Vulnerability · Published Feb 24, 2026 · Assigned by GitHub_M
CVE-2026-27586Caddy's mTLS client authentication silently fails open when CA certificate file is missing or malformed
High 8.8
In plain English
A high-severity security flaw in caddy from caddyserver. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 22% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Alpine:v3.23 | caddy | 2.11.2-r0 | ALPINE-CVE-2026-27586 |
| Alpine:v3.24 | caddy | 2.11.1-r0 | ALPINE-CVE-2026-27586 |
| Debian:14 | caddy | 2.11.2-1 | DEBIAN-CVE-2026-27586 |
| Go | github.com/caddyserver/caddy/v2 | 2.11.1 | GO-2026-4539 |
| Go | github.com/caddyserver/caddy/v2 | 2.11.1 | GHSA-hffm-g8v7-wrv7 |
| Ubuntu:25.10 | caddy | no fix listed | UBUNTU-CVE-2026-27586 |
| Ubuntu:26.10 | caddy | no fix listed | UBUNTU-CVE-2026-27586 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| caddyserver | caddy | < 2.11.1 |
Weakness: CWE-755 · Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:P
References
- https://github.com/caddyserver/caddy/security/advisories/GHSA-hffm-g8v7-wrv7 x_refsource_CONFIRM
- https://gist.github.com/moscowchill/9566c79c76c0b64c57f8bd0716f97c48 x_refsource_MISC
- https://github.com/caddyserver/caddy/releases/tag/v2.11.1 x_refsource_MISC