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Showcase app vulnerability allows remote command execution
Who should read this
All Struts 2 developers
Impact of vulnerability
Remote command execution
Maximum security rating
Developers should immediately upgrade to Struts 18.104.22.168
Struts Showcase App 2.0.0 - Struts Showcase App 2.3.13
Xgc Kxlzx, Alibaba Security Team
Reported directly to email@example.com
OGNL provides, among other features, extensive expression evaluation capabilities.
A request that included a specially crafted request parameter could be used to inject arbitrary OGNL code into a property, afterward used as request parameter of a redirect address, which will cause a further evaluation.
OGNL evaluation was already addressed in S2-003 and S2-005 and S2-009, but, since it involved just the parameter's name, it turned out that the resulting fixes based on whitelisting acceptable parameter names and denying evaluation of the expression contained in parameter names, closed the vulnerability only partially.
The second evaluation happens when redirect result reads it from the stack and uses the previously injected code as redirect parameter.
This lets malicious users put arbitrary OGNL statements into any unsanitized String variable exposed by an action and have it evaluated as an OGNL expression to enable method execution and execute arbitrary methods, bypassing Struts and OGNL library protections.
The issue, in order to work, need a redirect result defined as the following:
The OGNLUtil class was changed to deny eval expressions by default.