CVE-2025-4476: libsoup HTTP Library DoS Vulnerability

CVE-2025-4476 Overview

A denial-of-service vulnerability has been identified in the libsoup HTTP client library. This flaw can be triggered when a libsoup client receives a 401 (Unauthorized) HTTP response containing a specifically crafted domain parameter within the WWW-Authenticate header. Processing this malformed header can lead to a crash of the client application using libsoup. An attacker could exploit this by setting up a malicious HTTP server. If a user's application using the vulnerable libsoup library connects to this malicious server, it could result in a denial-of-service. Successful exploitation requires tricking a user's client application into connecting to the attacker's malicious server.

Critical Impact

Client applications using the libsoup library may crash when connecting to malicious HTTP servers that send crafted WWW-Authenticate headers, resulting in denial-of-service conditions.

Affected Products

• libsoup HTTP client library (vulnerable versions)
• Applications built using libsoup for HTTP client functionality
• GNOME-based applications utilizing libsoup for web requests

Discovery Timeline

• May 16, 2025 - CVE-2025-4476 published to NVD
• May 19, 2025 - Last updated in NVD database

Technical Details for CVE-2025-4476

Vulnerability Analysis

This vulnerability is classified as a Null Pointer Dereference (CWE-476), which occurs when the libsoup library attempts to process a malformed WWW-Authenticate header in an HTTP 401 response. The flaw exists in the header parsing logic that handles the domain parameter within authentication challenge responses. When a specially crafted domain value is encountered, the library fails to properly validate the input before dereferencing it, leading to a null pointer dereference and subsequent application crash.

The attack requires user interaction in that the victim's application must be induced to connect to a malicious server controlled by the attacker. This reduces the severity of the vulnerability but still presents a meaningful risk for applications that may connect to untrusted or user-supplied URLs.

Root Cause

The root cause of this vulnerability is improper input validation when parsing the domain parameter in WWW-Authenticate headers. The libsoup library does not adequately verify that the domain parameter contains valid data before attempting to process it. When encountering a malformed or specially crafted value, the parsing function returns a null pointer that is subsequently dereferenced without proper null checks, triggering a crash.

This is a classic CWE-476 (NULL Pointer Dereference) scenario where defensive programming practices were not applied to handle edge cases in HTTP header parsing.

Attack Vector

The attack vector requires network access and user interaction. An attacker must:

1. Set up a malicious HTTP server that returns 401 Unauthorized responses
2. Craft a WWW-Authenticate header with a malformed domain parameter designed to trigger the null pointer dereference
3. Entice a victim to use an application with the vulnerable libsoup library to connect to the malicious server

When the victim's application connects and receives the crafted 401 response, the libsoup library crashes while attempting to parse the malicious WWW-Authenticate header, causing a denial-of-service condition for the client application.

The attack scenario might involve phishing, social engineering, or compromising a legitimate service to redirect users to the malicious server. Applications that automatically follow redirects or process user-supplied URLs are at higher risk.

Detection Methods for CVE-2025-4476

Indicators of Compromise

• Unexpected application crashes in programs using libsoup for HTTP requests
• Crash dumps indicating null pointer dereference in libsoup authentication header parsing functions
• Network traffic showing connections to suspicious servers followed by 401 responses with unusual WWW-Authenticate headers
• Application logs showing authentication failures immediately before crashes

Detection Strategies

• Monitor for application crashes with stack traces pointing to libsoup authentication handling code
• Implement network monitoring to detect HTTP 401 responses with abnormally long or malformed WWW-Authenticate headers
• Use application crash reporting systems to identify patterns of null pointer dereference crashes in libsoup-dependent applications
• Deploy intrusion detection rules to identify suspicious WWW-Authenticate header patterns in HTTP responses

Monitoring Recommendations

• Enable verbose logging for applications using libsoup to capture HTTP response headers before potential crashes
• Configure crash monitoring and reporting for all applications that utilize the libsoup library
• Implement network-level inspection for anomalous HTTP 401 responses, particularly those with unusual domain parameters
• Review application logs for repeated authentication failures or connection attempts to unknown servers

How to Mitigate CVE-2025-4476

Immediate Actions Required

• Identify all applications and systems using the libsoup HTTP client library
• Review the Red Hat CVE-2025-4476 Advisory for vendor-specific guidance and patch availability
• Monitor the Red Hat Bug #2366513 for updates on the fix
• Consider restricting applications from connecting to untrusted or user-supplied URLs where possible
• Update libsoup to the latest patched version when available from your distribution

Patch Information

Consult the Red Hat CVE-2025-4476 Advisory for the latest patch information and affected package versions. Organizations should apply vendor-provided patches as soon as they become available through their respective Linux distributions or package managers.

For GNOME and related ecosystems, monitor upstream libsoup releases for security fixes addressing this null pointer dereference vulnerability.

Workarounds

• Implement URL whitelisting to prevent applications from connecting to untrusted servers
• Use a proxy or gateway that can filter or sanitize HTTP responses before they reach vulnerable clients
• Deploy application-level sandboxing to limit the impact of crashes on the overall system
• Consider disabling automatic authentication handling if the application allows, requiring manual user intervention for authentication challenges

# Example: Check installed libsoup version on RPM-based systems
rpm -qa | grep libsoup

# Example: Check for updates to libsoup packages
dnf check-update libsoup*

# Example: Update libsoup to latest available version
sudo dnf update libsoup*