CVE-2021-31808: Squid-cache Squid DoS Vulnerability

CVE-2021-31808 Overview

CVE-2021-31808 is an Integer Overflow vulnerability affecting Squid caching proxy server versions before 4.15 and 5.x before 5.0.6. Due to an input-validation bug in the HTTP Range request handling, a malicious client can send specially crafted requests that trigger a Denial of Service condition, impacting all clients using the affected proxy server.

Critical Impact

Authenticated attackers can exploit this vulnerability remotely to cause a complete Denial of Service against the Squid proxy, disrupting network connectivity for all clients relying on the proxy infrastructure.

Affected Products

• Squid-cache Squid (versions before 4.15 and 5.x before 5.0.6)
• Debian Linux 9.0 and 10.0
• NetApp Cloud Manager
• Fedora 33 and 34

Discovery Timeline

• 2021-05-27 - CVE-2021-31808 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-31808

Vulnerability Analysis

This vulnerability stems from improper input validation in Squid's HTTP Range request processing logic. When handling Range headers, the affected code fails to properly validate integer boundaries, leading to an integer overflow condition (CWE-190). The flaw allows an authenticated attacker to craft malicious HTTP Range requests that cause the proxy server to enter an unstable state, resulting in service disruption for all connected clients.

The vulnerability is network-accessible and requires low privileges to exploit, with no user interaction needed. While the attack does not compromise confidentiality or integrity, it severely impacts service availability by crashing or hanging the proxy server process.

Root Cause

The root cause is an integer overflow vulnerability (CWE-190) in the Range request header parsing code. When processing HTTP Range requests, Squid performs arithmetic operations on user-supplied range values without adequate boundary checking. This allows attackers to provide range values that cause integer overflow when processed, leading to unexpected behavior and service disruption.

Attack Vector

The attack is executed remotely over the network by an authenticated client sending specially crafted HTTP Range requests to the Squid proxy server. The attacker must have valid credentials to access the proxy (low privilege requirement), but once authenticated, can trigger the vulnerability by submitting malformed Range headers that exploit the integer overflow condition in the request processing logic.

The exploitation process involves crafting HTTP Range request headers with values designed to trigger arithmetic overflow during processing. When the vulnerable code performs calculations on these values, the overflow results in incorrect memory access patterns or infinite loops that cause the proxy service to become unresponsive.

Detection Methods for CVE-2021-31808

Indicators of Compromise

• Unusual volume of HTTP Range requests from specific client IP addresses
• Squid proxy service crashes or unexpected restarts
• Memory consumption anomalies in the Squid process
• Client connectivity issues across the proxy infrastructure

Detection Strategies

• Monitor Squid access logs for abnormal Range header patterns or unusually large range values
• Implement alerting on Squid process crashes or service interruptions
• Deploy network intrusion detection rules to identify malformed HTTP Range requests
• Review system logs for out-of-memory errors or segmentation faults related to the Squid process

Monitoring Recommendations

• Configure real-time monitoring for Squid service availability and process health
• Establish baseline metrics for HTTP Range request frequency and alert on deviations
• Enable detailed logging of Range request headers for forensic analysis
• Implement automated service recovery procedures with alerting on restart events

How to Mitigate CVE-2021-31808

Immediate Actions Required

• Upgrade Squid to version 4.15 or later for the 4.x branch
• Upgrade Squid to version 5.0.6 or later for the 5.x branch
• Apply vendor-supplied patches immediately if upgrade is not possible
• Review and restrict proxy access to trusted clients only

Patch Information

Official patches and updated packages are available from Squid-cache and major Linux distributions:

• Squid Cache Patch File - Official patch for Squid 4.x
• GitHub Security Advisory - Detailed advisory with remediation guidance
• Debian Security Advisory DSA-4924 - Debian package updates
• NetApp Security Advisory - NetApp Cloud Manager guidance

Workarounds

• Implement strict access control lists (ACLs) to limit proxy access to trusted clients only
• Consider temporarily disabling Range request support if operationally feasible
• Deploy a web application firewall (WAF) to filter malicious Range headers
• Implement rate limiting on incoming requests to reduce DoS impact

# Example Squid ACL to restrict client access
# Add to squid.conf to limit proxy access to trusted networks
acl trusted_networks src 10.0.0.0/8 172.16.0.0/12 192.168.0.0/16
http_access allow trusted_networks
http_access deny all