader. When Firefox parses a JavaScript module as JSON, specific code paths can perform object access across compartment boundaries. JavaScript compartments isolate memory regions belonging to different security principals within the engine. Cross-compartment access without proper wrapping can leave dangling references to objects whose underlying memory has been freed.
Subsequent use of these references constitutes a use-after-free condition. The CWE-416 classification confirms the memory management flaw category. According to Mozilla's advisories MFSA-2025-01, MFSA-2025-02, MFSA-2025-04, and MFSA-2025-05, the fix shipped alongside other module-handling corrections in the January 2025 release cycle.
Root Cause
The root cause stems from improper lifetime management of JavaScript objects when the module loader treats a module import as a JSON resource. Cross-compartment pointers can outlive the garbage-collected objects they reference, producing a dangling pointer that the engine later dereferences.
Attack Vector
Exploitation requires a local attack context. An attacker needs to deliver content that triggers the specific module-as-JSON parse path within the victim's browser or mail client. Successful exploitation produces a limited integrity impact through memory corruption, with no direct confidentiality or availability consequences reported by Mozilla.
The vulnerability manifests inside the SpiderMonkey module loader when a JavaScript module import resolves to a resource parsed as JSON. See Mozilla Bug Report #1929623 for technical details on the affected code path.
Detection Methods for CVE-2025-0240
Indicators of Compromise
- Unexpected Firefox or Thunderbird process crashes referencing SpiderMonkey module loading or JSON parsing in crash reports
- Browser telemetry showing anomalous module import operations against JSON-typed resources
- Presence of outdated Firefox (<134), Firefox ESR (<128.6), or Thunderbird (<134 or <128.6) binaries on managed endpoints
Detection Strategies
- Inventory installed Firefox and Thunderbird versions across the fleet using endpoint software inventory tools
- Correlate browser crash telemetry with web page or email content delivered immediately before the fault
- Monitor for suspicious child process creation originating from firefox.exe or thunderbird.exe following crashes
Monitoring Recommendations
- Alert on execution of unsigned or unexpected code loaded by Firefox or Thunderbird processes
- Track endpoints that remain on vulnerable Firefox and Thunderbird versions beyond patch windows
- Review Debian LTS advisory feeds for downstream package updates referenced in the Debian LTS Announcement
How to Mitigate CVE-2025-0240
Immediate Actions Required
- Upgrade Firefox to version 134 or later and Firefox ESR to 128.6 or later
- Upgrade Thunderbird to version 134 or later, or ESR branch 128.6 or later
- Apply Debian and other Linux distribution security updates referencing MFSA-2025-01, MFSA-2025-02, MFSA-2025-04, and MFSA-2025-05
- Validate that enterprise browser deployment tools have pushed the updates to all endpoints
Patch Information
Mozilla released fixes in Firefox 134, Firefox ESR 128.6, Thunderbird 134, and Thunderbird 128.6. Full details are available in Mozilla Security Advisory MFSA-2025-01, MFSA-2025-02, MFSA-2025-04, and MFSA-2025-05.
Workarounds
- No vendor-supplied workaround exists; apply the official patch
- Restrict users from loading untrusted web content or opening HTML email from unknown senders until the patch is deployed
- Enforce enterprise policies that disable automatic rendering of remote content in Thunderbird where feasible
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
