Vulnerability in Caddyserver Caddy
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 9.1 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N.
EPSS exploit prediction
EPSS: 0.004 (32.4th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 32nd percentile — 32.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.004 · change: +0.001
Affected products
- Caddyserver Caddy — versions < 2.11.1
Weakness classification (CWE)
References
- github.com/caddyserver/caddy/security/advisories/GHSA-hffm-g8v7-wrv7 (Exploit, Vendor Advisory)
- gist.github.com/moscowchill/9566c79c76c0b64c57f8bd0716f97c48 (Exploit)
- github.com/caddyserver/caddy/releases/tag/v2.11.1 (Release Notes)
Frequently asked questions
- What is CVE-2026-27586?
- CVE-2026-27586 is a critical-severity vulnerability in Caddyserver Caddy, classified under Improper Handling of Exceptional Conditions. CVSS score: 9.1/10. Published 2026-02-24.
- How severe is CVE-2026-27586?
- Critical severity. CVSS v3 base score is 9.1 out of 10.