Deserialization of Untrusted Data
Manipulating classpath asset file URLs in Tapestry before 5.4.4, an attacker could guess the path to a known file in the classpath and have it downloaded. If the attacker found the file with the value of the tapestry.hmac-passphrase configuration symbol, most probably the webapp's AppModule class, the value of this symbol could be used to craft a Java deserialization attack, thus running malicious injected Java code. The vector would be the t:formdata parameter from the Form component.
CWE-502 - Deserialization of Untrusted Data
Deserialization of untrusted data vulnerabilities enable an attacker to replace or manipulate a serialized object, replacing it with malicious data. When the object is deserialized at the victim's end the malicious data is able to compromise the victim’s system. The exploit can be devastating, its impact may range from privilege escalation, broken access control, or denial of service attacks to allowing unauthorized access to the application's internal code and logic which can compromise the entire system.