How severe is CVE-2026-55166?

CVE-2026-55166 has a CVSS 3.x base score of 9.9, rated critical severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability low.

Is CVE-2026-55166 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (12th percentile), an estimate of the probability of exploitation in the next 30 days.

How do I fix CVE-2026-55166?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

When was CVE-2026-55166 published?

CVE-2026-55166 was published on 2026-08-18.

CVE-2026-55166

CVE-2026-55166 is a critical-severity vulnerability with a CVSS 3.x base score of 9.9. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-285.

Key facts

Severity: Critical (CVSS 3.x base score 9.9)
EPSS exploit prediction: 0% (12th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-285
Published: 2026-08-18
Last modified: 2026-08-18

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.

Frequently asked questions

What is CVE-2026-55166?: 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.
How severe is CVE-2026-55166?: CVE-2026-55166 has a CVSS 3.x base score of 9.9, rated critical severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability low.
Is CVE-2026-55166 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (12th percentile), an estimate of the probability of exploitation in the next 30 days.
How do I fix CVE-2026-55166?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
When was CVE-2026-55166 published?: CVE-2026-55166 was published on 2026-08-18.

References

Other CWE-285 (Improper Authorization) vulnerabilities

CVE-2025-65041 — Critical (CVSS 10.0): Improper authorization in Microsoft Partner Center allows an unauthorized attacker to elevate privileges over a network.
CVE-2023-33189 — Critical (CVSS 10.0): Pomerium is an identity and context-aware access proxy. With specially crafted requests, incorrect authorization…
CVE-2022-2595 — Critical (CVSS 10.0): Improper Authorization in GitHub repository kromitgmbh/titra prior to 0.79.1.
CVE-2021-28799 — Critical (CVSS 10.0): An improper authorization vulnerability has been reported to affect QNAP NAS running HBS 3 (Hybrid Backup Sync. ) If…
CVE-2026-34048 — Critical (CVSS 9.9): Coolify is an open-source and self-hostable tool for managing servers, applications, and databases. Prior to…
CVE-2026-5412 — Critical (CVSS 9.9): In Juju versions prior to 2.9.57 and 3.6.21, an authorization issue exists in the Controller facade. An authenticated…

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