Caddy is an extensible server platform that uses TLS by default. From version 2.7.5 to before version 2.11.2, the vars_regexp matcher in vars.go:337 double-expands user-controlled input through the Caddy replacer. When vars_regexp matches against a placeholder like {http.request.header.X-Input}, the header value gets resolved once (expected), then passed through repl.ReplaceAll() again (the bug). This means an attacker can put {env.DATABASE_URL} or {file./etc/passwd} in a request header and the server will evaluate it, leaking environment variables, file contents, and system info. This issue has been patched in version 2.11.2.
Exploitability
AV:NAC:LAT:NPR:NUI:NVulnerable System
VC:LVI:NVA:NSubsequent System
SC:NSI:NSA:N5.5/CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N/E:PInformation Disclosure
Input Validation