flowchart TD
    Start([Attacker sends HTTP POST /upload.action<br/>Content-Type: &#37;&#123;OGNL_PAYLOAD&#125;.multipart/form-data])
    Start --> PIPE

    PIPE["<b>Struts2 filter → wrap → parse pipeline</b><br/>doFilter() → PrepareOperations.wrapRequest()<br/>→ Dispatcher.wrapRequest() → new MultiPartRequestWrapper()"]
    PIPE --> D_CT{"Content-Type contains<br/>&quot;multipart/form-data&quot;?"}
    D_CT -- "No → normal request ..." --> OUT_NORM([non-upload path ...])
    D_CT -- "Yes (attacker appends<br/>.multipart/form-data to payload)" --> PARSE

    PARSE[/"<b>JakartaMultiPartRequest.parse()</b><br/>→ Commons FileUpload.parseRequest()"/]
    PARSE --> D_VALID{"FileUpload can parse<br/>Content-Type?"}
    D_VALID -- "Yes → normal file upload ..." --> OUT_OK([files extracted, action executes ...])
    D_VALID -- "No — malformed Content-Type<br/>throws InvalidContentTypeException<br/>(message = raw Content-Type string)" --> CATCH

    CATCH["parse() catch block<br/>calls buildErrorMessage(e, &#123;&#125;)"]
    CATCH --> FIND

    FIND[/"<b>buildErrorMessage()</b><br/>→ LocalizedTextUtil.findText(class, errorKey, locale, e.getMessage(), args)"/]
    FIND --> D_KEY{"Resource bundle has key<br/>struts.messages.upload.error.*?"}
    D_KEY -- "Yes → safe localised message ..." --> OUT_SAFE([error displayed safely ...])
    D_KEY -- "No — falls back to<br/>e.getMessage() as the default<br/>(attacker's raw Content-Type)" --> TRANSLATE

    TRANSLATE["TextParseUtil.translateVariables()<br/>scans string for &#37;&#123;...&#125; expressions"]
    TRANSLATE --> D_OGNL{"String contains<br/>&#37;&#123;...&#125; OGNL expression?"}
    D_OGNL -- "No → plain error string ..." --> OUT_PLAIN([safe error message ...])
    D_OGNL -- "Yes — evaluates OGNL<br/>inside attacker content" --> SANDBOX

    SANDBOX[/"<b>OGNL sandbox bypass</b><br/>#ognlUtil.getExcludedClasses().clear()<br/>#ognlUtil.getExcludedPackageNames().clear()"/]
    SANDBOX --> ACCESS

    ACCESS[/"<b>Unrestricted reflection</b><br/>#context.setMemberAccess(DEFAULT_MEMBER_ACCESS)"/]
    ACCESS --> RCE

    RCE[/"<b>RCE</b><br/>Runtime.getRuntime().exec(cmd)"/]
    RCE --> Done([Command executed as Tomcat process user])

    style Start fill:#d32f2f,color:#fff
    style Done fill:#d32f2f,color:#fff
    style OUT_NORM fill:#9e9e9e,color:#fff
    style OUT_OK fill:#9e9e9e,color:#fff
    style OUT_SAFE fill:#9e9e9e,color:#fff
    style OUT_PLAIN fill:#9e9e9e,color:#fff
    style SANDBOX fill:#b71c1c,color:#fff
    style ACCESS fill:#b71c1c,color:#fff
    style RCE fill:#b71c1c,color:#fff
    style CATCH fill:#e65100,color:#fff
    style TRANSLATE fill:#e65100,color:#fff