CVE-2021-31806 Overview
CVE-2021-31806 is a Denial of Service (DoS) vulnerability discovered in Squid proxy server versions before 4.15 and 5.x before 5.0.6. The vulnerability stems from a memory-management bug in the HTTP Range request processing functionality. When exploited, this flaw allows an authenticated attacker to disrupt proxy services for all clients using the affected Squid instance.
Critical Impact
Successful exploitation enables attackers to deny service to all clients using the vulnerable Squid proxy through malicious HTTP Range requests, potentially disrupting network access for entire organizations.
Affected Products
- Squid-cache Squid (versions before 4.15 and 5.x before 5.0.6)
- Debian Linux 9.0 and 10.0
- Fedora 33 and 34
- NetApp Cloud Manager
Discovery Timeline
- 2021-05-27 - CVE-2021-31806 published to NVD
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2021-31806
Vulnerability Analysis
This vulnerability is classified under CWE-116 (Improper Encoding or Escaping of Output), though its practical impact manifests as a memory management issue. The flaw resides in how Squid handles HTTP Range request processing, a feature commonly used for partial content retrieval and resumable downloads.
When processing specially crafted HTTP Range requests, Squid's memory management routines fail to properly handle certain edge cases. This improper handling leads to memory corruption or exhaustion that affects the stability of the entire proxy service. Since Squid operates as a centralized proxy for potentially hundreds or thousands of client connections, a successful exploit impacts all users relying on the proxy server.
The network-based attack vector with low complexity makes this vulnerability particularly concerning for organizations using Squid as their primary web proxy infrastructure. Authentication is required to exploit this vulnerability, which provides some mitigation against anonymous internet-based attacks.
Root Cause
The root cause of CVE-2021-31806 lies in improper memory management within Squid's HTTP Range request handler. When processing Range headers, the code fails to correctly validate or handle certain memory allocation scenarios, leading to memory corruption. This bug exists in the content delivery mechanism that handles byte-range specifications in HTTP requests.
Attack Vector
The attack is executed over the network without requiring user interaction. An authenticated attacker can send maliciously crafted HTTP requests containing specific Range header values to the vulnerable Squid proxy. Upon processing these requests, the memory management bug is triggered, causing service disruption.
The exploitation scenario involves an attacker with valid proxy authentication credentials sending requests with manipulated Range headers. While the exact manipulation required depends on the specific code path, the general approach targets the boundary conditions in Range request parsing where memory allocation assumptions may be violated.
Detection Methods for CVE-2021-31806
Indicators of Compromise
- Unexpected Squid proxy crashes or service restarts
- Elevated memory usage patterns on Squid servers prior to service failures
- Anomalous HTTP Range request patterns in proxy access logs
- Client complaints about intermittent proxy unavailability
Detection Strategies
- Monitor Squid process stability and implement alerting on unexpected service restarts
- Analyze HTTP access logs for unusual Range header patterns or excessive Range requests from single sources
- Implement network-level inspection for malformed or suspicious HTTP Range headers
- Deploy version detection scanning to identify unpatched Squid installations
Monitoring Recommendations
- Configure Squid cache manager to expose health metrics for continuous monitoring
- Set up log aggregation to correlate Range request patterns across time periods
- Implement memory utilization alerts on Squid servers to detect resource exhaustion attempts
- Monitor client-reported errors that may indicate proxy service degradation
How to Mitigate CVE-2021-31806
Immediate Actions Required
- Upgrade Squid to version 4.15 or later (for 4.x branch) or version 5.0.6 or later (for 5.x branch)
- Apply vendor-provided security patches from distribution repositories (Debian, Fedora, etc.)
- Review and restrict proxy authentication to minimize potential attackers
- Implement monitoring for Squid service availability and resource utilization
Patch Information
Official patches are available from multiple sources. The Squid project has released the fix as patch squid-4-e7cf864f938f24eea8af0692c04d16790983c823 for the 4.x branch. Detailed information about the vulnerability and remediation is available in the GitHub Squid Security Advisory.
Distribution-specific updates are available:
- Debian users should reference DSA-4924 and the Debian LTS Announcement
- Fedora users can obtain updates through Fedora Package Announcements
- NetApp customers should consult the NetApp Security Advisory NTAP-20210716-0007
Workarounds
- Restrict proxy access to trusted, authenticated users only to reduce attack surface
- Implement rate limiting on HTTP requests with Range headers
- Consider deploying a web application firewall (WAF) in front of Squid to filter suspicious Range requests
- Segment Squid proxy infrastructure to limit blast radius of potential DoS attacks
# Verify current Squid version
squid -v | head -n 1
# Check for available security updates (Debian/Ubuntu)
apt-get update && apt-cache policy squid
# Apply security patches (Debian/Ubuntu)
apt-get update && apt-get install --only-upgrade squid
# Apply security patches (RHEL/Fedora)
dnf update squid
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
