CVE-2026-74977: Firefox Graphics Buffer Overflow Flaw

CVE-2026-74977 is a buffer overflow flaw in the Firefox Graphics component caused by an integer overflow. This vulnerability affects Firefox and Thunderbird browsers, potentially allowing code execution. This article covers technical details, affected versions, security impact, and mitigation strategies.

CVE-2026-74977 Overview

CVE-2026-74977 is an integer overflow vulnerability [CWE-190] in the Graphics component shared by Mozilla Firefox, Firefox ESR, and Thunderbird. A remote attacker can trigger the flaw over the network without authentication or user interaction, leading to a high-impact availability condition. Mozilla addressed the issue in Firefox 154, Firefox ESR 153.1, Thunderbird 154, and Thunderbird 153.1.

Critical Impact
Remote, unauthenticated attackers can trigger an integer overflow in the Graphics component to disrupt availability of Firefox and Thunderbird clients.

Affected Products

Mozilla Firefox versions prior to 154
Mozilla Firefox ESR versions prior to 153.1
Mozilla Thunderbird versions prior to 154 and prior to 153.1

Discovery Timeline

2026-08-18 - CVE-2026-74977 published to NVD
2026-08-18 - Last updated in NVD database

Technical Details for CVE-2026-74977

Vulnerability Analysis

The vulnerability resides in the Graphics component of the Gecko rendering engine used by Firefox and Thunderbird. Integer overflow conditions in graphics processing paths typically arise when size or dimension values are combined arithmetically without proper bounds checking. When the resulting value wraps, subsequent memory allocations or loop counters operate on values inconsistent with the underlying buffer state.

An attacker exploits the flaw by serving crafted content that reaches the affected Graphics routines during rendering. In Firefox this is a web page; in Thunderbird it is HTML email or remote content processed by the same engine. The vulnerability produces a high availability impact, consistent with a crash of the content or parent process.

Root Cause

The root cause is an arithmetic operation in the Graphics component that permits an integer overflow before values are used in size, offset, or allocation logic. Missing or insufficient overflow validation on attacker-influenced dimensions allows the calculation to wrap into an unexpected range.

Attack Vector

Exploitation is remote and requires no privileges or user interaction beyond loading attacker-controlled content. In Firefox, visiting a malicious page triggers the vulnerable graphics path. In Thunderbird, processing a crafted message with remote content enabled reaches the same code. Successful exploitation results in denial of service against the browser or mail client.

No public proof-of-concept, exploit code, or in-the-wild exploitation has been reported. See the Mozilla Bug #2028440 tracker and the Mozilla Security Advisory MFSA-2026-74 for vendor detail.

Detection Methods for CVE-2026-74977

Indicators of Compromise

Unexpected crashes of firefox.exe, firefox, or thunderbird processes correlated with rendering of remote content.
Crash reports referencing the Graphics component or GPU/compositor threads submitted via Mozilla crash telemetry.
Repeated tab or content-process terminations across multiple endpoints originating from the same URL or sender.

Detection Strategies

Inventory Firefox, Firefox ESR, and Thunderbird versions across managed endpoints and flag builds below the fixed versions.
Correlate browser and mail client crash events in endpoint telemetry with recently visited URLs or received messages.
Monitor for repeated content-process crash signatures that indicate attempted exploitation of rendering flaws.

Monitoring Recommendations

Ingest Windows Application, macOS ReportCrash, and Linux coredump events for Mozilla binaries into your SIEM.
Alert on crash bursts of the same Mozilla process across multiple users within a short time window.
Track outbound web and mail traffic to newly observed domains preceding client crashes.

How to Mitigate CVE-2026-74977

Immediate Actions Required

Upgrade Firefox to version 154 or later on all managed endpoints.
Upgrade Firefox ESR deployments to version 153.1 or later.
Upgrade Thunderbird to version 154 or Thunderbird ESR 153.1 or later.
Prioritize systems that process untrusted web content or external email, including kiosks and mail gateways.

Patch Information

Mozilla fixed the vulnerability in Firefox 154, Firefox ESR 153.1, Thunderbird 154, and Thunderbird 153.1. Refer to the Mozilla Security Advisory MFSA-2026-74, MFSA-2026-77, MFSA-2026-78, and MFSA-2026-80 for build metadata and cross-referenced fixes.

Workarounds

Disable remote content loading in Thunderbird until patches are applied.
Restrict browsing to trusted sites through web proxy or DNS filtering policies where immediate patching is not feasible.
Enforce automatic update channels for Firefox and Thunderbird to shorten exposure windows on future advisories.

bash

# Configuration example
# Verify installed Firefox and Thunderbird versions across Linux endpoints
firefox --version
thunderbird --version