GHSA-h75p-j8xm-m278

Опубликовано: 06 мар. 2026

Источник: github

Github: Прошло ревью

CVSS3: 7.5

Описание

CoreDNS Loop Detection Denial of Service Vulnerability

Executive Summary

A Denial of Service vulnerability exists in CoreDNS's loop detection plugin that allows an attacker to crash the DNS server by sending specially crafted DNS queries. The vulnerability stems from the use of a predictable pseudo-random number generator (PRNG) for generating a secret query name, combined with a fatal error handler that terminates the entire process.

Technical Details

Vulnerability Description

The CoreDNS loop plugin is designed to detect forwarding loops by performing a self-test during server startup. The plugin generates a random query name (qname) using Go's math/rand package and sends an HINFO query to itself. If the server receives multiple matching queries, it assumes a forwarding loop exists and terminates.

The vulnerability arises from two design flaws:

Predictable PRNG Seed: The random number generator is seeded with time.Now().UnixNano(), making the generated qname predictable if an attacker knows the approximate server start time.
Fatal Error Handler: When the plugin detects what it believes is a loop (3+ matching HINFO queries), it calls log.Fatalf() which invokes os.Exit(1), immediately terminating the process without cleanup or recovery.

Affected Code

File: plugin/loop/setup.go

// PRNG seeded with predictable timestamp var r = rand.New(time.Now().UnixNano()) // Qname generation using two consecutive PRNG calls func qname(zone string) string { l1 := strconv.Itoa(r.Int()) l2 := strconv.Itoa(r.Int()) return dnsutil.Join(l1, l2, zone) }

File: plugin/loop/loop.go

func (l *Loop) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) { // ... validation checks ... if state.Name() == l.qname { l.inc() // Increment counter } if l.seen() > 2 { // FATAL: Terminates entire process log.Fatalf("Loop (%s -> %s) detected for zone %q...", ...) } // ... }

File: plugin/pkg/log/log.go

func Fatalf(format string, v ...any) { logf(fatal, format, v...) os.Exit(1) // Immediate process termination }

Exploitation Window

The loop plugin remains active during the following conditions:

Condition	Window Duration	Attack Feasibility
Healthy startup	2 seconds	Requires precise timing
Self-test failure (upstream unreachable)	30 seconds	HIGH - Extended window
Network degradation	Variable	Depends on retry behavior

Attack Scenario

Primary Attack Vector: Network Degradation

When the upstream DNS server is unreachable (network partition, misconfiguration, outage), the loop plugin's self-test fails repeatedly. During this period:

The loop plugin remains active for up to 30 seconds
Each self-test attempt generates an HINFO query visible in CoreDNS logs
An attacker with log access (shared Kubernetes cluster, centralized logging) can observe the qname
The attacker sends 3 HINFO queries with the observed qname
The server immediately crashes

┌──────────────────────────────────────────────────────────────────────────┐ │ ATTACK TIMELINE │ ├──────────────────────────────────────────────────────────────────────────┤ │ T+0s CoreDNS starts, PRNG seeded with UnixNano() │ │ T+0.5s Self-test HINFO query sent (visible in logs) │ │ T+2s Self-test fails (upstream timeout) │ │ T+3s Retry #1 - counter resets, qname unchanged │ │ T+5s Retry #2 - attacker observes qname in logs │ │ T+5.1s ATTACKER: Send HINFO #1 → counter = 1 │ │ T+5.2s ATTACKER: Send HINFO #2 → counter = 2 │ │ T+5.3s ATTACKER: Send HINFO #3 → counter = 3 → os.Exit(1) │ │ T+5.3s SERVER CRASHES │ └──────────────────────────────────────────────────────────────────────────┘

Impact Assessment

Attack Requirements

Requirement	Notes
Network Access	Must be able to send UDP packets to CoreDNS port
Log Access	Required to observe the qname (common in shared clusters)
Timing	Extended window during network degradation
Authentication	None required

Real-World Impact

CoreDNS is the default DNS server for Kubernetes clusters. A successful attack would:

Disruption: All DNS resolution fails within the cluster
Cascading Failures: Services unable to discover each other
Restart Loop: If attack persists, CoreDNS enters crash-restart cycle
Data Plane Impact: Application-level failures across the cluster

References

CoreDNS GitHub: https://github.com/coredns/coredns
Loop Plugin Documentation: https://coredns.io/plugins/loop/
Go math/rand Documentation: https://pkg.go.dev/math/rand

Ссылки

Пакеты

Наименование

github.com/coredns/coredns

Затронутые версииВерсия исправления

< 1.14.2

1.14.2

EPSS

Процентиль: 16%

0.00052

Низкий

7.5 High

CVSS3

CVE ID

CVE-2026-26018

Дефекты

CWE-337

Связанные уязвимости

CVE-2026-26018

CVSS3: 7.5

redhat

21 день назад

CoreDNS is a DNS server that chains plugins. Prior to version 1.14.2, a denial of service vulnerability exists in CoreDNS's loop detection plugin that allows an attacker to crash the DNS server by sending specially crafted DNS queries. The vulnerability stems from the use of a predictable pseudo-random number generator (PRNG) for generating a secret query name, combined with a fatal error handler that terminates the entire process. This issue has been patched in version 1.14.2.

CVE-2026-26018

CVSS3: 7.5

nvd

20 дней назад

CVE-2026-26018

CVSS3: 7.5

msrc

18 дней назад

CoreDNS Loop Detection Denial of Service Vulnerability

CVE-2026-26018

CVSS3: 7.5

debian

20 дней назад

CoreDNS is a DNS server that chains plugins. Prior to version 1.14.2, ...

EPSS

Процентиль: 16%

0.00052

Низкий

7.5 High

CVSS3

CVE ID

CVE-2026-26018

Дефекты

CWE-337