CVE-2026-109760: Firefox HTTP Auth Bypass Vulnerability

CVE-2026-109760 Overview

CVE-2026-109760 is a mitigation bypass vulnerability in the Networking: HTTP component of Mozilla Firefox and Thunderbird. The flaw allows attackers to circumvent an existing security mitigation within the HTTP networking stack. Mozilla addressed the issue in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5. The vulnerability is documented across multiple Mozilla Foundation Security Advisories, including MFSA-2026-106, MFSA-2026-108, MFSA-2026-109, and MFSA-2026-110.

Critical Impact

An attacker who leverages this bypass can weaken browser-level HTTP protections, potentially enabling follow-on attacks against users of unpatched Firefox and Thunderbird builds.

Affected Products

• Mozilla Firefox versions prior to 158
• Mozilla Firefox ESR versions prior to 153.5
• Mozilla Thunderbird versions prior to 158 and prior to 153.5

Discovery Timeline

• 2026-10-13 - CVE-2026-109760 published to the National Vulnerability Database
• 2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-109760

Vulnerability Analysis

The vulnerability exists in the Networking: HTTP component shared across Mozilla's Gecko-based products. The defect lets an attacker bypass a security mitigation that the HTTP stack enforces during request or response handling. Mozilla has not published full exploitation details, consistent with its coordinated disclosure practice while users migrate to patched builds. Tracking metadata for the underlying defect is maintained in Mozilla Bugzilla #2070633.

Because the issue affects both the browser and the mail client, exposure extends beyond active web browsing. Thunderbird renders remote content and interacts with HTTP-based services, meaning the same networking code path is reachable from message rendering and account activity.

Root Cause

Mozilla classifies the defect as a mitigation bypass rather than a direct memory-safety or logic bug. The upstream advisories indicate that an existing protection within the Networking: HTTP code did not fully constrain attacker-controlled inputs or state transitions. Specific root-cause details are not disclosed in the public advisory text at this time.

Attack Vector

Exploitation requires a victim to interact with attacker-influenced HTTP content. For Firefox, this typically means visiting a crafted web page or loading a hostile resource embedded in a trusted site. For Thunderbird, loading remote content in an HTML message provides a comparable path. The NVD entry does not list a CVSS vector, and Mozilla has not reported in-the-wild exploitation.

No verified proof-of-concept code has been published. See the referenced Mozilla advisories for authoritative technical context.

Detection Methods for CVE-2026-109760

Indicators of Compromise

• No file hashes, domains, or network IOCs have been published for CVE-2026-109760.
• Presence of Firefox builds below 158, Firefox ESR below 153.5, or Thunderbird below 158/153.5 on managed endpoints indicates exposure.

Detection Strategies

• Inventory browser and mail client versions across the fleet and flag any Mozilla product below the fixed releases.
• Monitor endpoint telemetry for Firefox or Thunderbird processes initiating unusual outbound HTTP connections or spawning child processes after rendering remote content.
• Correlate web proxy logs with endpoint process execution to identify suspicious page loads preceding anomalous network behavior.

Monitoring Recommendations

• Alert on execution of outdated firefox.exe, firefox-bin, or thunderbird binaries after the patch deadline.
• Track HTTP response anomalies at the egress proxy, including malformed headers and unexpected redirects that may align with mitigation-bypass behavior.
• Review Thunderbird remote-content loads from untrusted senders and treat frequent fetches as investigative leads.

How to Mitigate CVE-2026-109760

Immediate Actions Required

• Upgrade all Firefox installations to version 158 or later and Firefox ESR deployments to 153.5 or later.
• Upgrade Thunderbird installations to version 158 or 153.5, matching the deployed release channel.
• Enforce automatic updates through enterprise policy to prevent regression to vulnerable builds.

Patch Information

Mozilla released fixes in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5. Patch details and affected code paths are referenced in MFSA-2026-106, MFSA-2026-108, MFSA-2026-109, and MFSA-2026-110.

Workarounds

• Disable automatic remote content loading in Thunderbird to reduce exposure from HTML messages.
• Restrict browsing to trusted sites via proxy allow-lists until patched builds are deployed.
• Apply enterprise policy to block installation or execution of unsupported Firefox and Thunderbird versions.

# Configuration example
# Enforce minimum Firefox version via Group Policy ADMX (Windows)
# Policy path: Computer Configuration > Administrative Templates > Mozilla > Firefox
# Set: DisableAppUpdate = false
# Set: AppAutoUpdate = true

# Linux: verify installed versions
firefox --version       # expect: Mozilla Firefox 158.0 or later
thunderbird --version   # expect: Thunderbird 158.0 or 153.5 or later