Manipulating classpath asset file URLs, 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.
The product deserializes untrusted data without sufficiently ensuring that the resulting data will be valid.