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AUSCERT External Security Bulletin Redistribution
CGI web servers assign Proxy header values from client
requests to internal HTTP_PROXY environment variables
19 July 2016
AusCERT Security Bulletin Summary
Product: Web Server products running CGI
Operating System: UNIX variants (UNIX, Linux, OSX)
Impact/Access: Provide Misleading Information -- Remote/Unauthenticated
Unauthorised Access -- Remote/Unauthenticated
CVE Names: CVE-2016-1000110 CVE-2016-1000109 CVE-2016-5388
CVE-2016-5387 CVE-2016-5386 CVE-2016-5385
Comment: Where applicable, patches should be applied to correct this issue.
Where patches have not yet been released, mitigations have been
provided in this bulletin for a number of products. For products where
mitigations have not yet been provided, contact the product vendor
for more information.
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Vulnerability Note VU#797896
CGI web servers assign Proxy header values from client requests to internal
HTTP_PROXY environment variables
Original Release date: 18 Jul 2016 | Last revised: 18 Jul 2016
Web servers running in a CGI or CGI-like context may assign client request
Proxy header values to internal HTTP_PROXY environment variables. This
vulnerability can be leveraged to conduct man-in-the-middle (MITM) attacks on
internal subrequests or to direct the server to initiate connections to
CWE-807: Reliance on Untrusted Inputs in a Security Decision, CWE-454:
External Initialization of Trusted Variables or Data Stores
Web servers running in a CGI or CGI-like context may assign client request
Proxy header values to internal HTTP_PROXY environment variables. The
vulnerable behavior is the result of a naming convention for meta-variables,
defined in RFC 3876, which leads to a name collision: "The HTTP header field
name is converted to upper case, has all occurrences of "-" replaced with "_"
and has "HTTP_" prepended to give the meta-variable name."
According to the researchers, a web server is vulnerable if:
1. A web server, programming language or framework (and in some limited
situations the application itself) sets the environmental variable HTTP_PROXY
from the user supplied Proxy header in the web request, or sets a similarly
used variable (essentially when the request header turns from harmless data
into a potentially harmful environmental variable).
2. A web application makes use of HTTP_PROXY or similar variable unsafely
(e.g. fails to check the request type) resulting in an attacker controlled
proxy being used (essentially when HTTP_PROXY is actually used unsafely).
By sending a specially crafted request to a vulnerable server, a remote,
unauthenticated attacker may be able to conduct MITM attacks on internal
server subrequests or direct the server to initiate connections to arbitrary
hosts. For more information, refer to httpoxy.org.
A remote, unauthenticated attacker may be able to conduct MITM attacks on
internal server subrequests or direct the server to initiate connections to
Apply an update
Where applicable, affected products and components should be updated to
address this vulnerability. Check with vendors for information about patching.
Where patches are unavailable or updating is not an option, consider the
Filter Proxy request headers
The researchers and community have identified several filtering strategies
that are product-dependent:
In this configuration, any language may be vulnerable (the HTTP_PROXY env
var is "real"). If you are using mod_headers , you can unset the "Proxy"
header with this directive:
RequestHeader unset Proxy
If you are using mod_security, you can use a rule like (vary the action to
SecRule &REQUEST_HEADERS:Proxy "@gt 0"
Refer to Apache's response for more information.
httprequest delheader Proxy
Use this to block the Proxy header from being passed on to PHPFPM, PHPPM,
fastcgi_param HTTP_PROXY "";
Nginx with proxy_pass
The following setting should work for people who are using "proxy_pass"
proxy_set_header Proxy "";
Microsoft has provided the following guidance for IIS servers utilizing
affected third-party frameworks:
Update apphost.config with the following rule:
<rule name=3D"Erase HTTP_PROXY" patternSyntax=3D"Wildcard">
<match url=3D"*.*" />
<set name=3D"HTTP_PROXY" value=3D"" />
<action type=3D"None" />
Vendor Information (Learn More)
Vendor Status Date Notified Date Updated
Apache HTTP Server Project Affected 12 Jul 2016 18 Jul 2016
Go Programming Language Affected - 18 Jul 2016
HAProxy Affected - 13 Jul 2016
HHVM Affected - 18 Jul 2016
Microsoft Corporation Affected 12 Jul 2016 13 Jul 2016
nginx Affected - 13 Jul 2016
Python Affected - 18 Jul 2016
The PHP Group Affected - 18 Jul 2016
EfficientIP SAS Not Affected 12 Jul 2016 12 Jul 2016
ACCESS Unknown 12 Jul 2016 12 Jul 2016
Alcatel-Lucent Unknown 12 Jul 2016 12 Jul 2016
Apple Unknown 12 Jul 2016 12 Jul 2016
Arista Networks, Inc. Unknown 12 Jul 2016 12 Jul 2016
ARRIS Unknown 12 Jul 2016 12 Jul 2016
Aruba Networks Unknown 12 Jul 2016 12 Jul 2016
If you are a vendor and your product is affected, let us know.
CVSS Metrics (Learn More)
Group Score Vector
Base 5.1 AV:N/AC:H/Au:N/C:P/I:P/A:P
Temporal 4.6 E:POC/RL:ND/RC:C
Environmental 1.1 CDP:ND/TD:L/CR:ND/IR:ND/AR:ND
Thanks to Dominic Scheirlinck and Scott Geary of Vend for reporting this
This document was written by Joel Land.
CVE IDs: CVE-2016-5385 CVE-2016-5386 CVE-2016-5387 CVE-2016-5388
Date Public: 18 Jul 2016
Date First Published: 18 Jul 2016
Date Last Updated: 18 Jul 2016
Document Revision: 59
If you have feedback, comments, or additional information about this
vulnerability, please send us email.
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