CVE-2026-74964: Mozilla Firefox Integer Overflow Vulnerability

CVE-2026-74964 Overview

CVE-2026-74964 is an integer overflow vulnerability [CWE-190] in the Graphics component of Mozilla Firefox and Thunderbird. The flaw allows a remote attacker to trigger memory corruption by delivering crafted graphics content processed by the browser or mail client. Exploitation requires no privileges and no user interaction beyond loading attacker-controlled content. Mozilla addressed the issue in Firefox 154, Firefox ESR 140.14, Firefox ESR 153.1, Thunderbird 154, Thunderbird 140.14, and Thunderbird 153.1.

Critical Impact
A network-reachable attacker can achieve memory corruption in the Graphics component, potentially leading to arbitrary code execution within the Firefox or Thunderbird process.

Affected Products

Mozilla Firefox (versions prior to 154)
Mozilla Firefox ESR (versions prior to 140.14 and 153.1)
Mozilla Thunderbird (versions prior to 154, 140.14, and 153.1)

Discovery Timeline

2026-08-18 - CVE-2026-74964 published to the National Vulnerability Database
2026-08-19 - Last updated in NVD database

Technical Details for CVE-2026-74964

Vulnerability Analysis

The vulnerability resides in the Graphics component shared by Firefox and Thunderbird. An integer overflow occurs during arithmetic operations on values that describe graphics buffers or geometry. When the computed value wraps around the integer boundary, subsequent allocations or bounds checks operate on a truncated size. This mismatch enables out-of-bounds memory access on the heap, producing exploitable memory corruption.

Because Thunderbird renders HTML mail using the same graphics stack, the issue extends beyond web browsing to email preview and display. Successful exploitation runs code in the content process, which an attacker can then combine with a sandbox escape to gain broader system access.

Root Cause

The root cause is unchecked integer arithmetic on attacker-influenced dimensions or offsets within the Graphics component. Mozilla classifies the issue under [CWE-190: Integer Overflow or Wraparound]. Details are tracked in the Mozilla Bug Report #2053327.

Attack Vector

Exploitation is network-based and requires no authentication. An attacker hosts a malicious web page containing crafted graphics content, such as an oversized image, canvas operation, or WebGL payload. When Firefox loads the page, the Graphics component processes the content and triggers the overflow. For Thunderbird, the equivalent trigger is a specially crafted HTML message rendered by the client.

See the Mozilla Security Advisory MFSA-2026-74 for vendor-provided technical context.

Detection Methods for CVE-2026-74964

Indicators of Compromise

Unexpected crashes of firefox.exe or thunderbird.exe processes with access-violation exit codes, particularly in graphics or renderer threads.
Content processes spawning unusual child processes such as command shells, PowerShell, or scripting hosts.
Outbound connections from browser or mail-client processes to unfamiliar hosts immediately after loading web or email content.

Detection Strategies

Inventory endpoints running Firefox or Thunderbird and flag hosts with versions below the patched builds (Firefox 154, ESR 140.14, ESR 153.1, and equivalent Thunderbird releases).
Alert on crash telemetry containing graphics-related stack frames followed by process restart loops.
Correlate browser crash events with subsequent process creation or network activity to identify likely exploitation chains.

Monitoring Recommendations

Monitor EDR telemetry for child-process creation, script interpreter launches, and memory-injection behavior originating from browser or mail-client processes.
Track file writes to user-writable directories such as %APPDATA% or /tmp sourced from Firefox or Thunderbird processes.
Log DNS and HTTP egress from browser processes and hunt for connections following visits to newly registered or low-reputation domains.

How to Mitigate CVE-2026-74964

Immediate Actions Required

Update Firefox to version 154 or newer on all managed endpoints without delay.
Update Firefox ESR deployments to 140.14 or 153.1 depending on the supported branch.
Update Thunderbird to 154, 140.14, or 153.1 to close the same code path in the mail client.
Restart affected applications after patching so the updated Graphics component is loaded.

Patch Information

Mozilla published fixed builds alongside advisories MFSA-2026-74, MFSA-2026-76, MFSA-2026-77, MFSA-2026-78, MFSA-2026-79, and MFSA-2026-80. Administrators should confirm silent-update policies are enabled and validate versions using package inventory tools.

Workarounds

Disable JavaScript and WebGL in high-risk environments until patches are deployed, accepting the reduction in site compatibility.
Configure Thunderbird to display messages in plain text to avoid rendering attacker-controlled HTML and graphics content.
Apply network-level filtering to block access to untrusted or newly observed domains from endpoints that cannot be immediately updated.

bash

# Configuration example: force plain-text rendering in Thunderbird
# Add the following preferences to prefs.js or set via Config Editor
user_pref("mailnews.display.prefer_plaintext", true);
user_pref("mailnews.display.html_as", 1);
user_pref("mailnews.display.disallow_mime_handlers", 3);