Would that make it only interoperable with itself, and not have the security a correct implementation of SHA1 provides? Is it a security bug (i.e. is there anything worth stealing by breaking their not-quite-SHA1-hash)?
The application is using a proprietary client/server protocol, so it already lacks lacks any kind of interoperability.
In this specific case, it's unclear whether the bug has direct security implications. The broken SHA-1 is used on some user-controlled data that gets XORed onto the server's decryption of a user-specified payload before being passed into an RC4 key schedule. It's certainly plausible that this might produce a server-assisted privacy compromise of other users' sessions.
In this specific case, it's unclear whether the bug has direct security implications. The broken SHA-1 is used on some user-controlled data that gets XORed onto the server's decryption of a user-specified payload before being passed into an RC4 key schedule. It's certainly plausible that this might produce a server-assisted privacy compromise of other users' sessions.