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Who should read this
All Struts 2 developers and users
Impact of vulnerability
Affects of a cross-site scripting vulnerability.
Maximum security rating
Upgrade runtime JRE to a recent major version, preferably 1.8. Alternatively upgrade to Struts 2.3.28
Struts 2.0.0 - Struts Struts 126.96.36.199
WhiteHat Security (whitehatsec.com)
When using a single byte page encoding such as ISO-8895-1, an attacker might submit a non-spec URL-encoded parameter value including multi-byte characters.
Struts 2 used the standard JRE URLDecoder to decode parameter values. Especially JRE 1.5's URLDecoder implementation seems to be broken to the point that this non-spec encoding isn't rejected / filtered. In later JREs the issue was fixed, best known solution is found in JRE 1.8.
Upgrade runtime JRE/JDK, preferably to the most recent 1.8 version.
Alternatively upgrade to Struts 2.3.28, which includes and uses a safe URLDecoder implementation from Apache Tomcat
No issues expected when upgrading to Struts 2.3.28
Use UTF-8 for page and parameter encoding.