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  • S2-015
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Summary

A vulnerability introduced by wildcard matching mechanism or double evaluation of OGNL Expression allows remote command execution.

Who should read this

All Struts 2 developers and users

Impact of vulnerability

Remote command execution, remote server context manipulation, injection of malicious client side code

Maximum security rating

Highly Critical

Recommendation

Developers should immediately upgrade to Struts 2.3.14.3

Affected Software

Struts 2.0.0 - Struts 2.3.14.2

Reporter

Jon Passki from Coverity Security Research Laboratory reported directly to security@struts.a.o and via blog post

CVE Identifier

CVE-2013-2135, CVE-2013-2134

Problem

Struts 2 allows define action mapping base on wildcards, like in example below:

If a request doesn't match any other defined action, it will be matched by * and requested action name will be used to load JSP file base on the name of action. And as value of {1} is threaten as an OGNL expression, thus allow to execute arbitrary Java code on server side. This vulnerability is combination of two problems:

  • requested action name isn't escaped or checked agains whitelist
  • double evaluation of an OGNL expression in TextParseUtil.translateVariables when combination of $ and % open chars is used.

Proof of concept

Wildcard matching

  1. Run struts2-blank app
  2. Open the following url, resulting in dynamic action name resolution based on passed value of #foo

As you can notice, action name is resolved based on user input and you can put any arbitrary code to perform attack.

Double evaluation of an expression

  1. Open example.xml present in the Struts Blank App and change result of HelloWorld action to one below:
  2. Open HelloWorld.java and change execute() method as below:
  3. Run struts2-blank app
  4. Open the following url (you must have a tool to check response headers)
  5. Check value of foobar header, it should be 3

As you can notice, passed value of message parameter was used to set value of foobar header and the value was double evaluated - first time when ${message} was evaluated, secondly when parsed value (${%{1+2}}) was evaluated again.

Solution

With the new version actions' names whitelisting was introduced and by default is set to accept actions that match the following regex:

user can change the definition by setting up a new constant in struts.xml as below:

Double evaluation of passed expression was removed from OgnlTextParser which is used by TextParseUtil.translateVariables.

Backward Compatibility

There should be no problems with migration from previous version.

It is strongly recommended to upgrade to Struts 2.3.14.3.

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4 Comments

  1. Anonymous

    It seems that struts2-core-2.0.11 is not affected by this problem?

    1. Maybe, but we haven't been testing that.

  2. Anonymous

    Double evaluation of passed expression was removed from OgnlTextParser which is used by TextParseUtil.translateVariables.

    May I ask how?

    1. Parser is going further, does not restart index on each nested open char.