Know every vulnerabilitybefore it knows you.
DevGuard continuously monitors your dependencies and alerts you when CVEs like this one affect your stack — with real-time threat intelligence built for developers.
OSEC-2026-12
No affected components available
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
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)\n" verified;
Printf.printf "secret left in the output buffer: %S\n" (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
The vulnerability requires local access to the device to be exploited. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a high impact on the integrity of the data.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
Probability that this vulnerability will be exploited in the wild within the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
Browse More
Continuously monitor your dependencies and get alerted when vulnerabilities like this one affect your stack.
Checkout DevGuard