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Vigil@nce: HTTP, incoherent handling of parameters

May 2009 by Vigil@nce

The HTTP protocol does not define the behavior of web servers when
a request contains the same variable several times, which can
generate vulnerabilities.

Severity: 1/4

Consequences: data reading, data creation/edition, data flow

Provenance: internet client

Means of attack: no proof of concept, no attack

Ability of attacker: expert (4/4)

Confidence: unique source (2/5)

Diffusion of the vulnerable configuration: high (3/3)

Creation date: 20/05/2009

IMPACTED PRODUCTS

 HTTP

DESCRIPTION OF THE VULNERABILITY

The RFC 2616 defines the HTTP protocol. The RFC 3986 defines the
syntax of uris. For example:
http://server/page?var1=val1&var2=val2

Both RFC do not define how to handle urls containing several times
the same variable name. For example:
http://server/page?var=val1&var=val2

Developers of HTTP service thus made different choices:
 ASP.NET : the value is the concatenation of parameters
("val1,val2")
 PHP : the value is the last parameter ("val2")
 JSP : the value is the first parameter ("val1")
 Zope : the value is an array ([’val1’, ’val2’])

Similarly, if a parameter is defined in the Query String and in a
Cookie, behavior diverge. For example:
POST /page?var=val1
Cookie: var=val2
\n
var=val3

An attacker can therefore use these incoherent behavior in order
to bypass an IDS or web filtering modules.

Theses vulnerabilities were named HPP (HTTP Parameter Pollution).

CHARACTERISTICS

Identifiers: VIGILANCE-VUL-8726

http://vigilance.fr/vulnerability/HTTP-incoherent-handling-of-parameters-8726


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