Keylime has a hardcoded attestation challenge nonce that allows replay attacks

CVE-2026-6420: Hardcoded attestation challenge nonce allows replay attacks

Impact

The CertificationParameters.generate_challenge() method in the push attestation protocol uses a hardcoded challenge nonce instead of generating a cryptographically random value. This removes the nonce-based replay protection from TPM quote attestation.

An attacker with root access on a monitored agent node can exploit this by stockpiling valid TPM quotes (using tpm2_quote with the known nonce) before compromising the system, then replaying them to evade detection by the verifier. The push attestation timeout (~10s) constrains the generation window, but TPM throughput allows stockpiling ~50-200 quotes, enabling approximately 8-33 minutes of undetected compromise with default settings.

The attack is limited to a single agent node (AK signature binding prevents cross-agent replay). The pull-mode (legacy) attestation path is not affected.

Affected versions: >= 7.14.0, <= 7.14.1

CVSS: 6.3 Medium (CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:L)

The base score does not fully capture the operational severity: Keylime exists to detect machine compromise, so 8-33 minutes of undetected compromise is operationally critical. The fix is a one-line change and should be applied immediately regardless of the base score.

Patches

The fix restores the original random nonce generation (one-line change in keylime/models/verifier/evidence.py):

# Before (vulnerable):
def generate_challenge(self, bit_length):
    # self.challenge = Nonce.generate(bit_length)
    self.challenge = bytes.fromhex("49beed365aac777dae23564f5ad0ec")

# After (fixed):
def generate_challenge(self, bit_length):
    self.challenge = Nonce.generate(bit_length)

Users should upgrade to the version containing this fix (7.14.2).

Workarounds

There is no complete workaround. The following existing mechanisms provide partial mitigation and are already active by default (no configuration needed):

1. TPM clock monotonicity check limits each distinct stockpiled quote to a single use, bounding the total evasion time.
2. Push attestation timeout (default 10s) prevents the attacker from going silent and constrains the quote generation window.

Reducing quote_interval increases the attestation frequency but does not prevent the stockpiling attack.

References

• CWE-329: Generation of Predictable IV/Nonce (primary -- hardcoded nonce in cryptographic attestation protocol)
• CWE-547: Use of Hard-Coded, Security-relevant Constants (hardcoded constant left in production code)
• CWE-294: Authentication Bypass by Capture-replay (consequence -- enables replay attacks)
• CWE-1241: Use of Predictable Algorithm in Random Number Generator
• Introducing commit: 2bf91197 via PR #1814
• TCG TPM 2.0 Library Specification, Part 1, Section 18.4 (TPM2_Quote)
• IETF RATS Architecture (RFC 9334), Section 8 (Freshness)