CVE-2026-6420
MEDIUMCVE-2026-6420 is a medium-severity (CVSS 6.3) vulnerability in keylime. O3 Security confirms whether CVE-2026-6420 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Keylime has a hardcoded attestation challenge nonce that allows replay attacks
Real-World Exposure
keylimeReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
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):
# 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):
- 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)
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | keylime | ≥ 7.14.0&&< 7.14.2 | 7.14.2 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for keylime. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update keylime to 7.14.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-6420 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-6420 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-6420. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-6420 in your dependencies?
O3 detects CVE-2026-6420 across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.