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Category:   Application (Web Server/CGI)  >   Squid Vendors:
(Red Hat Issues Fix) Squid ESI Response Processing Bugs Let Remote Users Deny Service
SecurityTracker Alert ID:  1035990
SecurityTracker URL:
CVE Reference:   CVE-2016-4556   (Links to External Site)
Date:  May 31 2016
Impact:   Denial of service via network
Fix Available:  Yes  Vendor Confirmed:  Yes  
Version(s): 3.x - 3.5.17, 4.x - 4.0.9
Description:   Two vulnerabilities were reported in Squid. A remote user can cause denial of service conditions on the target system.

A remote server can return specially crafted ESI response data to trigger pointer handling errors and cause denial of service conditions on all connected clients.

The vulnerabilities reside in '' [CVE-2016-4555] and '' [CVE-2016-4556].

"bfek-18" and "@vftable" reported these vulnerabilities.

Impact:   A remote server can cause denial of service conditions on all connected clients.
Solution:   Red Hat has issued a fix for CVE-2016-4556.

The Red Hat advisory is available at:

Vendor URL: (Links to External Site)
Cause:   Access control error
Underlying OS:  Linux (Red Hat Enterprise)
Underlying OS Comments:  6

Message History:   This archive entry is a follow-up to the message listed below.
May 6 2016 Squid ESI Response Processing Bugs Let Remote Users Deny Service

 Source Message Contents

Subject:  [RHSA-2016:1138-01] Moderate: squid security update

Hash: SHA1

                   Red Hat Security Advisory

Synopsis:          Moderate: squid security update
Advisory ID:       RHSA-2016:1138-01
Product:           Red Hat Enterprise Linux
Advisory URL:
Issue date:        2016-05-31
CVE Names:         CVE-2016-4051 CVE-2016-4052 CVE-2016-4053 
                   CVE-2016-4054 CVE-2016-4554 CVE-2016-4556 

1. Summary:

An update for squid is now available for Red Hat Enterprise Linux 6.

Red Hat Product Security has rated this update as having a security impact
of Moderate. A Common Vulnerability Scoring System (CVSS) base score, which
gives a detailed severity rating, is available for each vulnerability from
the CVE link(s) in the References section.

2. Relevant releases/architectures:

Red Hat Enterprise Linux Server (v. 6) - i386, ppc64, s390x, x86_64
Red Hat Enterprise Linux Workstation (v. 6) - i386, x86_64

3. Description:

Squid is a high-performance proxy caching server for web clients,
supporting FTP, Gopher, and HTTP data objects.

Security Fix(es):

* A buffer overflow flaw was found in the way the Squid cachemgr.cgi
utility processed remotely relayed Squid input. When the CGI interface
utility is used, a remote attacker could possibly use this flaw to execute
arbitrary code. (CVE-2016-4051)

* Buffer overflow and input validation flaws were found in the way Squid
processed ESI responses. If Squid was used as a reverse proxy, or for
TLS/HTTPS interception, a remote attacker able to control ESI components on
an HTTP server could use these flaws to crash Squid, disclose parts of the
stack memory, or possibly execute arbitrary code as the user running Squid.
(CVE-2016-4052, CVE-2016-4053, CVE-2016-4054)

* An input validation flaw was found in Squid's mime_get_header_field()
function, which is used to search for headers within HTTP requests. An
attacker could send an HTTP request from the client side with specially
crafted header Host header that bypasses same-origin security protections,
causing Squid operating as interception or reverse-proxy to contact the
wrong origin server. It could also be used for cache poisoning for client
not following RFC 7230. (CVE-2016-4554)

* An incorrect reference counting flaw was found in the way Squid processes
ESI responses. If Squid is configured as reverse-proxy, for TLS/HTTPS
interception, an attacker controlling a server accessed by Squid, could
crash the squid worker, causing a Denial of Service attack. (CVE-2016-4556)

4. Solution:

For details on how to apply this update, which includes the changes
described in this advisory, refer to:

After installing this update, the squid service will be restarted

5. Bugs fixed (

1329126 - CVE-2016-4051 squid: buffer overflow in cachemgr.cgi
1329136 - CVE-2016-4052 CVE-2016-4053 CVE-2016-4054 squid: multiple issues in ESI processing
1334241 - CVE-2016-4554 squid: Header Smuggling issue in HTTP Request processing
1334786 - CVE-2016-4556 squid: SIGSEGV in ESIContext response handling

6. Package List:

Red Hat Enterprise Linux Server (v. 6):






Red Hat Enterprise Linux Workstation (v. 6):




These packages are GPG signed by Red Hat for security.  Our key and
details on how to verify the signature are available from

7. References:

8. Contact:

The Red Hat security contact is <>. More contact
details at

Copyright 2016 Red Hat, Inc.
Version: GnuPG v1


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