Do not use Python's == operator to compare HMAC digests. The == operator is
not designed to be used for cryptographic comparisons, and it can be
vulnerable to timing attacks. Instead, use the hmac.compare_digest() function
to compare HMAC digests.

The == operator works by comparing the length and contents of two objects.
However, this can be a problem for HMAC digests, because the length of an
HMAC digest is not necessarily unique. For example, two different messages
with the same key will have the same HMAC digest.

A timing attack is a type of attack that exploits the time it takes to
execute a piece of code. In the case of HMAC digests, a timing attack could
be used to determine whether two messages have the same HMAC digest. This
could be used to break the security of an HMAC-protected system.

The hmac.compare_digest() function is designed to be used for cryptographic
comparisons. It works by comparing the binary representations of two HMAC
digests. This makes it more resistant to timing attacks.

Example

import hmac


received_digest = (
    b"\xe2\x93\x08\x19T8\xdc\x80\xef\x87\x90m\x1f\x9d\xf7\xf2"
    "\xf5\x10>\xdbf\xa2\xaf\xf7x\xcdX\xdf"
)

key = b"my-super-duper-secret-key-string"
password = b"pass"
digest = hmac.digest(key, password, digest="sha224")

print(digest == received_digest)

??? example "Example Output"
> precli tests/unit/rules/python/stdlib/hmac/examples/hmac_timing_attack.py ⛔️ Error on line 13 in tests/unit/rules/python/stdlib/hmac/examples/hmac_timing_attack.py PY005: Observable Timing Discrepancy Comparing digests with the '==' operator is vulnerable to timing attacks.

Remediation

The recommendation is to replace the == operator with the function
compare_digest.

import hmac


received_digest = (
    b"\xe2\x93\x08\x19T8\xdc\x80\xef\x87\x90m\x1f\x9d\xf7\xf2"
    "\xf5\x10>\xdbf\xa2\xaf\xf7x\xcdX\xdf"
)

key = b"my-secret-key"
password = b"pass"
digest = hmac.digest(key, password, digest="sha224")

print(hmac.compare_digest(digest, received_digest))

Default Configuration

enabled = true
level = "error"

See also

!!! info
- hmac — Keyed-Hashing for Message Authentication
- CWE-208: Observable Timing Discrepancy

New in version 0.1.4