The HTTP/2 protocol allows a denial of service (server resource consumption) because request cancellation can reset many streams quickly, as exploited in the wild in August through October 2023.

The caddy-geo-ip (aka GeoIP) middleware through 0.6.0 for Caddy 2, when trust_header X-Forwarded-For is used, allows attackers to spoof their source IP address via an X-Forwarded-For header, which may bypass a protection mechanism (trusted_proxy directive in reverse_proxy or IP address range restrictions).

Caddy v2.4.6 was discovered to contain an open redirection vulnerability which allows attackers to redirect users to phishing websites via crafted URLs.

An out-of-bounds read in the rewrite function at /modules/caddyhttp/rewrite/rewrite.go in Caddy v2.5.1 allows attackers to cause a Denial of Service (DoS) via a crafted URI. Note: This has been disputed as a bug, not a security vulnerability, in the Caddy web server that emerged when an administrator's bad configuration containing a malformed request URI caused the server to return an empty reply instead of a valid HTTP response to the client.

Caddy v2.4 was discovered to contain an open redirect vulnerability. A remote unauthenticated attacker may exploit this vulnerability to redirect users to arbitrary web URLs by tricking the victim users to click on crafted links.

Caddy before 0.10.13 mishandles TLS client authentication, as demonstrated by an authentication bypass caused by the lack of the StrictHostMatching mode.

Caddy through 0.11.0 sends incorrect certificates for certain invalid requests, making it easier for attackers to enumerate hostnames. Specifically, when unable to match a Host header with a vhost in its configuration, it serves the X.509 certificate for a randomly selected vhost in its configuration. Repeated requests (with a nonexistent hostname in the Host header) permit full enumeration of all certificates on the server. This generally permits an attacker to easily and accurately discover the existence of and relationships among hostnames that weren't meant to be public, though this information could likely have been discovered via other methods with additional effort.