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Core Security Technologies - CoreLabs Advisory
http://www.coresecurity.com/corelabs/
Multiple Vulnerabilities with 8.3 Filename Pseudonyms in Web Servers
1. *Advisory Information*
Title: Multiple Vulnerabilities with 8.3 Filename Pseudonyms in Web Servers
Advisory Id: CORE-2010-0121
Advisory URL:
http://www.coresecurity.com/content/filename-pseudonyms-vulnerabilities
Date published: 2010-02-05
Date of last update: 2010-02-05
Vendors contacted: Cherokee team, Nginx team, Mongoose team
Release mode: User release
2. *Vulnerability Information*
Class: Improper Handling of Exceptional Conditions [CWE-755], Path
traversal [CWE-22]
Impact: Security bypass
Remotely Exploitable: Yes
Locally Exploitable: No
Bugtraq ID: N/A
CVE Name: N/A
3. *Vulnerability Description*
When a file is created on a Windows system, a DOS-compatible 8.3 short
file name (hereafter referred to as '8.3 alias') is generated for
backwards compatibility reasons. Both names can be used to refer to the
same file. Applications which allow users to specify file names on
Windows systems should be aware of these aliases and handle them
appropriately.
Often, by using 8.3 aliases for files, one can bypass IDS/IPS detection,
and evade filters and file restrictions. This can be a result of the
fact that only the long versions of file and folder names will be
restricted and the alias will not match the long filename.
Referencing files using their 8.3 aliases can even change how the files
are handled, due to truncation of the file extension in the event that
the file extension is longer than three characters. This problem is
exacerbated by the fact that intermediary systems used for things like
load balancing and caching do not have access to the actual file system
being accessed and need to convert any filenames and pathnames with
restrictions to their 8.3 alias before comparing to user data, which,
given the presence of other files or folders on the system with similar
names, may not be possible.
This advisory describes multiple vulnerabilities based on quirks in how
Windows handles file names. The affected software is the Windows version
of the following web servers:
. Nginx Web Server [1]. The way Nginx handles files may differ when
they are requested using their 8.3 alias, and short file or path names
are not correctly handled when applying file handling rules or access
restrictions. By abusing of these flaws an attacker can bypass security
options implemented in the web server. For instance, 'file.shtml' will
become 'FILE~1.SHT'. This will cause the file to be handled as a '.sht'
file, not a '.shtml' file. The result of this is that instead of
processing SSI directives as would normally be the case with a '.shtml'
file, the file would be served unprocessed. Additionally, Nginx does not
correctly handle extraneous spaces after file extensions when applying
preprocessing rules or access restrictions.
. Cherokee Web Server [2]. On Cherokee Web Server for Windows, short
file and folder names are not correctly handled when applying file
handling rules, IP access restrictions or authentication rules.
Extraneous trailing spaces in file names are not correctly handled when
applying file handling rules or access restrictions. By abusing of these
flaws, Cherokee configuration folders can be accessed from any IP,
without authentication bypassing the web server security protection
mechanisms.
. Mongoose Web Server [3]. Mongoose does not correctly handle Windows
short file names (and folder names) when applying preprocessing rules or
access restrictions. As a result, an attacker can bypass the web server
security protection mechanisms, and protected files can be accessed
unprocessed without authentication.
. LightTPD Web Server. The WLMP build of LightTPD for Windows [4]
does not correctly handle Windows short file or path names when applying
preprocessing rules, file extension restrictions or access restrictions.
It also does not correctly handle extraneous trailing periods when
applying file extension restrictions, but does properly apply
preprocessing rules.
4. *Vulnerable packages*
. Nginx Web Server v0.7.63
. Nginx Web Server v0.8.29
. Cherokee Web Server v0.99.28
. Mongoose Web Server v2.8
. WLMP web server package for Windows v1.1.6.1171
. Older versions are probably affected too, but they were not checked.
5. *Non-vulnerable packages*
. Nginx Web Server v0.8.33
. Contact Cherokee, Mongoose and WLMP Web Server projects for
additional information.
6. *Vendor Information, Solutions and Workarounds*
Attacks using 8.3 aliases can be mitigated by turning off 8.3 alias
generation, by setting the value of 'NtfsDisable8dot3NameCreation' to 1
in 'HKLM\System\CurrentControlSet\Control\FileSystem'.
7. *Credits*
These vulnerabilities were discovered and researched by Dan Crowley from
Core Security Technologies.
8. *Technical Description / Proof of Concept Code*
8.1. *Nginx Web Server*
The following configuration snippet for Nginx Web Server will process
any file with an extension of '.phtml' or '.php' by passing it to
another service running locally on port 8080 for processing. It will
deny requests for files beginning with '.ht' and the directory
'longfoldername'.
