CVE-2026-27588 Caddy: MatchHost becomes case-sensitive for large host lists (>100), enabling host-based route/auth bypass
Attackers can bypass host-based routing and access controls by manipulating Host header casing when large host lists are configured.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-27588?
CVE-2026-27588 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, Caddy's HTTP `host` request matcher is documented as case-insensitive, but when configured with a large host list (>100 entries) it becomes case-sensitive due to an optimized matching path. An attacker can bypass host-based routing and any access controls attached to that route by changing the casing of the `Host` header. Version 2.11.1 contains a fix for the issue.
Weakness: CWE-178
Which versions are affected by CVE-2026-27588?
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-2026-27588?
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-2026-27588 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-27588 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-2026-27586 Caddy's mTLS client authentication silently fails open when CA certificate file is missing or malformed
- 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`
Sources
Also known as DEBIAN-CVE-2026-27588, GHSA-x76f-jf84-rqj8, GO-2026-4541, MINI-h35g-r2q2-vh37, UBUNTU-CVE-2026-27588.