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AEADs/aes-gcm: Plaintext exposed in decrypt_in_place_detached even on tag verification failure

Moderate severity GitHub Reviewed Published Sep 21, 2023 in RustCrypto/AEADs • Updated Feb 16, 2024

Package

cargo aes-gcm (Rust)

Affected versions

>= 0.10.0, <= 0.10.2

Patched versions

0.10.3

Description

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);

References

@tarcieri tarcieri published to RustCrypto/AEADs Sep 21, 2023
Published to the GitHub Advisory Database Sep 22, 2023
Reviewed Sep 22, 2023
Published by the National Vulnerability Database Sep 22, 2023
Last updated Feb 16, 2024

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Local
Attack complexity
High
Privileges required
High
User interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
High
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:H/A:N

EPSS score

0.046%
(17th percentile)

Weaknesses

CVE ID

CVE-2023-42811

GHSA ID

GHSA-423w-p2w9-r7vq

Source code

Credits

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