Netflix Lemur up to 1.9.1 Acme Client AcmeHandler.setup_acme_client acme_url server-side request forgery

Summaryinfo

A vulnerability classified as critical has been found in Netflix Lemur up to 1.9.1. Affected is the function AcmeHandler.setup_acme_client of the component Acme Client. Performing a manipulation of the argument acme_url results in server-side request forgery. This vulnerability is cataloged as CVE-2026-55166. It is possible to initiate the attack remotely. There is no exploit available. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability was found in Netflix Lemur up to 1.9.1. It has been classified as critical. This affects the function AcmeHandler.setup_acme_client of the component Acme Client. The manipulation of the argument acme_url with an unknown input leads to a server-side request forgery vulnerability. CWE is classifying the issue as 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. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

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.

The advisory is shared at github.com. This vulnerability is uniquely identified as CVE-2026-55166 since 06/16/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. Technical details are known, but no exploit is available.

Upgrading to version 1.9.2 eliminates this vulnerability.

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Productinfo

Vendor

• Netflix

Name

• Lemur

Version

• 1.9.0
• 1.9.1

Website

• Product: https://github.com/Netflix/lemur/

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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