PT-2026-88811 · Ocaml+1 · Mirage-Crypto

CVE-2026-87732

·

Published

2026-07-27

·

Updated

2026-09-09

CVSS v3.1

6.2

Medium

VectorAV:L/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
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 string forged in
 let dst = Bytes.make len 'x00' in
 let verified =
  GCM.authenticate decrypt into ~key ~nonce forged ~src off:0 ~tag off:len dst
   ~dst off:0 len
 in
 Printf.printf "tag verified: %b (the message is rejected as forged)
" verified;
 Printf.printf "secret left in the output buffer: %S
" (Bytes.to string dst)

Timeline

  • June 25th 2026: report to ocaml/security-advisories
  • June 29th: acknowledgement of issue with several questions for the reporter
  • July 6th: answers from reporter, including a patch
  • July 27th: release of mirage-crypto 2.2.0 and security advisory

Exploit

Fix

Improper Verification of Cryptographic Signature

Found an issue in the description? Have something to add? Feel free to write us 👾
dbugs@ptsecurity.com

Weakness Enumeration

CWE-347

Related Identifiers

CVE-2026-87732
OSEC-2026-12

Affected Products

Mirage-Crypto