/-----
location ~ \.php$ {
proxy_pass http://127.0.0.1:8080;
}
location ~ \.phtml$ {
proxy_pass http://127.0.0.1:8080;
}
location ~ /\.ht {
deny all;
}
#deny longfoldername directory explicitly
location ~ /longfoldername/ {
deny all;
}
- -----/
The 'longfoldername' folder can be accessed when using the following URI:
/-----
http://www.example.com/LONGFO~1/
- -----/
Files denied by matching the extension or the file name can be accessed
using a similar technique. A file named '.htpasswd' could be retrieved
as demonstrated below:
/-----
http://www.example.com/HTPASS~1
- -----/
Nginx Web Server will serve the unprocessed source code for files
normally intended to be preprocessed when using URIs similar to the
following (these examples assume the presence of the files 'file.phtml'
and 'file.php':
/-----
http://www.example.com/FILE~1.PHT
http://www.example.com/file.php%20
- -----/
Similarly, if a file can be uploaded whose extension, when truncated to
three characters, matches an extension which Nginx is set to process as
code, it can be run as code by referring to it with an 8.3 alias. For
instance, if a file named 'file.phpx' is uploaded to the webroot, it can
be executed as PHP code with the following URI:
/-----
http://www.example.com/FILE~1.PHP
- -----/
8.2. *Cherokee Web Server*
The following configuration snippet for Cherokee Web Server will allow
only requests from '1.2.3.4' to reach the directory 'longfoldername' and
additionally, requires a password defined in '/Program
Files/Octality/Cherokee/etc/passwd' in order to gain access.
/-----
vserver!1!rule!1!match = directory
vserver!1!rule!1!match!directory = /longfoldername
vserver!1!rule!1!match!allow_from = 1.2.3.4
vserver!1!rule!1!auth = plain
vserver!1!rule!1!auth!methods = basic
vserver!1!rule!1!auth!passwdfile = /Program
Files/Octality/Cherokee/etc/passwd
vserver!1!rule!1!auth!realm = secret
- -----/
The folder can be accessed from any IP, without authentication, when
accessing the file using either of the following URIs:
/-----
http://www.example.com/longfoldername./
http://www.example.com/LONGFO~1/
- -----/
Files denied by matching the extension or the file name can be accessed
using similar techniques, as demonstrated below:
/-----
http://www.example.com/longfilename.inc.
http://www.example.com/longfilename.inc%20
http://www.example.com/LONGFI~1.INC
- -----/
In the following rule, the extensions '.php' and '.phtml' are both
configured to be handled by FastCGI and preprocessed before being served.
/-----
vserver!1!rule!99999!match = extensions
vserver!1!rule!99999!match!extensions = php,phtml
vserver!1!rule!99999!encoder!gzip = 1
vserver!1!rule!99999!handler = fcgi
vserver!1!rule!99999!handler!balancer = round_robin
vserver!1!rule!99999!handler!balancer!source!1 = 1
- -----/
Cherokee Web Server will serve the unprocessed source code for these
file extensions when using URIs similar to the following:
/-----
http://www.example.com/EXAMPL~1.PHT
http://www.example.com/example.phtml%2e
http://www.example.com/example.php%20
- -----/
Similarly, if a file can be uploaded whose extension, when truncated to
three characters, matches an extension which Cherokee is set to process
as code, it can be run as code by referring to it with an 8.3 alias. For
instance, if a file named 'file.phpx' is uploaded to the webroot, it can
be executed as PHP code with the following URI:
/-----
http://www.example.com/FILE~1.PHP
- -----/
8.3. *Mongoose Web Server*
The following configuration snippet will cause Mongoose to process files
with extensions of '.php' or '.phtml' as PHP code, and files with
extensions of '.shtml' will be processed for SSI directives. It will
also require a password (stored in 'C:\passwords.txt') when attempting
to access the folder '/longfoldername'.
/-----
cgi_ext php,phtml
ssi_ext shtml
protect /longfoldername/=C:\passwords.txt
- -----/
Given that the files 'file.phtml' and 'file.shtml' exist and the folder
'/longfoldername' exists, these items can be accessed unprocessed,
without authentication, using the following URIs:
/-----
http://www.example.com/LONGFO~1/
http://www.example.com/FILE~1.PHT
http://www.example.com/FILE~1.SHT
- -----/
Similarly, if a file can be uploaded whose extension, when truncated to
three characters, matches an extension which Mongoose is set to process
as code, it can be run as code by referring to it with an 8.3 alias. For
instance, if a file named 'file.phpx' is uploaded to the webroot, it can
be executed as PHP code with the following URI:
/-----
http://www.example.com/FILE~1.PHP
- -----/
8.4. *LightTPD Web Server (WLMP package)*
The following configuration snippet for WLMP will forbid any file with
the extensions '.inc' or '.exte', attempt to process files with a
'.phtml' or '.php' extension for PHP code, and authenticate users
attempting to access the directory '/longfoldername'.
