CVE-2025-8037: Mozilla Firefox XSS Vulnerability

CVE-2025-8037 Overview

CVE-2025-8037 is a critical cookie security bypass vulnerability affecting Mozilla Firefox and Thunderbird. Setting a nameless cookie with an equals sign in the value allows an attacker to shadow other cookies, including those with the Secure attribute. This cookie confusion attack enables a nameless cookie set over an insecure HTTP connection to override secure cookies, potentially leading to session hijacking, authentication bypass, and other serious security implications.

Critical Impact

Attackers can bypass the Secure cookie attribute protection by setting a nameless cookie over HTTP that shadows legitimate secure cookies, enabling potential session hijacking and authentication bypass attacks.

Affected Products

• Mozilla Firefox versions prior to 141
• Mozilla Firefox ESR versions prior to 140.1
• Mozilla Thunderbird versions prior to 141
• Mozilla Thunderbird ESR versions prior to 140.1

Discovery Timeline

• July 22, 2025 - CVE-2025-8037 published to NVD
• July 28, 2025 - Last updated in NVD database

Technical Details for CVE-2025-8037

Vulnerability Analysis

This vulnerability exploits a flaw in how Mozilla Firefox and Thunderbird handle cookie parsing, specifically when processing nameless cookies that contain an equals sign (=) in the value. The cookie parsing logic fails to properly differentiate between nameless cookies with values containing equals signs and legitimate named cookies, creating a shadowing condition.

The core issue relates to CWE-614 (Sensitive Cookie in HTTPS Session Without 'Secure' Attribute), where the browser's cookie handling mechanism allows an attacker to circumvent the Secure attribute protection. When a nameless cookie is crafted with a value that includes an equals sign, the parser interprets the content in a way that shadows other cookies in the cookie jar.

Root Cause

The root cause stems from improper cookie string parsing in the affected Mozilla products. When a cookie is set without a name but with a value containing an equals sign (e.g., =foo=bar), the browser's cookie handling code incorrectly processes this string. The equals sign in the value causes the parser to misinterpret the cookie boundaries, allowing the nameless cookie to effectively replace or shadow legitimate cookies—including those protected with the Secure attribute.

This parsing inconsistency means that even cookies that were properly set over HTTPS with the Secure flag can be overwritten by malicious cookies set over plain HTTP, completely bypassing the security protections that the Secure attribute is designed to provide.

Attack Vector

The attack is network-based and requires no privileges or user interaction. An attacker positioned to inject HTTP traffic (such as through a man-in-the-middle position on an insecure network) can exploit this vulnerability by:

1. Intercepting HTTP traffic from a victim's browser
2. Injecting a response containing a maliciously crafted nameless cookie with an equals sign in the value
3. The malicious cookie shadows the victim's legitimate secure cookies
4. Subsequent requests to the target domain include the attacker-controlled cookie instead of the legitimate one

This attack is particularly dangerous because it allows an attacker on an insecure channel (HTTP) to compromise sessions that were established over a secure channel (HTTPS), undermining the fundamental security guarantees of the Secure cookie attribute.

Detection Methods for CVE-2025-8037

Indicators of Compromise

• Unusual cookie values in browser requests containing nameless cookies with equals signs in the value
• Session inconsistencies or unexpected authentication failures for users on affected browser versions
• Network traffic showing HTTP responses setting cookies with the pattern Set-Cookie: =value=content
• User reports of session hijacking or unexpected logouts on secure sites

Detection Strategies

• Monitor web application logs for anomalous cookie patterns, particularly nameless cookies or cookies with unusual formatting
• Implement server-side cookie validation to detect and reject malformed or suspicious cookie structures
• Deploy network intrusion detection rules to identify HTTP responses attempting to set nameless cookies with equals signs in values
• Audit browser versions across the organization to identify vulnerable Firefox and Thunderbird installations

Monitoring Recommendations

• Enable enhanced logging on web application firewalls to capture Set-Cookie header anomalies
• Monitor authentication and session management systems for unusual patterns indicating cookie-based attacks
• Track browser version telemetry to ensure all endpoints are running patched versions
• Review network traffic for HTTP responses on domains that should only receive HTTPS traffic

How to Mitigate CVE-2025-8037

Immediate Actions Required

• Update Mozilla Firefox to version 141 or later immediately
• Update Mozilla Firefox ESR to version 140.1 or later
• Update Mozilla Thunderbird to version 141 or later
• Update Mozilla Thunderbird ESR to version 140.1 or later
• Enforce HSTS (HTTP Strict Transport Security) on all web properties to reduce the attack surface for HTTP-based cookie injection

Patch Information

Mozilla has released security updates addressing this vulnerability. Detailed information is available in the following security advisories:

• Mozilla Security Advisory MFSA-2025-56
• Mozilla Security Advisory MFSA-2025-59
• Mozilla Security Advisory MFSA-2025-61
• Mozilla Security Advisory MFSA-2025-63

Technical details about the underlying issue can be found in Mozilla Bug Report #1964767.

Workarounds

• Deploy HSTS preloading for all critical domains to prevent any HTTP traffic that could be exploited for cookie injection
• Implement server-side validation to reject or sanitize suspicious cookie patterns before processing
• Use network-level controls to block HTTP traffic to sensitive domains, forcing all communications over HTTPS
• Consider implementing additional session validation mechanisms that don't rely solely on cookies

# Enable HSTS header on web server (Apache example)
Header always set Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"