CVE-2026-74965: Mozilla Firefox Privilege Escalation Flaw

CVE-2026-74965 Overview

CVE-2026-74965 is a privilege escalation vulnerability in the Shell Integration component of Mozilla Firefox and Mozilla Thunderbird. The flaw is classified under CWE-269: Improper Privilege Management. An attacker can leverage the vulnerability over the network with user interaction to achieve high impact on confidentiality, integrity, and availability. Mozilla fixed the issue in Firefox 154, Firefox ESR 140.14, Firefox ESR 153.1, Thunderbird 154, Thunderbird 140.14, and Thunderbird 153.1.

Critical Impact
Successful exploitation lets an attacker escalate privileges through the Shell Integration component, potentially bypassing sandbox boundaries and gaining elevated access on the host system.

Affected Products

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

Discovery Timeline

2026-08-18 - CVE-2026-74965 published to NVD
2026-08-21 - Last updated in NVD database

Technical Details for CVE-2026-74965

Vulnerability Analysis

The vulnerability resides in the Shell Integration component, which handles interactions between the Mozilla application and the operating system shell. Improper privilege management (CWE-269) allows an attacker-controlled input path to influence operations executed with higher privilege than intended. This weakens the isolation between the browser or mail client and the host environment.

Exploitation requires user interaction, such as opening a crafted resource, message, or link inside Firefox or Thunderbird. Once triggered, the flaw allows the attacker to escalate privileges within the affected process context, breaking the trust boundary the Shell Integration layer is meant to enforce.

Root Cause

The root cause is improper enforcement of privilege boundaries in the Shell Integration code path. The component performs an action on behalf of the user without sufficiently constraining the caller or validating the operation, allowing lower-privileged content to influence higher-privileged shell operations. Refer to the Mozilla Bug Report #2053455 for the internal tracking record.

Attack Vector

The attack vector is network-based. An attacker hosts crafted content that the victim interacts with through Firefox or Thunderbird. When the Shell Integration component processes the crafted input, the privilege check is bypassed and the attacker obtains elevated privileges in the application context. No prior authentication is required. See the Mozilla Security Advisory MFSA-2026-74 and companion advisories for scope details.

No public proof-of-concept code has been published. The vulnerability mechanism is described in the referenced Mozilla advisories.

Detection Methods for CVE-2026-74965

Indicators of Compromise

Firefox or Thunderbird child processes spawning unexpected shell commands, cmd.exe, powershell.exe, or shell utilities without a user-driven action.
Unusual file writes or registry modifications originating from firefox.exe or thunderbird.exe outside of the standard profile directory.
Outbound connections from browser or mail client processes to attacker-controlled infrastructure immediately followed by privilege-elevation activity.

Detection Strategies

Inventory endpoint browser and mail client versions and alert on hosts still running Firefox below 154, Firefox ESR below 140.14 or 153.1, or Thunderbird below the equivalent fixed builds.
Build behavioral rules that flag Shell Integration-related process launches, such as browser processes invoking OS shell handlers with unexpected arguments.
Correlate user interaction events (link clicks, attachment opens) with subsequent privileged operations to surface exploitation attempts.

Monitoring Recommendations

Forward endpoint process, file, and network telemetry to a centralized analytics platform for cross-host correlation.
Enable module and script logging on Windows to capture PowerShell or WScript invocations from Mozilla processes.
Track Mozilla advisory publications (MFSA-2026-74 through MFSA-2026-80) for related follow-up fixes.

How to Mitigate CVE-2026-74965

Immediate Actions Required

Upgrade Firefox to version 154 or later on all endpoints.
Upgrade Firefox ESR deployments to 140.14 or 153.1.
Upgrade Thunderbird to 154, 140.14, or 153.1 depending on the deployed branch.
Restart affected applications after patching to ensure the vulnerable Shell Integration binaries are unloaded.

Patch Information

Mozilla addressed CVE-2026-74965 in Firefox 154, Firefox ESR 140.14, Firefox ESR 153.1, Thunderbird 154, Thunderbird 140.14, and Thunderbird 153.1. Full advisory content is available in the Mozilla Security Advisory MFSA-2026-74, MFSA-2026-76, MFSA-2026-77, MFSA-2026-78, MFSA-2026-79, and MFSA-2026-80.

Workarounds

Restrict Firefox and Thunderbird from being registered as default handlers for high-risk URI schemes until patching is complete.
Enforce least-privilege user accounts so the impact of any residual Shell Integration abuse is limited.
Use application allowlisting to block child processes such as shells or scripting hosts from being launched by Mozilla applications.
Deliver security awareness reminders instructing users not to interact with untrusted links or attachments during the patch window.

bash

# Configuration example: enumerate Mozilla versions across a fleet (Windows PowerShell)
Get-ItemProperty "HKLM:\SOFTWARE\Mozilla\Mozilla Firefox" -ErrorAction SilentlyContinue |
  Select-Object CurrentVersion
Get-ItemProperty "HKLM:\SOFTWARE\Mozilla\Mozilla Thunderbird" -ErrorAction SilentlyContinue |
  Select-Object CurrentVersion