CVE-2026-12305: Mozilla Firefox Buffer Overflow Vulnerability

CVE-2026-12305 Overview

CVE-2026-12305 is a memory safety vulnerability affecting Mozilla Firefox and Mozilla Thunderbird. Mozilla addressed the flaw in Firefox 152, Firefox ESR 140.12, Thunderbird 152, and Thunderbird 140.12. The issue is classified under [CWE-119] (Improper Restriction of Operations within the Bounds of a Memory Buffer). The vulnerability is reachable over the network, requires no privileges, and needs no user interaction beyond standard browsing or message rendering. Successful exploitation primarily impacts availability, leading to process crashes that may be leveraged for further memory corruption attacks.

Critical Impact
Remote attackers can trigger memory corruption in Firefox or Thunderbird through crafted web content or messages, causing application crashes and potentially enabling further exploitation.

Affected Products

Mozilla Firefox (versions prior to 152)
Mozilla Firefox ESR (versions prior to 140.12)
Mozilla Thunderbird (versions prior to 152 and 140.12)

Discovery Timeline

2026-06-16 - CVE-2026-12305 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2026-12305

Vulnerability Analysis

The flaw is a memory safety bug present in the shared codebase of Firefox and Thunderbird. Mozilla's advisories indicate that engineers identified memory corruption signatures that, with sufficient effort, could be exploited to run arbitrary code. The vulnerability is reachable through normal content processing paths, meaning a malicious web page or HTML email can introduce the malformed structures that trigger the bug. The primary observable outcome is process instability, but the underlying corruption increases the attack surface for follow-on exploitation chains. The EPSS probability for this issue is 0.374%, placing it in the 29th percentile of vulnerabilities by predicted exploitation likelihood.

Root Cause

The root cause is improper restriction of operations within a memory buffer, mapped to [CWE-119]. Memory safety bugs in browser engines typically arise from incorrect bounds tracking, lifetime management errors in C++ object graphs, or unchecked allocator interactions. Mozilla has not disclosed the precise component in the public advisory. The bug report referenced in Mozilla Bugzilla #2037290 tracks the underlying defect.

Attack Vector

An attacker delivers crafted content over the network to a vulnerable client. For Firefox, this means hosting a malicious page or compromising an existing site to serve hostile HTML, JavaScript, CSS, or media. For Thunderbird, exploitation requires the user to view a crafted message in a context where remote content rendering is permitted. No authentication is required, and the attack does not depend on user interaction beyond loading the content. See Mozilla Security Advisory MFSA-2026-57 for vendor-confirmed details.

// No verified proof-of-concept code is available for CVE-2026-12305.
// Refer to Mozilla Bugzilla #2037290 and MFSA-2026-57 through MFSA-2026-61 for technical details.

Detection Methods for CVE-2026-12305

Indicators of Compromise

Unexpected crashes of firefox.exe, firefox, thunderbird.exe, or thunderbird processes, particularly with access violation or segmentation fault signatures.
Crash reporter telemetry referencing memory corruption in content or rendering processes after visiting untrusted sites or opening untrusted messages.
Child process spawning or shellcode-like behavior originating from a browser or mail client process.

Detection Strategies

Inventory installed Firefox, Firefox ESR, Thunderbird, and Thunderbird ESR versions across managed endpoints and flag any build below the fixed releases.
Correlate process crash events on endpoints with recent web navigation or message-open events to identify potential exploitation attempts.
Monitor egress traffic from browser and mail client processes for anomalous connections that follow crashes or hangs.

Monitoring Recommendations

Forward Windows Application crash events, macOS ReportCrash logs, and Linux core dump records to a centralized analytics platform for correlation.
Track Mozilla advisories MFSA-2026-57, MFSA-2026-58, MFSA-2026-60, and MFSA-2026-61 for follow-up guidance.
Alert on browser or mail client processes launching command interpreters such as cmd.exe, powershell.exe, /bin/sh, or bash.

How to Mitigate CVE-2026-12305

Immediate Actions Required

Upgrade Firefox to version 152 or later and Firefox ESR to 140.12 or later on all managed endpoints.
Upgrade Thunderbird to version 152 or later and Thunderbird ESR to 140.12 or later.
Enforce automatic update channels for Mozilla products through enterprise policy where feasible.

Patch Information

Mozilla released fixes in Firefox 152, Firefox ESR 140.12, Thunderbird 152, and Thunderbird 140.12. Patch details are documented in MFSA-2026-57, MFSA-2026-58, MFSA-2026-60, and MFSA-2026-61.

Workarounds

Disable JavaScript on untrusted sites using browser policy until patches are deployed.
Configure Thunderbird to block remote content in messages by default to reduce exposure from email-borne content.
Restrict end-user ability to downgrade Firefox or Thunderbird builds through endpoint management policies.

bash

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

# Windows: query installed versions via registry
reg query "HKLM\SOFTWARE\Mozilla\Mozilla Firefox" /v CurrentVersion
reg query "HKLM\SOFTWARE\Mozilla\Mozilla Thunderbird" /v CurrentVersion