Denial of Service Vulnerability in Squid Proxy Server

squid Updated 02 Oct 2025

Summary

27/05/2021

Disclosed on

N/A

Severity

Yes i

Fix available

Medium

Severity

6.5

CVSS Score

ALPINE-CVE-2021-31806: An issue was discovered in Squid before 4.15 and 5.x before 5.0.6. Affects squid versions < 4.15-r0. Fixed in 4.15-r0. How to fix ↓

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Background

Squid is a widely used caching proxy for the web that supports HTTP, HTTPS, FTP, and more. It is designed to improve web performance by caching frequently requested content and reducing bandwidth usage. Squid is often deployed in enterprise environments to manage web traffic and enhance security. However, it has been found to be vulnerable to a Denial of Service (DoS) attack due to a memory-management bug, which can affect all clients using the proxy.

Vulnerability Detail

The vulnerability, identified as CVE-2021-31806, arises from improper handling of HTTP Range requests in Squid versions prior to 4.15 and 5.x before 5.0.6. This flaw allows an attacker to exploit the memory-management bug, potentially leading to a Denial of Service condition. When exploited, it can cause the Squid proxy server to become unresponsive, impacting all clients relying on it for web access. The CVSS score for this vulnerability is 6.5, indicating a moderate severity level.

How To Fix

Minimum fixed version

4.15-r0

This is the first release that contains the fix.

sudo apk add --upgrade squid

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Workaround

As a temporary workaround, you can limit the use of HTTP Range requests or implement firewall rules to block suspicious traffic targeting the Squid proxy. However, this is not a permanent solution and should only be used until the package is updated.

Conclusion

The vulnerability CVE-2021-31806 in the Squid proxy server poses a significant risk of Denial of Service attacks due to a memory-management issue. It is crucial for users to update their installations to the latest patched versions to mitigate this risk. Monitoring tools like Vulert can assist in identifying such vulnerabilities in your software.

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