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Vulnerability record · CVE-2016-4555 · published 10 May 2016

CVE-2016-4555: Squid proxy crash via crafted ESI responses

Squid Cache · Squid

Squid 3.x before 3.5.18 and 4.x before 4.0.10 mishandles crafted Edge Side Includes (ESI) responses in client_side_request.cc, causing improper input validation that crashes the proxy. Because Squid is a shared forward/reverse proxy, a crash disrupts all users and services routed through it.

7.5 CVSS 3.0 High EPSS 54% · top 1.0% CWE-20 · Improper input validation
7.5CVSS 3.0 base score, v2 5.0
54%EPSS exploitation probability, 30 days
NoNot in CISA KEV
3Affected product versions listed by NVD
32References, 2 tagged exploit
17 Jun 2026Last modified by NVD

Description

client_side_request.cc in Squid 3.x before 3.5.18 and 4.x before 4.0.10 allows remote servers to cause a denial of service (crash) via crafted Edge Side Includes (ESI) responses.

CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Automated analysis

Generated by VULONE's analysis model from the NVD record, CISA KEV and EPSS data on 20 September 2026. Confidence: high.

high priorityRemote, unauthenticated crash of a shared proxy with a high EPSS score and public exploit references, though no KEV listing or code execution.

What it is

Squid 3.x before 3.5.18 and 4.x before 4.0.10 mishandles crafted Edge Side Includes (ESI) responses in client_side_request.cc, causing improper input validation that crashes the proxy. Because Squid is a shared forward/reverse proxy, a crash disrupts all users and services routed through it.

Impact

An attacker gains denial of service: the Squid process crashes, interrupting proxied traffic for all clients until it is restarted. There is no confidentiality or integrity impact per the CVSS vector.

Attack surface

Reachable remotely over the network with no authentication and no user interaction (AV:N/AC:L/PR:N/UI:N); the malicious content is delivered in an ESI response from a remote server that Squid processes.

Exploitation

Not listed in CISA KEV, but EPSS is 0.5392 (98.95th percentile) and the vendor bug report is tagged Exploit, indicating public exploit material exists.

What to do

  • Upgrade Squid to 3.5.18 or 4.0.10 or later, or apply the vendor SQUID-2016_9 patches for the 3.4 and 3.5 branches.
  • Apply distribution updates (Ubuntu USN-2995-1, Debian DSA-3625, Red Hat RHSA-2016:1139/1140, Gentoo GLSA 201607-01, openSUSE advisories).
  • If ESI processing is not required, disable it in squid.conf to remove the vulnerable code path.
  • Restrict which upstream servers and clients can send responses through the proxy to reduce exposure to untrusted ESI content.
  • Monitor and auto-restart the Squid service so crashes are detected and recovered quickly.

Detection

  • Monitor Squid process exits or core dumps and correlate them with requests carrying ESI-related content.
  • Alert on repeated Squid restarts or service failures within short windows.
  • Inspect proxy logs for responses containing ESI markup from untrusted or unusual upstream servers.
  • Track Squid version inventory to confirm hosts are on 3.5.18/4.0.10 or later.

This assessment is produced automatically and is not human-reviewed. Verify against the vendor advisory before acting on it.

Affected products

3 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
http://bugs.squid-cache.org/show_bug.cgi?id=4455 ExploitIssue TrackingPatchVendor Advisory
http://lists.opensuse.org/opensuse-security-announce/2016-08/msg00010.html
http://lists.opensuse.org/opensuse-security-announce/2016-08/msg00040.html
http://lists.opensuse.org/opensuse-updates/2016-08/msg00069.html
http://www.debian.org/security/2016/dsa-3625
http://www.openwall.com/lists/oss-security/2016/05/06/3 PatchRelease Notes
http://www.openwall.com/lists/oss-security/2016/05/06/5 PatchThird Party Advisory
http://www.oracle.com/technetwork/topics/security/linuxbulletinapr2016-2952096.html Third Party Advisory
http://www.securitytracker.com/id/1035770 Third Party Advisory
http://www.squid-cache.org/Advisories/SQUID-2016_9.txt Vendor Advisory
http://www.squid-cache.org/Versions/v3/3.4/changesets/SQUID-2016_9.patch Patch
http://www.squid-cache.org/Versions/v3/3.5/changesets/SQUID-2016_9.patch Patch
http://www.ubuntu.com/usn/USN-2995-1 Third Party Advisory
https://access.redhat.com/errata/RHSA-2016:1139
https://access.redhat.com/errata/RHSA-2016:1140
https://security.gentoo.org/glsa/201607-01
http://bugs.squid-cache.org/show_bug.cgi?id=4455 ExploitIssue TrackingPatchVendor Advisory
http://lists.opensuse.org/opensuse-security-announce/2016-08/msg00010.html
http://lists.opensuse.org/opensuse-security-announce/2016-08/msg00040.html
http://lists.opensuse.org/opensuse-updates/2016-08/msg00069.html
http://www.debian.org/security/2016/dsa-3625
http://www.openwall.com/lists/oss-security/2016/05/06/3 PatchRelease Notes
http://www.openwall.com/lists/oss-security/2016/05/06/5 PatchThird Party Advisory
http://www.oracle.com/technetwork/topics/security/linuxbulletinapr2016-2952096.html Third Party Advisory
http://www.securitytracker.com/id/1035770 Third Party Advisory
http://www.squid-cache.org/Advisories/SQUID-2016_9.txt Vendor Advisory
http://www.squid-cache.org/Versions/v3/3.4/changesets/SQUID-2016_9.patch Patch
http://www.squid-cache.org/Versions/v3/3.5/changesets/SQUID-2016_9.patch Patch
http://www.ubuntu.com/usn/USN-2995-1 Third Party Advisory
https://access.redhat.com/errata/RHSA-2016:1139
https://access.redhat.com/errata/RHSA-2016:1140
https://security.gentoo.org/glsa/201607-01

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Source: NIST National Vulnerability Database (record CVE-2016-4555), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.