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Keylime has a hardcoded attestation challenge nonce that allows replay attacks

Moderate severity GitHub Reviewed Published May 6, 2026 in keylime/keylime • Updated May 11, 2026

Package

pip keylime (pip)

Affected versions

>= 7.14.0, <= 7.14.1

Patched versions

7.14.2

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):

  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)

References

@ansasaki ansasaki published to keylime/keylime May 6, 2026
Published to the GitHub Advisory Database May 11, 2026
Reviewed May 11, 2026
Last updated May 11, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Local
Attack complexity
Low
Privileges required
High
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
Low

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:L

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(2nd percentile)

Weaknesses

Authentication Bypass by Capture-replay

A capture-replay flaw exists when the design of the product makes it possible for a malicious user to sniff network traffic and bypass authentication by replaying it to the server in question to the same effect as the original message (or with minor changes). Learn more on MITRE.

Generation of Predictable IV with CBC Mode

The product generates and uses a predictable initialization Vector (IV) with Cipher Block Chaining (CBC) Mode, which causes algorithms to be susceptible to dictionary attacks when they are encrypted under the same key. Learn more on MITRE.

Use of Hard-coded, Security-relevant Constants

The product uses hard-coded constants instead of symbolic names for security-critical values, which increases the likelihood of mistakes during code maintenance or security policy change. Learn more on MITRE.

Use of Predictable Algorithm in Random Number Generator

The device uses an algorithm that is predictable and generates a pseudo-random number. Learn more on MITRE.

CVE ID

CVE-2026-6420

GHSA ID

GHSA-q8w6-w55c-ccv5

Source code

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