GHSA-p9w4-585h-g3c7 biscuit-auth vulnerable to public key confusion in third party block

• crates.io biscuit-auth: >= 4.0.0, < 5.0.0 → fixed in 5.0.0

Third-party blocks can be generated without transferring the whole token to the third-party authority. Instead, a `ThirdPartyBlock` request can be sent, providing only the necessary info to generate a third-party block and to sign it: • the public key of the previous block (used in the signature) • the public keys part of the token symbol table (for public key interning in datalog expressions) A third-part block request forged by a malicious user can trick the third-party authority into generating datalog trusting the wrong keypair. Consider the following example (nominal case) • Authority `A` emits the following token: `check if thirdparty("b") trusting ${pubkeyB}` • The well-behaving holder then generates a third-party block request based on the token and sends it to third-party authority `B` • Third-party `B` generates the following third-party block `thirdparty("b"); check if thirdparty("c") trusting ${pubkeyC}` • The token holder now must obtain a third-party block from third party `C` to be able to use the token Now, with a malicious user: • Authority `A` emits the following token: `check if thirdparty("b") trusting ${pubkeyB}` • The holder then attenuates the token with the following third party block `thirdparty("c")`, signed with a keypair `pubkeyD, privkeyD)` they generate • The holder then generates a third-party block request based on this token, but alter the `ThirdPartyBlockRequest` `publicKeys` field and replace `pubkeyD` with `pubkeyC` • Third-party `B` generates the following third-party block `thirdparty("b"); check if thirdparty("c") trusting ${pubkeyC}` • Due to the altered symbol table, the actual meaning of the block is `thirdparty("b"); check if thirdparty("c") trusting ${pubkeyD}` • The attacker can now use the token without obtaining a third-party block from `C`. Impact Tokens with third-party blocks containing `trusted` annotations generated through a third party block request