[squid-announce] [ADVISORY] SQUID-2026:3 Out of Bounds Read in ICP message handling (CVE-2026-33515)

[Amos Jeffries]

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     Squid Proxy Cache Security Update Advisory SQUID-2026:3
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Advisory ID:       | SQUID-2026:3 (CVE-2026-33515)
Date:              | March 25, 2026
Summary:           | Out of Bounds Read in ICP message handling
Affected versions: | Squid 3.x -> 3.5.28
                    | Squid 4.x -> 4.17
                    | Squid 5.x -> 5.9
                    | Squid 6.x -> 6.14
                    | Squid 7.x -> 7.4
Fixed in version:  | Squid 7.5
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Problem Description:

  Due to improper input validation bugs, Squid is vulnerable to out
  of bounds reads when handling ICP traffic.

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Severity:

  This problem allows a remote attacker to
  receive small amounts of memory potentially containing sensitive
  information when responding with errors to invalid ICP requests.

  This attack is limited to Squid deployments that explicitly
  enable ICP support (i.e. configure non-zero icp_port).

  This problem _cannot_ be mitigated by denying ICP queries
  using icp_access rules.

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Updated Packages

  These bugs were fixed in Squid version 7.5.

  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/8138e909d2058d4401e0ad49b583afaec912b165>

Note: we are aware this patch may not apply cleanly.
   Ensure that the patch for SQUID-2026:1 is applied first.

  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

  Run the following command to identify whether your Squid
  has been configured with ICP enabled:

   squid -k parse 2>&1 | grep -E "(icp|udp)_port" | tail -n1

  All Squid configured with port 0 are not vulnerable.

  All Squid-3.0 up to and including 7.4 configured with
  a non-zero port should be assumed to be vulnerable.

  All Squid-3.2 up to and including 7.4 configured without
  any port value can be assumed to be not vulnerable.

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Workaround

Either,

   * Do not enable ICP support,

Or,

  * explicitly disable ICP using "icp_port 0".


  Warning: These problems _cannot_ be mitigated by denying ICP
           queries using icp_access rules.

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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

  Discovered and Fixed by:

   * Joshua Rogers with ZeroPath
   * Alex Rousskov, The Measurement Factory

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Revision history:

2025-09-07 20:22:00 EDT Report of the first set of vulnerabilities
2026-01-26 08:48:00 EDT Report of additional vulnerabilities
2026-02-12 20:28:43 UTC official fixes in master branch
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