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GHSA-423w-p2w9-r7vq
No affected components available
Summary
In the AES GCM implementation of decrypt_in_place_detached, the decrypted ciphertext (i.e. the correct plaintext) is exposed even if tag verification fails.
Impact
If a program using the aes-gcm crate's decrypt_in_place* APIs accesses the buffer after decryption failure, it will contain a decryption of an unauthenticated input. Depending on the specific nature of the program this may enable Chosen Ciphertext Attacks (CCAs) which can cause a catastrophic breakage of the cipher including full plaintext recovery.
Details
As seen in the implementation of decrypt_in_place_detached for AES GCM, if the tag verification fails, an error is returned. Because the decryption of the ciphertext is done in place, the plaintext contents are now exposed via buffer.
This should ideally not be the case - as noted in page 17 of NIST's publication Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC: In Step 8, the result of Step 7 is compared with the authentication tag that was received as an input: if they are identical, then the plaintext is returned; otherwise,FAIL is returned.
This is seems correctly addressed in the AES GCM SIV implementation, where the decrypted buffer is encrypted again before the error is returned - this fix is straightforward to implement in AES GCM. To ensure that these types of cases are covered during testing, it would be valuable to add test cases like 23, 24 etc from project wycheproof to ensure that when a bad tag is used, there is an error on decryption and that the plaintext value is not exposed.
PoC
To reproduce this issue, I'm using test case 23 from project wycheproof.
let key = GenericArray::from_slice(&hex!("000102030405060708090a0b0c0d0e0f"));
let nonce = GenericArray::from_slice(&hex!("505152535455565758595a5b"));
let tag = GenericArray::from_slice(&hex!("d9847dbc326a06e988c77ad3863e6083")); // bad tag
let mut ct = hex!("eb156d081ed6b6b55f4612f021d87b39");
let msg = hex!("202122232425262728292a2b2c2d2e2f");
let aad = hex!("");
let cipher = Aes128Gcm::new(&key);
let _plaintext = cipher.decrypt_in_place_detached(&nonce, &aad, &mut ct, &tag);
assert_eq!(ct, msg);
The vulnerability requires local access to the device to be exploited. It is difficult for an attacker to exploit this vulnerability and may require special conditions. An attacker needs high-level or administrative privileges. 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 low impact on the confidentiality of the information. There is a high impact on the integrity of the data.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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