/-----
## deny access the file-extensions
#
# ~ is for backupfiles from vi, emacs, joe, ...
# .inc is often used for code includes which should in general not be part
# of the document-root
url.access-deny = ( ".inc", ".exte" )
# which extensions should not be handle via static-file transfer
#
# .php, .pl, .fcgi are most often handled by mod_fastcgi or mod_cgi
static-file.exclude-extensions = ( ".phtml" )
#### CGI module
cgi.assign = ( ".phtml"=> "C:/WLMP/PHP/php-cgi.exe", ".php"=>
"C:/WLMP/PHP/php-cgi.exe" )
auth.backend = "plain"
auth.backend.plain.userfile = "C:/WLMP/LightTPD/conf/lighttpd-auth.conf"
auth.require = ( "/longfoldername/" =>
(
"method" => "basic",
"realm" => "Restricted folder",
"require" => "user=admin"
)
)
- -----/
The folder '/longfoldername' can be accessed without authentication,
when accessing the file using the following URI:
/-----
http://www.example.com/LONGFO~1/
- -----/
The unprocessed contents of a '.inc', '.phtml', or '.exte' file under
this configuration could be accessed using URIs like the following
(these URIs assume files named 'file.inc', 'longfilename.exte' and
'file.phtml' exist:
/-----
http://www.example.com/file.inc.
http://www.example.com/longfilename.exte%20
http://www.example.com/FILE~1.PHT
- -----/
Similarly, if a file can be uploaded whose extension, when truncated to
three characters, matches an extension which LightTPD is set to process
as code, it can be run as code by referring to it with an 8.3 alias. For
instance, if a file named 'file.phpx' is uploaded to the webroot, it can
be executed as PHP code with the following URI:
/-----
http://www.example.com/FILE~1.PHP
- -----/
9. *Report Timeline*
. 2010-01-12:
Core Security Technologies notifies the Cherokee, Nginx and Mongoose
teams of the vulnerability. Core also informs that Dan Crowley will be
giving a presentation explaining these flaws at the security conference
Shmoocon 2010 [5].
. 2010-01-12:
The Cherokee and Mongoose teams ask Core for a technical description of
the vulnerability.
. 2010-01-12:
Technical details sent to Cherokee and Mongoose teams by Core.
. 2010-01-12:
Cherokee team notifies Core that the issues have been evaluated and
considered security vulnerabilities. Cherokee team also informs us that
they are not currently shipping Windows binaries because they are aware
of all this sort of issues. The Windows port has not received much
attention for the last few years and it is far from being ready for
production. Cherokee team also states that they will link Core advisory
from their bug-tracker as soon as it is published. Currently the Windows
port is still low-priority for Cherokee project.
. 2010-01-13:
The Nginx team asks Core for a technical description of the vulnerability.
. 2010-01-13:
Technical details sent to Nginx team by Core.
. 2010-01-15:
Nginx team informs us that the investigation about the reported issues
has finished, the issues have been evaluated and Nginx team agrees that
they are security vulnerabilities. The Nginx team also notifies us that
they will fix it in next versions that will be released on Jan 18 or 25.
. 2010-02-01:
Nginx team releases a new version of the web server (nginx-0.8.33) [6].
. 2010-02-05:
The flaws are disclosed by Dan Crowley at the security conference
Shmoocon 2010 [5].
. 2010-02-05:
The advisory CORE-2010-0121 is published.
10. *References*
[1] Nginx Web Server. http://wiki.nginx.org/.
[2] Cherokee Project. http://www.cherokee-project.com/.
[3] Mongoose project. http://code.google.com/p/mongoose/.
[4] WMLP project. http://en.wlmp-project.net/.
[5] Shmoocon 2010. http://www.shmoocon.org/presentations-all.html.
[6] Nginx mail list.
http://nginx-maillist.blogspot.com/2010/02/nginx-0833.html.
11. *About CoreLabs*
CoreLabs, the research center of Core Security Technologies, is charged
with anticipating the future needs and requirements for information
security technologies. We conduct our research in several important
areas of computer security including system vulnerabilities, cyber
attack planning and simulation, source code auditing, and cryptography.
Our results include problem formalization, identification of
vulnerabilities, novel solutions and prototypes for new technologies.
CoreLabs regularly publishes security advisories, technical papers,
project information and shared software tools for public use at:
http://www.coresecurity.com/corelabs.
12. *About Core Security Technologies*
Core Security Technologies develops strategic solutions that help
security-conscious organizations worldwide develop and maintain a
proactive process for securing their networks. The company's flagship
product, CORE IMPACT, is the most comprehensive product for performing
enterprise security assurance testing. CORE IMPACT evaluates network,
endpoint and end-user vulnerabilities and identifies what resources are
exposed. It enables organizations to determine if current security
investments are detecting and preventing attacks. Core Security
Technologies augments its leading technology solution with world-class
security consulting services, including penetration testing and software
security auditing. Based in Boston, MA and Buenos Aires, Argentina, Core
Security Technologies can be reached at 617-399-6980 or on the Web at
http://www.coresecurity.com.
13. *Disclaimer*
The contents of this advisory are copyright (c) 2010 Core Security
Technologies and (c) 2010 CoreLabs, and may be distributed freely
provided that no fee is charged for this distribution and proper credit
is given.
14. *PGP/GPG Keys*
This advisory has been signed with the GPG key of Core Security
Technologies advisories team, which is available for download at
http://www.coresecurity.com/files/attachments/core_security_advisories.asc.
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