CVE-2026-71303
Received Received - Intake

Lemur ACME URL Validation Bypass in Authority Update

Publication date: 2026-08-18

Last updated on: 2026-08-18

Assigner: GitHub, Inc.

Description
Lemur manages TLS certificate creation. Prior to 1.9.3, _validate_acme_url enforced ACME_DIRECTORY_HOST_ALLOWLIST when an authority was created, but PUT /api/1/authorities/ passed options to lemur/authorities/service.py without applying the same check. A user holding an authority role could replace the stored acme_url with an internal service or instance-metadata URL such as 169.254.169.254. The next issuance operation loaded that value and passed it to ClientV2.get_directory, causing an outbound request from the Lemur backend. This bypassed the creation-time mitigation for CVE-2026-55166 and could expose internal services or cloud metadata. The fix revalidates acme_url whenever authority options are updated. This issue is fixed in version 1.9.3.

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Meta Information

Published
2026-08-18
Last Modified
2026-08-18
Generated
2026-08-19
AI Q&A
2026-08-18
EPSS Evaluated
N/A
NVD
CVE-2026-71303
EUVD
EUVD-2026-61154

Affected Vendors & Products

Showing 3 associated CPEs
Vendor Product Version / Range
lemur lemur 1.9.3
netflix lemur 1.9.3
netflix lemur to 1.9.3 (exc)

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-918 The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.

Attack-Flow Graph

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Executive Summary

CVE-2026-71303 is a Server-Side Request Forgery (SSRF) vulnerability in the Lemur certificate management tool. It stems from an incomplete fix for a previous vulnerability where acme_url validation was only applied during authority creation but not during updates. Authenticated users with authority roles could overwrite the acme_url with internal or cloud metadata URLs, bypassing allowlist checks. When certificates are issued, Lemur fetches the attacker-controlled URL, enabling SSRF attacks.

Detection Guidance

Check Lemur logs for outbound requests to unexpected internal or cloud metadata URLs (e.g., 169.254.169.254). Inspect authority configurations for unauthorized acme_url changes via API logs or database queries. Monitor network traffic from Lemur servers to detect SSRF attempts.

Impact Analysis

This vulnerability allows attackers with authority roles to access internal services or cloud metadata endpoints like AWS IMDS, GCP metadata, or Azure IMDS. This could expose sensitive data such as IAM credentials or enable probing of internal systems. The attack requires low privileges and no user interaction, making it relatively easy to exploit.

Compliance Impact

This vulnerability could lead to unauthorized access to sensitive data, potentially violating GDPR's data protection requirements or HIPAA's security rules. Exposure of internal services or cloud metadata may result in data breaches, non-compliance with regulatory standards, and legal consequences.

Mitigation Strategies

Upgrade Lemur to version 1.9.3 or later to apply the fix. Review and restrict authority role memberships to trusted users. Audit existing authorities for unauthorized acme_url modifications and revert any suspicious changes.

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