Summary
CVE-2026-6420: Hardcoded attestation challenge nonce allows replay attacks
Workarounds
There is no complete workaround. The following existing mechanisms provide partial mitigation and are already active by default (no configuration needed):
- TPM clock monotonicity check limits each distinct stockpiled quote to a single use, bounding the total evasion time.
- 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:
2bf91197via PR #1814 - TCG TPM 2.0 Library Specification, Part 1, Section 18.4 (TPM2_Quote)
- IETF RATS Architecture (RFC 9334), Section 8 (Freshness)
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.
CVE-2026-6420 has a CVSS score of 6.3 (Medium). The vector is requires local access, high privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (7.14.2); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
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).
Frequently Asked Questions
- What is CVE-2026-6420? CVE-2026-6420 is a medium-severity security vulnerability in keylime (pip), affecting versions >= 7.14.0, <= 7.14.1. It is fixed in 7.14.2.
- How severe is CVE-2026-6420? CVE-2026-6420 has a CVSS score of 6.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of keylime are affected by CVE-2026-6420? keylime (pip) versions >= 7.14.0, <= 7.14.1 is affected.
- Is there a fix for CVE-2026-6420? Yes. CVE-2026-6420 is fixed in 7.14.2. Upgrade to this version or later.
- Is CVE-2026-6420 exploitable, and should I be worried? Whether CVE-2026-6420 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-6420 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-6420? Upgrade
keylimeto 7.14.2 or later.