Description

Lemur manages TLS certificate creation. Prior to 1.9.2, authenticated users could influence an ACME authority acme_url without an effective server-side destination restriction and trigger AcmeHandler.setup_acme_client to make backend requests. An attacker could target cloud instance metadata or internal services from Lemur network context, potentially obtaining credentials available to the host. The advisory also identifies creator-equality authorization behavior that could preserve access to certificate key material after ownership or role changes, with insufficient export_private_key audit context to distinguish that access path. Together, the acme_url server-side request forgery and authorization weakness could expose cloud credentials and long-lived PKI private-key access. The fix adds ACME_DIRECTORY_HOST_ALLOWLIST validation and enriches key-export audit events with creator and current-owner context. This issue is fixed in version 1.9.2.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "9.9",
  "pubDate": "2026-08-18T19:16:58.783Z",
  "pubdate": "2026-08-18T19:16:58.783Z",
  "executiveSummary": "A security vulnerability affecting Lemur prior to version 1.9.2 combines a Server-Side Request Forgery (SSRF) flaw in ACME authority handling with an authorization weakness in certificate key material access. The vulnerability allows authenticated users to supply arbitrary acme_url values without server-side destination restrictions, triggering backend requests via AcmeHandler.setup_acme_client. This enables attackers to target internal services and cloud instance metadata endpoints from the Lemur network context to harvest sensitive host credentials.\nAdditionally, creator-equality authorization logic may inadvertently preserve access to private key material even after ownership or role changes, compounded by insufficient export_private_key audit logging context. Exploitation requires authenticated user access to manipulate ACME authority parameters. The risk implications include potential compromise of cloud infrastructure credentials and unauthorized exposure of long-lived PKI private keys. The identified issues are fully addressed in version 1.9.2 through strict destination allowlisting and enhanced audit context.",
  "technicalDetails": "The vulnerability stems from inadequate input validation and missing egress filtering within Lemur prior to version 1.9.2. Specifically, authenticated users can manipulate the acme_url parameter associated with an ACME authority without effective server-side destination restrictions. When processed, this unvalidated URL is passed to the vulnerable component AcmeHandler.setup_acme_client, which initiates outbound backend requests from the host network context.\nThe attack flow proceeds as follows: An authenticated attacker submits a crafted ACME authority configuration containing a malicious or internal acme_url, such as a link-local cloud instance metadata service endpoint (e.g., AWS, GCP, or Azure metadata IPs) or other sensitive internal microservices. Upon triggering the ACME handler setup, Lemur executes an outbound HTTP request to the attacker-specified destination. The application processes the response or exposes internal service data to the attacker, potentially yielding sensitive cloud provider credentials, API tokens, or internal network intelligence available to the underlying host.\nFurthermore, the application suffers from an authorization flaw characterized by creator-equality behavior. This logic permits original creators to retain access to certificate key material despite subsequent ownership transfers or role modifications. Compounding this authorization weakness, the export_private_key function lacks sufficient audit context to distinguish or trace this specific access path.\nPrerequisites for exploitation include authenticated access to the Lemur platform and the ability to configure or influence ACME authority parameters. The post-exploitation impact includes unauthorized acquisition of cloud infrastructure credentials, lateral movement within the internal network context, and potential exfiltration of long-lived PKI private-key material without adequate traceability in audit logs."
}