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)
| Metric | Value | Rationale |
|---|---|---|
| AV | Local | Exploitation requires local access to the agent machine (stop agent, access TPM, run replacement). The network transmission of quotes to the verifier is normal protocol operation. |
| AC | Low | Deterministic attack: publicly visible nonce, standard tpm2-tools, no race conditions. |
| PR | High | Root on a legitimate enrolled node is required. The vulnerability does not help gain access -- it only helps evade detection after root is obtained. No value against a machine the attacker already controls. |
| UI | None | Fully automated after initial setup. |
| S | Unchanged | AK signature binding confines impact to the single compromised agent. |
| C | High | Compromised node continues receiving bootstrap keys, payloads, and secrets intended for trusted nodes. |
| I | High | Verifier cannot distinguish a healthy system from a fully compromised one during the evasion window. |
| A | Low | Only the compromised agent's revocation and incident response are suppressed; the system as a whole remains operational. |
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):
```python
# 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](https://github.com/keylime/keylime/commit/2bf91197) via [PR #1814](https://github.com/keylime/keylime/pull/1814) - TCG TPM 2.0 Library Specification, Part 1, Section 18.4 (TPM2_Quote)
- IETF RATS Architecture (RFC 9334), Section 8 (Freshness)
