CVE-2023-50269: Squid-cache Squid DOS Vulnerability

CVE-2023-50269 Overview

CVE-2023-50269 is a Denial of Service vulnerability in Squid, a widely-used caching proxy for the Web. The flaw stems from an Uncontrolled Recursion bug (CWE-674) in the HTTP Request parsing functionality. When the follow_x_forwarded_for feature is configured, a remote attacker can exploit this vulnerability by sending a crafted HTTP request with a large X-Forwarded-For header, causing the Squid proxy to enter an uncontrolled recursive state and become unresponsive.

Critical Impact

Remote attackers can perform Denial of Service attacks against Squid proxy servers by exploiting recursive parsing of X-Forwarded-For headers, potentially disrupting network operations and web traffic for organizations relying on Squid for caching and access control.

Affected Products

• Squid versions 2.6 through 2.7.STABLE9
• Squid versions 3.1 through 5.9
• Squid versions 6.0.1 through 6.5

Discovery Timeline

• 2023-12-14 - CVE-2023-50269 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-50269

Vulnerability Analysis

This vulnerability exists in the HTTP Request parsing component of Squid proxy when processing the X-Forwarded-For header. The X-Forwarded-For header is commonly used to identify the originating IP address of a client connecting through an HTTP proxy or load balancer. When Squid is configured with the follow_x_forwarded_for directive, it processes this header to determine client identity for access control and logging purposes.

The root cause lies in how Squid handles recursively nested or excessively long X-Forwarded-For headers. The parser does not implement proper recursion depth limits, allowing an attacker to craft a malicious request that triggers excessive recursive function calls. This leads to stack exhaustion and service disruption.

Root Cause

The vulnerability is classified as CWE-674 (Uncontrolled Recursion). When parsing the X-Forwarded-For header, Squid's parsing logic recursively processes each IP address entry in the header chain. Without proper bounds checking on the recursion depth or header length, an attacker can force the proxy into a deep recursive loop that exhausts system resources. This design flaw allows unauthenticated remote attackers to cause service disruption with a single crafted HTTP request.

Attack Vector

The attack can be executed remotely over the network without requiring authentication. An attacker needs only the ability to send HTTP requests to a vulnerable Squid proxy. The attack is performed by including a maliciously crafted X-Forwarded-For header with an excessive number of entries or deeply nested values in an HTTP request.

The vulnerability is only exploitable when the follow_x_forwarded_for configuration directive is enabled. This feature is commonly used in environments where Squid sits behind load balancers or other reverse proxies, making many production deployments potentially vulnerable.

The attack results in a complete denial of service condition where the Squid proxy becomes unresponsive, blocking legitimate traffic and disrupting network operations until the service is manually restarted.

Detection Methods for CVE-2023-50269

Indicators of Compromise

• Unusually large HTTP requests containing extremely long X-Forwarded-For headers
• Squid proxy service crashes or becoming unresponsive without apparent cause
• Elevated CPU usage on Squid servers followed by service failure
• Log entries showing malformed or abnormally long header values in access logs

Detection Strategies

• Implement network monitoring to detect HTTP requests with abnormally large headers exceeding typical thresholds
• Configure Web Application Firewall (WAF) rules to block requests with X-Forwarded-For headers exceeding reasonable length limits
• Monitor Squid process resource consumption for anomalous spikes in CPU or memory usage
• Deploy intrusion detection signatures that identify requests with excessive comma-separated values in X-Forwarded-For headers

Monitoring Recommendations

• Enable detailed access logging on Squid proxies to capture header information for forensic analysis
• Set up automated alerting for Squid service restarts or crashes
• Monitor network traffic patterns for unusual request volumes targeting the proxy infrastructure
• Implement health checks that detect and alert on Squid service unavailability

How to Mitigate CVE-2023-50269

Immediate Actions Required

• Upgrade Squid to version 6.6 or later which contains the official fix for this vulnerability
• Apply the appropriate patch for your Squid version from the official patch archives
• If patching is not immediately possible, consider disabling follow_x_forwarded_for if not strictly required
• Implement request header size limits at upstream load balancers or firewalls

Patch Information

Official patches are available from Squid-cache for affected versions:

• Squid Patch v5 SQUID-2023_10 - For Squid version 5.x
• Squid Patch v6 SQUID-2023_10 - For Squid version 6.x

Additional security information is available in the GitHub Security Advisory GHSA-wgq4-4cfg-c4x3.

Distribution-specific updates are available through Debian LTS Security Announcement, Fedora Package Announcements, and NetApp Security Advisory.

Workarounds

• Disable the follow_x_forwarded_for directive in squid.conf if client IP tracking through this header is not essential
• Implement upstream filtering to limit the size of X-Forwarded-For headers before they reach Squid
• Deploy a reverse proxy or load balancer in front of Squid with header validation capabilities
• Use access control lists (ACLs) to restrict which sources can send requests with X-Forwarded-For headers

# Disable follow_x_forwarded_for in squid.conf as a temporary workaround
# Edit /etc/squid/squid.conf and comment out or remove:
# follow_x_forwarded_for allow all

# Add explicit denial if the directive exists:
follow_x_forwarded_for deny all

# Restart Squid to apply changes:
systemctl restart squid