Insufficient Verification of Proofs generated by the immudb server in client SDK.

Impact

In certain scenario a malicious immudb server can provide a falsified proof that will be accepted by the client SDK signing a falsified transaction replacing the genuine one. This situation can not be triggered by a genuine immudb server and requires the client to perform a specific list of verified operations resulting in acceptance of an invalid state value.

This vulnerability only affects immudb client SDKs, the immudb server itself is not affected by this vulnerability.

Detailed description

immudb uses Merkle Tree enhanced with additional linear part to perform consistency proofs between two transactions. The linear part is built from the last leaf node of the Merkle Tree compensating for transactions that were not yet consumed by the Merkle Tree calculation.

The Merkle Tree part is then used to perform proofs for things that are in transaction range covered by the Merkle Tree where the linear part is used to check those that are not yet in the Merkle Tree.

When doing consistency checks between two immudb states, the linear proof part is not fully checked. In fact only the first (last Merkle Tree leaf) and the last (current DB state value) are checked against new Merkle Tree without ensuring that elements in the middle of that chain are correctly added as Merkle Tree leafs.

This lack of check means that the database can present different set of hashes on the linear proof part to what would later be used once those become part of the Merkle Tree. This property can be exploited by the database to expose two different transaction entries depending on the other transaction that the user requested consistency proof for.

In practice this could lead to a following scenario:

• a client requests a verified write operation
• the server responds with a proof for the transaction
• client stores the state value retrieved from the server and expects it to be a confirmation of that write and all the history of the database before that transaction
• a series of validated read / write operations is performed by the client, each accompanied by a successfully validated consistency proof and update of the client state
• the client requests verified get operation on the transaction it has written before (and that was verified with a proof from the server)
• the server replies with a completely different transaction that can be properly validated according to the currently stored db state on the client side

Patches

The following Go SDK versions is not vulnerable:

SDK	Version
go	1.4.1

Workarounds

Invalid proofs can not be generated in a normal immudb server and will be detected by a genuine replica server. To ensure that the server does not produce invalid proofs and to check that the history presented by the server does not contain falsified transactions, one should run a genuine immudb replica server in a safe environment and fully synchronize all databases with the primary.

References

• https://github.com/codenotary/immudb/tree/master/docs/security/vulnerabilities/linear-fake

For more information

If you have any questions or comments about this advisory:

• Open a discussion in immudb Discussions
• Email us at [email protected]