CVE-2026-16402: Firefox ImageLib Buffer Overflow Flaw

CVE-2026-16402 Overview

CVE-2026-16402 is an integer overflow vulnerability [CWE-190] in the Graphics: ImageLib component used by Mozilla Firefox and Thunderbird. The flaw affects image processing routines and can be triggered remotely when a victim loads attacker-controlled image content. Mozilla addressed the issue in Firefox 153 and Thunderbird 153. The vulnerability is network-exploitable without authentication or user interaction beyond loading a page or message that renders the malicious image.

Critical Impact
Remote attackers can trigger an integer overflow in ImageLib through crafted image data, potentially leading to memory corruption and arbitrary code execution in the browser or mail client process.

Affected Products

Mozilla Firefox versions prior to 153
Mozilla Thunderbird versions prior to 153
Applications embedding the Gecko Graphics: ImageLib component

Discovery Timeline

2026-07-21 - CVE-2026-16402 published to NVD
2026-07-22 - Last updated in NVD database

Technical Details for CVE-2026-16402

Vulnerability Analysis

The vulnerability resides in the Graphics: ImageLib component, which decodes and renders image formats within Gecko-based applications. An integer overflow occurs during size or offset calculations while processing image data. When an arithmetic operation exceeds the maximum value of its integer type, the result wraps around to a small or negative value.

This undersized value is then used to allocate or index into a memory buffer. Subsequent read or write operations exceed the actual allocation boundary, producing heap corruption. Attackers who control the overflow inputs can shape adjacent memory to hijack control flow or leak sensitive data from the renderer process.

The attack surface includes any content path that reaches ImageLib decoders: web pages, HTML email in Thunderbird, and embedded resources. The vulnerability is scored 9.8 based on network reachability, no privileges required, and full impact to confidentiality, integrity, and availability. The EPSS probability is 0.319% as of 2026-07-23.

Root Cause

The root cause is an unchecked arithmetic operation on attacker-controlled dimensions or length fields inside Graphics: ImageLib. Missing overflow validation before allocation permits a wrapped size to reach downstream memory operations. This aligns with [CWE-190] Integer Overflow or Wraparound.

Attack Vector

An attacker hosts a crafted image or delivers it inside HTML email content. When Firefox loads the page or Thunderbird renders the message, ImageLib parses the malformed dimensions and triggers the overflow. No authentication is required, and exploitation occurs during normal content rendering. See the Mozilla Bug Report #2052703 for technical details on the affected code path.

No public proof-of-concept code is available at the time of publication. Refer to Mozilla Security Advisory MFSA-2026-68 and Mozilla Security Advisory MFSA-2026-71 for vendor guidance.

Detection Methods for CVE-2026-16402

Indicators of Compromise

Firefox or Thunderbird processes (firefox.exe, thunderbird.exe) crashing while decoding image content, with crash signatures pointing to ImageLib decoders
Renderer or content processes spawning unexpected child processes such as cmd.exe, powershell.exe, or /bin/sh
Outbound network connections from browser or mail client processes to previously unseen infrastructure shortly after image content is rendered

Detection Strategies

Inventory endpoints running Firefox or Thunderbird versions below 153 and flag them as exposed
Alert on browser and mail client process trees that deviate from baseline, especially child process creation from renderer sandboxes
Inspect web proxy and email gateway telemetry for images with anomalous dimension or length metadata targeting known ImageLib decoders

Monitoring Recommendations

Collect endpoint process, file, and network telemetry from all workstations running Gecko-based applications
Correlate application crash events (WerFault, macOS crash reports, Linux core dumps) with subsequent suspicious process or network activity
Track patch state of Firefox and Thunderbird across the fleet and generate alerts for hosts that remain on vulnerable builds

How to Mitigate CVE-2026-16402

Immediate Actions Required

Upgrade Firefox to version 153 or later and Thunderbird to version 153 or later on all managed endpoints
Prioritize systems that handle untrusted email or browse arbitrary web content, including executive and finance workstations
Restart applications after patching to ensure the vulnerable ImageLib code is unloaded from memory

Patch Information

Mozilla fixed the vulnerability in Firefox 153 and Thunderbird 153. Patch details are published in Mozilla Security Advisory MFSA-2026-68 and Mozilla Security Advisory MFSA-2026-71. Enterprise administrators should deploy the updated MSI, PKG, or Linux packages through their standard software distribution tooling.

Workarounds

Disable automatic image loading in Thunderbird by unchecking remote content options until patching completes
Route web traffic through a filtering proxy that inspects and blocks malformed image responses
Restrict high-risk users to text-only email rendering where operationally feasible

bash

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

# Verify installed Thunderbird version
thunderbird --version

# Example: force policy update via Firefox enterprise policy
# /etc/firefox/policies/policies.json
# {
#   "policies": {
#     "AppAutoUpdate": true,
#     "DisableAppUpdate": false
#   }
# }