CVE-2026-32936

CoreDNS is a DNS server that chains plugins. In versions prior to 1.14.3, the DNS-over-HTTPS (DoH) GET path accepts oversized dns= query parameter values and performs URL query parsing, base64 decoding, and DNS message unpacking before rejecting the request. Unlike the POST path, which applies a bounded read via http.MaxBytesReader limited to 65536 bytes, the GET path has no equivalent size validation before expensive processing. A remote, unauthenticated attacker can repeatedly send oversized DoH GET requests to force high CPU usage, large transient memory allocations, and elevated garbage-collection pressure, leading to denial of service. This issue has been fixed in version 1.14.3.

CoreDNS is a DNS server that chains plugins. In versions prior to 1.14.3, the DNS-over-HTTPS (DoH) GET path accepts oversized dns= query parameter values and performs URL query parsing, base64 decoding, and DNS message unpacking before rejecting the request. Unlike the POST path, which applies a bounded read via http.MaxBytesReader limited to 65536 bytes, the GET path has no equivalent size validation before expensive processing. A remote, unauthenticated attacker can repeatedly send oversized DoH GET requests to force high CPU usage, large transient memory allocations, and elevated garbage-collection pressure, leading to denial of service. This issue has been fixed in version 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:

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:

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:

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:

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:

https://127.0.0.1:8443 {
    whoami
    log
    errors
    tls cert.pem key.pem
    pprof 127.0.0.1:6060
}

Run CoreDNS:

./coredns -conf Corefile

Proof-of-concept script

#!/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 =…

CoreDNS is a DNS server that chains plugins. In versions prior to 1.14.3, the DNS-over-HTTPS (DoH) GET path accepts oversized dns= query parameter values and performs URL query parsing, base64 decoding, and DNS message unpacking before rejecting the request. Unlike the POST path, which applies a bounded read via http.MaxBytesReader limited to 65536 bytes, the GET path has no equivalent size validation before expensive processing. A remote, unauthenticated attacker can repeatedly send oversized DoH GET requests to force high CPU usage, large transient memory allocations, and elevated garbage-collection pressure, leading to denial of service. This issue has been fixed in version 1.14.3.

CoreDNS is a DNS server that chains plugins. In versions prior to 1.14.3, the DNS-over-HTTPS (DoH) GET path accepts oversized dns= query parameter values and performs URL query parsing, base64 decoding, and DNS message unpacking before rejecting the request. Unlike the POST path, which applies a bounded read via http.MaxBytesReader limited to 65536 bytes, the GET path has no equivalent size validation before expensive processing. A remote, unauthenticated attacker can repeatedly send oversized DoH GET requests to force high CPU usage, large transient memory allocations, and elevated garbage-collection pressure, leading to denial of service. This issue has been fixed in version 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:

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:

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:

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:

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:

https://127.0.0.1:8443 {
    whoami
    log
    errors
    tls cert.pem key.pem
    pprof 127.0.0.1:6060
}

Run CoreDNS:

./coredns -conf Corefile

Proof-of-concept script

#!/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 =…