Input validation error in Squid - CVE-2021-28651

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Main Vulnerability Database Vulnerabilities #VU53017

Input validation error in Squid - CVE-2021-28651

Published: May 10, 2021 / Updated: May 1, 2026


Vulnerability identifier: #VU53017
CSH Severity: Medium
CVSS v4: 6.9 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N]
CVE-ID: CVE-2021-28651
CWE-ID: CWE-20
Exploitation vector: Remote access
Exploit availability: No public exploit available

Vulnerability details

The vulnerability allows a remote attacker to perform a denial of service (DoS) attack.

The vulnerability exists due to insufficient input validation when resolving "urn:" resource identifiers. A remote attacker can trick a user behind the proxy server to click on a specially crafted "urn:" link that leads to a server under attacker's control and force Squid to consume arbitrarily large amounts of memory on the server. 


Affected software

Squid
Gentoo Linux
Amazon Linux AMI
Arch Linux
SUSE Manager Retail Branch Server
SUSE CaaS Platform
SUSE Manager Proxy
SUSE Manager Server
SUSE Enterprise Storage
Red Hat Enterprise Linux for ARM 64
Red Hat Enterprise Linux for Power, little endian
Red Hat Enterprise Linux for IBM z Systems
Red Hat Enterprise Linux for x86_64
Anolis OS
SUSE OpenStack Cloud Crowbar
SUSE OpenStack Cloud
SUSE Linux Enterprise Point of Sale
SUSE Linux Enterprise Debuginfo
SUSE Linux Enterprise Server
SUSE Linux Enterprise Server for SAP
SUSE Linux Enterprise High Performance Computing
SUSE Linux Enterprise Module for Server Applications
Ubuntu
openEuler
Fedora
squid (Debian package)
squid3 (Ubuntu package)
squid (Ubuntu package)
libecap-devel
libecap
squid3-debugsource
squid3-debuginfo
squid3
squid (Red Hat package)
squid
squid-debugsource
squid-debuginfo

How to mitigate CVE-2021-28651

Install updates from vendor's website.

Squid - addressed in versions 4.15, 5.0.6
squid (Debian package) - update to 4.6-1+deb10u6
squid3 (Ubuntu package) - update to Ubuntu Pro
squid (Ubuntu package) - addressed in versions Ubuntu Pro (Infra-only), 3.5.27-1ubuntu1.11, 4.10-1ubuntu1.4, 4.13-1ubuntu2.2, 4.13-1ubuntu4.1
libecap-devel - update to 1.0.1-2.0.1
libecap - update to 1.0.1-2.0.1
squid3-debugsource - update to 3.1.23-8.16.37.18.1
squid3-debuginfo - update to 3.1.23-8.16.37.18.1
squid3 - update to 3.1.23-8.16.37.18.1
squid (Red Hat package) - update to 3.5.20-17.el7_9.14
squid - update to 3.5.20-17.44
squid-debugsource - addressed in versions 3.5.21-26.35.1, 4.15-4.18.1, 4.15-5.26.1
squid-debuginfo - addressed in versions 3.5.21-26.35.1, 4.15-4.18.1, 4.15-5.26.1
squid - addressed in versions 3.5.21-26.35.1, 4.15-4.18.1, 4.15-5.26.1
squid-debugsource - update to 4.9-8
squid - update to 4.9-8
squid-debuginfo - update to 4.9-8
squid - addressed in versions 4.15-1.fc33, 5.0.6-1.fc34
squid - update to 4.15-3

External References

Related Security Bulletins