liboqs is a C-language cryptographic library that provides implementations of post-quantum cryptography algorithms. A correctness error has been identified in the reference implementation of the HQC key encapsulation mechanism. Due to an indexing error, part of the secret key is incorrectly treated as non-secret data. This results in an incorrect shared secret value being returned when the decapsulation function is called with a malformed ciphertext. This vulnerability is fixed in 0.12.0.