[squid-announce] [ADVISORY] SQUID-2026:6 Request Smuggling in HTTP Transfer-Encoding

[Amos Jeffries]

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     Squid Proxy Cache Security Update Advisory SQUID-2026:6
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  Advisory ID:       | SQUID-2026:6
  Date:              | 2026-06-23
  Summary:           | Request Smuggling in HTTP Transfer-Encoding
  Affected versions: | Squid 3.3 -> 3.5.28
                     | Squid 4.x -> 4.17
                     | Squid 5.x -> 5.9
                     | Squid 6.x -> 6.14
                     | Squid 7.x -> 7.5
  Fixed in version:  | Squid 7.6
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Problem Description:

  Due to a CWE-841 Improper Enforcement of Behavioral Workflow
  bug Squid is vulnerable to a Request Smuggling attack against
  HTTP/1.1 Transfer-Encoding.

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Severity:

  This problem allows a trusted client to perform an HTTP Request
  Smuggling attack when HTTP/1.1 is used. Bypassing security
  mechanisms that may be in place between attacker and Squid.

  When there is an HTTP cache operating prior to the affected
  Squid, this Request Smuggling attack also allows the attacker
  to poison that web cache and store arbitrary malicious content
  at any URL for delivery to other clients future requests.

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Updated Packages:

  This bug is fixed by Squid version 7.6.

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

  Squid 7:
  <https://github.com/squid-cache/squid/commit/d06f48de8b8cc9a378cb91ecfe8e89b6e81696d2.patch>


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

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Determining if your version is vulnerable:

  Squid older than 3.3 have not been tested and should be
  assumed to be not vulnerable.

  All Squid-3.3 up to and including 3.5.28 have not been tested
  and should be assumed to be vulnerable.

  All Squid-4.x up to and including 4.17 have not been tested
  and should be assumed to be vulnerable.

  All Squid-5.x up to and including 5.9 have not been tested
  and should be assumed to be vulnerable.

  All Squid-6.x up to and including 6.14 are vulnerable.

  All Squid-7.x up to and including 7.5 are vulnerable.

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Workaround:

  There is no workaround for these issues.

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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 <squid-users at lists.squid-cache.org> mailing list is
  your primary support point. For subscription details see
  <https://www.squid-cache.org/Support/mailing-lists.html>.

  For reporting of non-security bugs in the latest STABLE release
  the squid bugzilla database should be used
  <https://bugs.squid-cache.org/>.

  For reporting of security sensitive bugs send an email to the
  <squid-bugs at lists.squid-cache.org> 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.

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Credits:

  This vulnerability was discovered by
    Mitchell Benjamin, Revamp Studio.

  Fixed by Amos Jeffries , Treehouse Networks Ltd.

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Revision history:

  2026-05-29 08:04:59 UTC Initial Report
  2026-05-31 08:29:04 UTC Patch Released
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