CVE-2026-27586 Caddy's mTLS client authentication silently fails open when CA certificate file is missing or malformed
Caddy silently bypasses mTLS client authentication if the CA certificate file is missing or malformed, allowing unauthorized access.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-27586?
CVE-2026-27586 is a high-severity authentication bypass vulnerability in caddyserver caddy, published on 24 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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.
Weakness: CWE-755
Which versions are affected by CVE-2026-27586?
3 open source packages ship an affected version, including github.com/caddyserver/caddy/v2. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
github.com/caddyserver/caddy/v2 Go | All versions | 2.11.1 |
github.com/caddyserver/caddy/v2 Go | All versions | 2.11.1 |
github.com/caddyserver/caddy/v2 Go | All versions | 2.11.1 |
Products include Caddyserver Caddy.
How do I fix CVE-2026-27586?
Upgrade github.com/caddyserver/caddy/v2 to 2.11.1 or later.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-27586 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-27586 exists.
Other vulnerabilities in caddyserver caddy
- CVE-2018-21246 Caddy vulnerable to Authentication Bypass due to mishandling of TLS client authentication
- CVE-2026-27590 Caddy: Unicode case-folding length expansion causes incorrect split_path index in FastCGI transport
- CVE-2026-45135 Caddy: Unsafe Unicode Handling in FastCGI splitPos Allows Execution of Non-PHP Files
- CVE-2026-52845 Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`
- CVE-2026-30851 Caddy forward_auth copy_headers Does Not Strip Client-Supplied Headers, Allowing Identity Injection and Privilege Escalation
Sources
Also known as DEBIAN-CVE-2026-27586, GHSA-hffm-g8v7-wrv7, GO-2026-4539, MINI-3rg5-95gh-5rjg, UBUNTU-CVE-2026-27586.