CVE-2026-3099: Libsoup Auth Bypass Vulnerability

CVE-2026-3099 Overview

A flaw was found in Libsoup. The server-side digest authentication implementation in the SoupAuthDomainDigest class does not properly track issued nonces or enforce the required incrementing nonce-count (nc) attribute. This vulnerability allows a remote attacker to capture a single valid authentication header and replay it repeatedly. Consequently, the attacker can bypass authentication and gain unauthorized access to protected resources, impersonating the legitimate user.

Critical Impact

Remote attackers can capture and replay valid authentication headers to bypass authentication and impersonate legitimate users, gaining unauthorized access to protected resources.

Affected Products

• Libsoup (GNOME HTTP library)
• Applications using SoupAuthDomainDigest for server-side digest authentication

Discovery Timeline

• 2026-03-12 - CVE CVE-2026-3099 published to NVD
• 2026-03-12 - Last updated in NVD database

Technical Details for CVE-2026-3099

Vulnerability Analysis

This vulnerability is classified under CWE-323 (Reusing a Nonce, Key Pair in Encryption). The flaw exists in Libsoup's server-side implementation of HTTP Digest authentication within the SoupAuthDomainDigest class. The fundamental issue is the library's failure to maintain proper state tracking for authentication sessions.

HTTP Digest authentication is designed to prevent replay attacks through two key mechanisms: server-issued nonces (number used once) and client-maintained nonce counts. Each authentication request should include a unique nonce-count value that increments with each request. The server must validate that each nonce-count is strictly greater than previously seen values for that nonce.

In vulnerable versions of Libsoup, the server does not maintain a record of issued nonces or track the nonce-count values received. This means an attacker who intercepts a single valid authentication exchange can replay the captured Authorization header indefinitely without the server detecting the replay.

Root Cause

The root cause lies in the incomplete implementation of RFC 2617 (HTTP Digest Authentication) in the SoupAuthDomainDigest class. The implementation fails to store and validate nonce state server-side, which is essential for detecting and preventing replay attacks. Without proper nonce tracking, the cryptographic protection offered by digest authentication is effectively nullified.

Attack Vector

This vulnerability is exploitable over the network but requires some preconditions for successful exploitation. An attacker must be in a position to intercept network traffic between a legitimate client and the vulnerable server (man-in-the-middle position). Once a valid authentication header is captured, the attacker can replay it to gain access to protected resources.

The attack complexity is considered high because it requires the attacker to intercept legitimate authentication traffic. However, in environments where network interception is possible (such as shared networks, compromised routers, or via ARP spoofing), the exploitation becomes straightforward. The attacker simply captures the Authorization header from a legitimate request and reuses it in their own requests to the server.

Detection Methods for CVE-2026-3099

Indicators of Compromise

• Multiple requests from different source IPs containing identical Authorization headers with the same nonce and nonce-count values
• Authentication requests arriving from geographic locations inconsistent with legitimate user activity
• Rapid authentication attempts using the same nonce value from different client sessions

Detection Strategies

• Implement network-level monitoring to detect duplicate Authorization headers being submitted from different source addresses
• Deploy intrusion detection rules to flag HTTP requests where the same nonce-count value appears multiple times for the same nonce
• Analyze authentication logs for patterns suggesting replay attacks, such as simultaneous sessions from disparate locations
• Monitor for unusual authentication patterns where the same credentials appear to be used across multiple distinct client fingerprints

Monitoring Recommendations

• Enable verbose logging for applications using Libsoup digest authentication to capture full Authorization header details
• Implement real-time alerting on authentication anomalies using SIEM solutions
• Review the GNOME libsoup Issue #495 for updates on detection guidance from the maintainers
• Monitor the Red Hat Bug Report #2442232 for additional security advisories

How to Mitigate CVE-2026-3099

Immediate Actions Required

• Review applications in your environment that depend on Libsoup for HTTP server functionality with digest authentication
• Consider temporarily disabling digest authentication in favor of alternative authentication mechanisms such as TLS client certificates or token-based authentication
• Implement additional network-layer protections such as TLS/HTTPS to encrypt authentication headers in transit
• Consult the Red Hat CVE-2026-3099 Advisory for vendor-specific remediation guidance

Patch Information

Monitor the official GNOME Libsoup repository and distribution package managers for security patches addressing this vulnerability. The GNOME libsoup Issue #495 tracks the upstream fix status. System administrators should apply patches as soon as they become available through their respective package management systems.

Workarounds

• Migrate affected services to use HTTPS/TLS encryption to protect authentication headers from interception
• Implement application-layer rate limiting to restrict the number of authentication attempts per session
• Deploy reverse proxy solutions that can enforce proper nonce-count validation at the edge
• Consider switching to alternative authentication mechanisms such as OAuth 2.0 or API key authentication that do not rely on the vulnerable digest authentication implementation

# Verify Libsoup version installed on Debian/Ubuntu systems
dpkg -l | grep libsoup
apt-cache policy libsoup-3.0-0

# Check for Libsoup usage in running applications
lsof | grep libsoup

# Monitor for available security updates
apt-get update && apt-cache policy libsoup*