CVE-2026-74935 Overview
CVE-2026-74935 is a privilege escalation vulnerability in the DOM: Networking component of Mozilla Firefox and Thunderbird. The flaw allows a remote attacker to elevate privileges within the browser process when a user visits a crafted web page. Mozilla addressed the issue in Firefox 154, Firefox ESR 115.39, Firefox ESR 140.14, Firefox ESR 153.1, Thunderbird 154, Thunderbird 140.14, and Thunderbird 153.1. The vulnerability is classified under CWE-269: Improper Privilege Management.
Critical Impact
A remote attacker who convinces a user to load a malicious page can escalate privileges within the browser and compromise confidentiality, integrity, and availability of the host session.
Affected Products
- Mozilla Firefox (versions prior to 154)
- Mozilla Firefox ESR (prior to 115.39, 140.14, and 153.1)
- Mozilla Thunderbird (prior to 154, 140.14, and 153.1)
Discovery Timeline
- 2026-08-18 - CVE-2026-74935 published to NVD
- 2026-08-21 - Last updated in NVD database
Technical Details for CVE-2026-74935
Vulnerability Analysis
The vulnerability resides in the DOM: Networking component of Gecko, the rendering engine shared by Firefox and Thunderbird. Improper privilege management in networking-related DOM interfaces allows content-level code paths to acquire capabilities that should remain restricted to privileged browser contexts. Successful exploitation requires the user to interact with attacker-controlled content, such as loading a page or opening a message that renders remote content.
Because Thunderbird shares the Gecko platform, HTML-rendered email content can also trigger the flaw when remote content loading is enabled. The impact aligns with a browser sandbox weakening rather than a native memory corruption, but the outcome enables execution paths outside the intended web-content boundary.
Root Cause
The root cause is improper privilege management [CWE-269] within networking-related DOM interfaces. The affected code paths did not correctly enforce the separation between web-content principals and privileged internal principals, allowing untrusted script to reach operations reserved for higher-privileged callers. Refer to Mozilla Bug Report #2051013 for the tracked defect.
Attack Vector
The attack vector is network-based and requires user interaction. An attacker hosts a crafted web page or delivers HTML email content that exercises the vulnerable DOM networking path. When the target loads the content in an unpatched Firefox or Thunderbird build, the attacker-controlled script gains elevated privileges within the application, enabling further actions against the user's session. See the Mozilla Security Advisory MFSA-2026-74 for vendor-confirmed details.
No public proof-of-concept exploit and no verified code examples are available at the time of publication. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.
Detection Methods for CVE-2026-74935
Indicators of Compromise
- Firefox or Thunderbird processes spawning unexpected child processes or writing to non-standard locations after visiting a web page or opening an email.
- Outbound connections from firefox.exe, thunderbird.exe, or their content processes to unfamiliar domains immediately following content rendering.
- Presence of unpatched Firefox and Thunderbird versions below the fixed releases identified by Mozilla.
Detection Strategies
- Inventory installed Firefox and Thunderbird versions across the fleet and flag any build older than Firefox 154, Firefox ESR 115.39/140.14/153.1, or the corresponding Thunderbird releases.
- Correlate browser process telemetry with subsequent script interpreter, shell, or LOLBin execution to surface post-exploitation activity.
- Alert on Thunderbird rendering remote HTML content followed by unusual network egress or process creation.
Monitoring Recommendations
- Continuously monitor endpoint telemetry for anomalous child processes descending from browser and mail client parents.
- Track HTTP and DNS activity from browser processes to newly registered or low-reputation domains.
- Maintain vulnerability management scans that include Mozilla component versioning to detect regressions after patch cycles.
How to Mitigate CVE-2026-74935
Immediate Actions Required
- Upgrade Firefox to version 154 or later, and Firefox ESR to 115.39, 140.14, or 153.1 as appropriate for the deployment channel.
- Upgrade Thunderbird to version 154, 140.14, or 153.1 across all managed endpoints.
- Restart affected applications after patching to ensure the vulnerable code is unloaded from memory.
Patch Information
Mozilla released fixes across seven advisories covering this issue. Review the vendor guidance in MFSA-2026-74, MFSA-2026-75, MFSA-2026-76, MFSA-2026-77, MFSA-2026-78, MFSA-2026-79, and MFSA-2026-80. Apply the update corresponding to the branch deployed in your environment.
Workarounds
- Disable remote content loading in Thunderbird to reduce exposure from HTML email until patches are deployed.
- Enforce browser policies that block untrusted sites and restrict script execution on unclassified domains.
- Use application allowlisting to prevent child processes from Firefox and Thunderbird when a business need does not exist.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
