CVE-2020-25097 Overview
CVE-2020-25097 is an HTTP Request Smuggling vulnerability discovered in the Squid caching proxy server. Due to improper input validation in certain uri_whitespace configuration settings, a trusted client can craft malicious HTTP requests that bypass security controls and access services that should be forbidden. This vulnerability affects Squid versions through 4.13 and 5.x through 5.0.4.
Critical Impact
Attackers can exploit this vulnerability to bypass access controls and reach otherwise protected backend services, potentially exposing sensitive data or internal systems.
Affected Products
- Squid-cache Squid (through version 4.13 and 5.x through 5.0.4)
- Debian Linux 10.0
- Fedora 32, 33, and 34
- NetApp Cloud Manager
Discovery Timeline
- 2021-03-19 - CVE-2020-25097 published to NVD
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2020-25097
Vulnerability Analysis
This vulnerability stems from improper input validation (CWE-20) in Squid's HTTP request parsing logic. When specific uri_whitespace configuration settings are in use, Squid fails to properly validate and normalize incoming HTTP requests. This allows an attacker with trusted client access to craft ambiguous HTTP requests that are interpreted differently by Squid and backend servers.
HTTP Request Smuggling attacks exploit discrepancies in how intermediary proxies and backend servers parse HTTP requests, particularly around Content-Length headers and Transfer-Encoding. In CVE-2020-25097, the vulnerability specifically involves whitespace handling in URIs, enabling attackers to inject additional requests that bypass Squid's access control mechanisms.
Root Cause
The root cause is improper input validation in Squid's URI parsing routines when handling whitespace characters. The uri_whitespace configuration directive controls how Squid handles whitespace in request URIs, but certain configurations allow malformed requests to pass through without proper sanitization. This creates ambiguity in request parsing between Squid and backend servers, enabling request smuggling attacks.
Attack Vector
The attack is network-based and requires the attacker to have trusted client access to the Squid proxy. The attacker crafts a specially formed HTTP request containing whitespace characters in the URI that exploits the parsing discrepancy. This malicious request is interpreted by Squid as one request but may be parsed by backend servers as multiple requests, allowing the smuggled request to bypass security controls.
The attack flow involves sending a crafted HTTP request with embedded whitespace characters in the URI. Squid processes what it believes is a legitimate request based on its uri_whitespace configuration, but the backend server interprets the whitespace differently, treating portions of the original request as a separate, smuggled request that can access restricted resources.
Detection Methods for CVE-2020-25097
Indicators of Compromise
- Unusual HTTP requests containing unexpected whitespace characters in URIs appearing in Squid access logs
- Access log entries showing requests to internal or restricted services from external client addresses
- Backend server logs showing requests that don't match corresponding Squid proxy logs
- Anomalous traffic patterns with malformed or ambiguous HTTP headers
Detection Strategies
- Implement HTTP request validation at the network perimeter to identify malformed requests containing unusual URI whitespace patterns
- Deploy web application firewalls (WAF) with HTTP request smuggling detection capabilities
- Configure logging to capture full HTTP request headers and compare Squid and backend server logs for discrepancies
- Use intrusion detection systems with signatures for HTTP request smuggling attack patterns
Monitoring Recommendations
- Enable verbose logging on Squid proxy servers to capture full request details including URI encoding
- Monitor for access to internal services that should not be reachable through the proxy
- Implement network traffic analysis to detect HTTP parsing anomalies between proxy and backend servers
- Set up alerts for unusual access patterns to sensitive backend resources
How to Mitigate CVE-2020-25097
Immediate Actions Required
- Update Squid to a patched version immediately (4.14+ or 5.0.5+)
- Review and restrict uri_whitespace configuration to use secure settings such as uri_whitespace deny
- Audit access control lists (ACLs) to ensure proper restrictions are in place
- Monitor proxy logs for any signs of exploitation attempts
Patch Information
Squid-cache has released patches to address this vulnerability. Patches are available for both affected version branches:
Additional security advisories and package updates have been released by major distributions:
For full technical details, see the GitHub Security Advisory GHSA-jvf6-h9gj-pmj6.
Workarounds
- Configure uri_whitespace deny in squid.conf to reject requests containing whitespace in URIs rather than encoding or stripping them
- Implement strict HTTP request validation at network ingress points before traffic reaches the Squid proxy
- Deploy additional access controls and network segmentation to limit the impact of potential bypass attacks
- Consider using a reverse proxy or WAF in front of Squid to normalize and validate HTTP requests
# Squid configuration hardening example
# Add to squid.conf to mitigate CVE-2020-25097
# Deny requests with whitespace in URIs
uri_whitespace deny
# Enforce strict HTTP request validation
relaxed_header_parser off
# Limit allowed HTTP methods
acl PURGE method PURGE
http_access deny PURGE !localhost
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
