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SQUID-2021:1 Denial of Service in URN processing

yadij published GHSA-ch36-9jhx-phm4 May 10, 2021


Squid (Squid)

Affected versions

2.0-4.14, 5.0.1-5.0.5

Patched versions

4.15, 5.0.6


Due to a buffer management bug Squid is vulnerable to a
Denial of service attack against the server it is operating on.

This attack is limited to proxies which attempt to resolve a
"urn:" resource identifier. Support for this resolving is enabled
by default in all Squid.


This problem allows a malicious server in collaboration with a
trusted client to consume arbitrarily large amounts of memory
on the server running Squid.

Lack of available memory resources impacts all services on the
machine running Squid. Once initiated the DoS situation will
persist until Squid is shutdown.

CVSS Score of 8.5

Updated Packages:

This bug is fixed by Squid versions 4.15 and 5.0.6.

In addition, patches addressing this problem for the stable
releases can be found in our patch archives:

Squid 4:

If you are using a prepackaged version of Squid then please refer
to the package vendor for availability information on updated

Determining if your version is vulnerable:

Squid older than 3.5.28 have not been tested and should be assumed to be

All Squid-4.x up to and including 4.14 are vulnerable.

All Squid-5.x up to and including 5.0.5 are vulnerable.


Disable URN processing by the proxy. Add these lines to

acl URN proto URN
http_access deny URN

Contact details for the Squid project:

For installation / upgrade support on binary packaged versions
of Squid: Your first point of contact should be your binary
package vendor.

If you install and build Squid from the original Squid sources
then the mailing list is your
primary support point. For subscription details see

For reporting of non-security bugs in the latest STABLE release
the squid bugzilla database should be used

For reporting of security sensitive bugs send an email to the mailing list. It's a closed
list (though anyone can post) and security related bug reports
are treated in confidence until the impact has been established.


This vulnerability was discovered by Joshua Rogers of Opera

Fixed by Amos Jeffries of Treehouse Networks Ltd.

Revision history:

2021-02-22 06:55:38 UTC Initial Report
2021-02-24 00:53:21 UTC Patch Released
2021-03-17 06:19:09 UTC CVE Assignment




CVSS Score

7.4 High