CVE-2025-0238: Mozilla Firefox Use-After-Free Vulnerability

CVE-2025-0238 Overview

CVE-2025-0238 is a use-after-free vulnerability [CWE-416] affecting Mozilla Firefox and Thunderbird. An attacker who can trigger a controlled failed memory allocation can cause the browser to reuse freed memory, resulting in a potentially exploitable crash. The flaw is reachable over the network with no privileges or user interaction, though impact is limited to confidentiality exposure through the crash conditions.

Mozilla addressed the issue in Firefox 134, Firefox ESR 128.6, Firefox ESR 115.19, Thunderbird 134, and Thunderbird 128.6. The vulnerability was published to the National Vulnerability Database (NVD) on January 7, 2025.

Critical Impact
Attackers who can force a failed allocation in the browser can trigger a use-after-free condition that leads to a crash with the potential for further exploitation, including information disclosure.

Affected Products

Mozilla Firefox versions prior to 134
Mozilla Firefox ESR versions prior to 128.6 and 115.19
Mozilla Thunderbird versions prior to 134 and 128.6

Discovery Timeline

2025-01-07 - CVE-2025-0238 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-0238

Vulnerability Analysis

The vulnerability is a use-after-free condition classified under [CWE-416]. Use-after-free bugs occur when a program continues to reference memory after that memory has been released back to the allocator. Subsequent access to the dangling pointer can read stale data, corrupt heap metadata, or hijack control flow if the freed region is reallocated with attacker-controlled contents.

In this case, the trigger is a controlled failed memory allocation. When the affected code path attempts to allocate memory and the allocation fails, error handling releases an object but leaves a pointer that is later dereferenced. An attacker who can shape allocator state, for example by exhausting memory through large web content, can make the failure deterministic.

Because the vulnerability is reachable through the browser's content rendering surface, a malicious webpage or HTML email opened in Thunderbird is sufficient to reach the vulnerable code without user interaction beyond visiting the resource.

Root Cause

The root cause is missing null-pointer or state validation on the failure path of a memory allocation. The code assumes allocation success and continues to operate on an object whose lifetime was terminated during error handling. Details on the specific component are tracked in Mozilla Bug Report #1915535.

Attack Vector

Exploitation requires the victim to load attacker-controlled content in Firefox or Thunderbird. The attacker crafts a page or message that pressures the allocator into a controlled failure, then interacts with the object graph that dereferences the freed pointer. No authentication or user interaction beyond loading the content is required. Successful exploitation yields a crash with potential for information disclosure or further memory-corruption primitives.

No public proof-of-concept or exploit code is available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. See Mozilla Security Advisory MFSA-2025-01 for vendor technical detail.

Detection Methods for CVE-2025-0238

Indicators of Compromise

Unexpected Firefox or Thunderbird process crashes with signatures pointing to freed-object access in the content or networking process.
Crash reporter submissions referencing the affected allocator path documented in Mozilla Bug Report #1915535.
Endpoint telemetry showing browser child processes terminating abnormally shortly after visiting untrusted origins.

Detection Strategies

Inventory installed browser and mail client versions and flag hosts running Firefox below 134, Firefox ESR below 128.6 or 115.19, and Thunderbird below 134 or 128.6.
Correlate browser crash events with outbound web traffic to newly observed or low-reputation domains to surface exploitation attempts.
Ingest Mozilla crash reporter and operating system application logs into a central data lake for retrospective hunting on repeated child-process failures.

Monitoring Recommendations

Track Firefox and Thunderbird versions across the estate using software inventory tooling and alert on regressions to unpatched builds.
Monitor for anomalous memory pressure events on user endpoints, which can indicate allocator-shaping activity preceding exploitation.
Subscribe to updates on Mozilla Security Advisory MFSA-2025-02 and related bulletins for follow-up guidance.

How to Mitigate CVE-2025-0238

Immediate Actions Required

Update Firefox to version 134 or later on all endpoints where the browser is installed.
Update Firefox ESR deployments to 128.6 or 115.19 depending on the supported branch.
Update Thunderbird to version 134 or 128.6 across all mail client installations.
On Debian-based fleets, apply the packages referenced in the Debian LTS Announcement.

Patch Information

Mozilla shipped the fix in Firefox 134, Firefox ESR 128.6, Firefox ESR 115.19, Thunderbird 134, and Thunderbird 128.6. Consult Mozilla Security Advisory MFSA-2025-03, MFSA-2025-04, and MFSA-2025-05 for the full advisory set covering the affected products.

Workarounds

Enforce browser auto-update policies through group policy or configuration management to prevent version drift.
Restrict browsing of untrusted content through egress filtering and DNS-based reputation controls until patches are deployed.
Disable HTML rendering of remote content in Thunderbird for high-risk users to reduce the attack surface for the mail client.

bash

# Verify installed Firefox and Thunderbird versions on Linux endpoints
firefox --version
thunderbird --version

# Example Debian/Ubuntu update commands
sudo apt-get update
sudo apt-get install --only-upgrade firefox-esr thunderbird