CVE-2026-12301: Mozilla Firefox Buffer Overflow Vulnerability

CVE-2026-12301 Overview

CVE-2026-12301 is a memory safety vulnerability affecting Mozilla Firefox and Mozilla Thunderbird prior to version 152. The flaw is categorized under [CWE-119] (Improper Restriction of Operations within the Bounds of a Memory Buffer). Mozilla addressed the issue in Firefox 152 and Thunderbird 152 through advisories MFSA-2026-57 and MFSA-2026-60.

The vulnerability is network-exploitable and requires no authentication or user interaction. Successful exploitation can expose limited confidential information from the browser process. No public proof-of-concept exists, and the issue is not listed in the CISA Known Exploited Vulnerabilities catalog.

Critical Impact
Remote attackers can trigger a memory safety error in Firefox or Thunderbird to disclose low-impact information from the affected process without user interaction.

Affected Products

Mozilla Firefox (versions prior to 152)
Mozilla Thunderbird (versions prior to 152)
Mozilla advisories: MFSA-2026-57 and MFSA-2026-60

Discovery Timeline

2026-06-16 - CVE-2026-12301 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2026-12301

Vulnerability Analysis

The vulnerability is a memory safety bug within the shared Gecko codebase used by both Firefox and Thunderbird. Mozilla classifies the issue under [CWE-119], indicating an out-of-bounds memory access that violates buffer boundary restrictions. The flaw is reachable over the network through standard web content rendering or HTML email parsing.

Exploitation does not require privileges or user interaction beyond visiting a crafted page or processing a crafted message. The impact is limited to confidentiality, with no integrity or availability consequences observed. The EPSS probability remains low, reflecting the absence of public exploit tooling.

Root Cause

The root cause is improper restriction of operations within a memory buffer in browser and mail client rendering code. Mozilla's bug report #2015647 tracks the underlying defect. Memory safety errors of this class typically arise from incorrect bounds checks, type confusion in object handling, or unsafe pointer arithmetic.

Attack Vector

An attacker delivers crafted web content or HTML email content that triggers the unsafe memory operation during parsing or rendering. The flaw can be reached through any vector that causes the client to process attacker-controlled markup or script. No authentication is required, and the attack complexity is low.

No verified exploit code is publicly available. Refer to the Mozilla Security Advisory MFSA-2026-57 and Mozilla Security Advisory MFSA-2026-60 for vendor-provided technical context.

Detection Methods for CVE-2026-12301

Indicators of Compromise

Firefox or Thunderbird processes crashing or exhibiting abnormal memory access patterns when rendering specific web pages or messages.
Outbound connections from browser or mail client processes to unfamiliar domains immediately after rendering untrusted content.
Presence of installed Firefox or Thunderbird builds reporting a version earlier than 152.

Detection Strategies

Inventory endpoints for Firefox and Thunderbird versions and flag any installation below 152 as vulnerable.
Monitor for unexpected child processes or memory-region anomalies originating from firefox.exe or thunderbird.exe.
Correlate browser crash telemetry with recent navigation events to identify potential exploitation attempts.

Monitoring Recommendations

Ingest browser and mail client crash logs into a centralized SIEM for trend analysis.
Track Mozilla security advisory feeds for follow-on disclosures related to MFSA-2026-57 and MFSA-2026-60.
Alert on Thunderbird processing of HTML email with active remote content, which remains a primary attack surface.

How to Mitigate CVE-2026-12301

Immediate Actions Required

Upgrade all Firefox installations to version 152 or later.
Upgrade all Thunderbird installations to version 152 or later.
Verify enterprise update channels (ESR, managed deployments) are pulling the patched builds.

Patch Information

Mozilla released fixes in Firefox 152 and Thunderbird 152. Refer to the Mozilla Security Advisory MFSA-2026-57 and Mozilla Security Advisory MFSA-2026-60 for full version details. The underlying defect is tracked in Mozilla Bug Report #2015647.

Workarounds

Disable JavaScript on untrusted sites where feasible until patched versions are deployed.
Configure Thunderbird to render messages as plain text and block remote content in HTML email.
Restrict browser usage on high-value endpoints to vetted sites until upgrades are complete.

bash

# Verify installed Firefox version on Linux endpoints
firefox --version

# Verify installed Thunderbird version on Linux endpoints
thunderbird --version

# Example: enforce minimum version via package manager (Debian/Ubuntu)
apt-get update && apt-get install --only-upgrade firefox thunderbird