An issue was discovered in the mirage-crypto package before 2.2.0 for OCaml. The AES.GCM.authenticate_decrypt_into and Chacha20.authenticate_decrypt_into functions write the decrypted plaintext into a caller-provided buffer and only then compares the tag. On a forged tag, the functions returns false, but the destination buffer already holds the full plaintext.

An issue was discovered in the mirage-crypto package before 2.2.0 for OCaml. The AES.GCM.authenticate_decrypt_into and Chacha20.authenticate_decrypt_into functions write the decrypted plaintext into a caller-provided buffer and only then compares the tag. On a forged tag, the functions returns false, but the destination buffer already holds the full plaintext.

Affected: opam: mirage-crypto

On the generic GCM path, `AES.GCM.authenticate_decrypt_into` (as well as `Chacha20.authenticate_decrypt_into` and `AES.CCM16.authenticate_decrypt_into`) writes the decrypted plaintext into the caller's destination buffer and only then compares the tag. On a forged tag the function returns false, but the destination buffer already holds the full plaintext. AEAD decryption is meant to be all-or-nothing: no plaintext should be released until the tag verifies. This is a release of unverified plaintext, and a caller that reads the buffer without first checking the return value sees forged-but-decrypted data. RustCrypto's aes-gcm fixed the same defect in CVE-2023-42811 (rated medium) by re-encrypting the buffer on tag failure.

## Solution

The tag is validated first, and only if the tag is valid, the decryption into the provided buffer is done. In the CCM code, the tag is computed over the plaintext - if the tag validation fails, the provided buffer is zeroed.

## Reproduction

```OCaml module GCM = Mirage_crypto.AES.GCM

let () = let key = GCM.of_secret (String.make 32 '\x00') in let nonce = String.make 12 '\x00' in let secret = "let password = 42" in let len = String.length secret in let blob = GCM.authenticate_encrypt ~key ~nonce secret in (* flip one bit of the 16-byte tag (at offset len); the ciphertext is untouched *) let forged = Bytes.of_string blob in Bytes.set forged len (Char.chr (Char.code (Bytes.get forged len) lxor 1)); let forged = Bytes.unsafe_to…

Affected: Ubuntu:22.04:LTS: ocaml-mirage-crypto, Ubuntu:24.04:LTS: ocaml-mirage-crypto, Ubuntu:26.04:LTS: ocaml-mirage-crypto

An issue was discovered in the mirage-crypto package before 2.2.0 for OCaml. The AES.GCM.authenticate_decrypt_into and Chacha20.authenticate_decrypt_into functions write the decrypted plaintext into a caller-provided buffer and only then compares the tag. On a forged tag, the functions returns false, but the destination buffer already holds the full plaintext.