CVE-2024-29736: Apache CXF SSRF Vulnerability

CVE-2024-29736 Overview

CVE-2024-29736 is a Server-Side Request Forgery (SSRF) vulnerability affecting the WADL (Web Application Description Language) service description functionality in Apache CXF, a popular open-source services framework. This vulnerability enables attackers to perform SSRF-style attacks against REST web services when a custom stylesheet parameter is configured.

Apache CXF is widely used for building and developing web services, making this vulnerability particularly concerning for organizations running affected versions. The SSRF flaw allows malicious actors to manipulate the server into making unauthorized requests to internal or external resources, potentially exposing sensitive data or enabling further attacks on internal infrastructure.

Critical Impact

Attackers can exploit this SSRF vulnerability to access internal network resources, exfiltrate sensitive data, or pivot to attack other systems from the trusted server context. The vulnerability requires a custom stylesheet parameter to be configured.

Affected Products

• Apache CXF versions before 4.0.5
• Apache CXF versions before 3.6.4
• Apache CXF versions before 3.5.9

Discovery Timeline

• 2024-07-19 - CVE-2024-29736 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2024-29736

Vulnerability Analysis

This vulnerability is classified as CWE-918 (Server-Side Request Forgery), a weakness where an application fetches remote resources based on user-supplied input without properly validating the destination. In the context of Apache CXF, the WADL service description feature processes stylesheet parameters that can be manipulated to force the server to make requests to arbitrary URLs.

The attack surface is limited to environments where a custom stylesheet parameter has been explicitly configured. When this configuration is present, an attacker can craft malicious requests that cause the CXF server to initiate connections to attacker-controlled or internal network resources.

Root Cause

The root cause of this vulnerability lies in insufficient validation of user-supplied input in the WADL service description handling code. When processing custom stylesheet parameters, Apache CXF fails to properly sanitize or restrict the URLs that can be specified, allowing attackers to specify arbitrary destinations for server-side requests.

The WADL functionality is designed to provide machine-readable descriptions of REST APIs, and the stylesheet parameter feature allows customization of the WADL output. However, the implementation did not adequately consider the security implications of allowing user-controlled URLs in this context.

Attack Vector

The attack is network-based and requires no authentication or user interaction to exploit. An attacker targeting a vulnerable Apache CXF deployment can manipulate the stylesheet parameter in requests to the WADL service endpoint.

The exploitation flow involves:

1. Identifying a target Apache CXF instance with WADL service description enabled and a custom stylesheet parameter configured
2. Crafting a malicious request that specifies an attacker-controlled or internal URL as the stylesheet parameter
3. The server processes the request and attempts to fetch the specified stylesheet URL
4. The attacker can leverage this to scan internal networks, access internal services, or exfiltrate data through the server's network context

For detailed technical analysis, refer to the Apache Mailing List Thread and the Openwall OSS-Security Discussion.

Detection Methods for CVE-2024-29736

Indicators of Compromise

• Unusual outbound requests from Apache CXF servers to internal IP addresses or unexpected external domains
• Requests to WADL endpoints containing suspicious or malformed stylesheet parameter values
• Network traffic patterns showing the CXF server accessing cloud metadata endpoints (e.g., 169.254.169.254)
• Log entries indicating failed connections to internal services that the CXF application should not access

Detection Strategies

• Monitor web application logs for requests to WADL service endpoints with unusual stylesheet parameter values
• Implement network-level monitoring to detect outbound connections from CXF servers to internal network segments or metadata services
• Deploy web application firewalls (WAF) with rules to detect SSRF patterns in request parameters
• Use SentinelOne Singularity to monitor for anomalous network behavior from application servers

Monitoring Recommendations

• Enable detailed logging for all WADL service description requests including full parameter values
• Configure network segmentation monitoring to alert on unexpected traffic flows from web application servers
• Implement DNS query logging to detect potential SSRF attempts using DNS-based data exfiltration
• Set up alerts for connections to RFC 1918 private IP ranges from externally-facing services

How to Mitigate CVE-2024-29736

Immediate Actions Required

• Upgrade Apache CXF to version 4.0.5, 3.6.4, or 3.5.9 or later immediately
• Review application configurations to determine if custom stylesheet parameters are enabled for WADL services
• If upgrading is not immediately possible, disable custom stylesheet parameters in WADL service configurations
• Implement network segmentation to limit the impact of potential SSRF exploitation

Patch Information

Apache has released patched versions addressing this vulnerability. Organizations should upgrade to the following versions based on their current branch:

• For CXF 4.x: Upgrade to version 4.0.5 or later
• For CXF 3.6.x: Upgrade to version 3.6.4 or later
• For CXF 3.5.x: Upgrade to version 3.5.9 or later

The official security advisory is available on the Apache Mailing List Thread. Additional vendor guidance is available in the NetApp Security Advisory ntap-20241115-0003.

Workarounds

• Disable the custom stylesheet parameter feature in WADL service configurations if not required
• Implement URL allowlisting for stylesheet parameters to restrict destinations to trusted sources only
• Deploy egress filtering rules to prevent the CXF server from making connections to internal network ranges
• Use network-level controls to block outbound connections from application servers to sensitive internal resources

# Example: Reviewing CXF configuration for custom stylesheet parameters
# Check your cxf.xml or beans.xml configuration files for stylesheet-related settings
grep -r "stylesheet" /path/to/webapp/WEB-INF/*.xml

# If found, consider removing or restricting the custom stylesheet configuration
# until the patch can be applied