How severe is CVE-2026-27586?

CVE-2026-27586 has a CVSS 3.x base score of 9.1, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.

Is CVE-2026-27586 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (18th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-27586?

CVE-2026-27586 affects Caddyserver Caddy. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-27586?

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.

Does CVE-2026-27586 have an EU (EUVD) identifier?

Yes. CVE-2026-27586 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-8541.

When was CVE-2026-27586 published?

CVE-2026-27586 was published on 2026-02-24 and last updated on 2026-06-17.

CVE-2026-27586

CVE-2026-27586 is a critical-severity vulnerability in Caddyserver Caddy with a CVSS 3.x base score of 9.1. 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-755.

Key facts

Severity: Critical (CVSS 3.x base score 9.1)
CVSS v4: 8.8
EPSS exploit prediction: 0% (18th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-8541
Weakness: CWE-755
Affected product: Caddyserver Caddy
Published: 2026-02-24
Last modified: 2026-06-17

Description

Caddy is an extensible server platform that uses TLS by default. Prior to version 2.11.1, two swallowed errors in `ClientAuthentication.provision()` cause mTLS client certificate authentication to silently fail open when a CA certificate file is missing, unreadable, or malformed. The server starts without error but accepts any client certificate signed by any system-trusted CA, completely bypassing the intended private CA trust boundary. Any deployment using `trusted_ca_cert_file` or `trusted_ca_certs_pem_files` for mTLS will silently degrade to accepting any system-trusted client certificate if the CA file becomes unavailable. This can happen due to a typo in the path, file rotation, corruption, or permission changes. The server gives no indication that mTLS is misconfigured. Version 2.11.1 fixes the vulnerability.

Frequently asked questions

What is CVE-2026-27586?: Caddy is an extensible server platform that uses TLS by default. Prior to version 2.11.1, two swallowed errors in `ClientAuthentication.provision()` cause mTLS client certificate authentication to silently fail open when a CA certificate file is missing, unreadable, or malformed. The server starts without error but accepts any client certificate signed by any system-trusted CA, completely bypassing the intended private CA trust boundary. Any deployment using `trusted_ca_cert_file` or `trusted_ca_certs_pem_files` for mTLS will silently degrade to accepting any system-trusted client certificate if the CA file becomes unavailable. This can happen due to a typo in the path, file rotation, corruption, or permission changes. The server gives no indication that mTLS is misconfigured. Version 2.11.1 fixes the vulnerability.
How severe is CVE-2026-27586?: CVE-2026-27586 has a CVSS 3.x base score of 9.1, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.
Is CVE-2026-27586 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (18th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-27586?: CVE-2026-27586 affects Caddyserver Caddy. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-27586?: 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.
Does CVE-2026-27586 have an EU (EUVD) identifier?: Yes. CVE-2026-27586 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-8541.
When was CVE-2026-27586 published?: CVE-2026-27586 was published on 2026-02-24 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:caddyserver:caddy:*:*:*:*:*:*:*:*

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