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CVE-2026-50251— Attacker supplied '0.0.0.0'/'::' glue triggers defensive full-cache flush

Quick assessment

Affected
NLnet Labs Unbound
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

NLnet Labs Unbound是荷兰NLnet Labs组织的域名系统解析服务器。 NLnet Labs Unbound 1.25.1及之前版本存在输入验证错误漏洞，该漏洞源于当启用“unwanted-reply-threshold”时，0.0.0.0/::0的粘合记录可导致Unbound短路，通过发送DNS查询并接收看似不需要的回复，循环触发缓存清除，可能导致拒绝服务。

CVSS 5.3 · Medium EPSS 0.25% · P17

Possible ATT&CK Techniques 1 AI

T1498.002 · Reflection Amplification

Affected Version Matrix 1

VendorProduct Version RangeStatus
NLnet Labs Unbound < 1.25.2 affected
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I. Basic Information for CVE-2026-50251

Vulnerability Information
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Vulnerability Title
Attacker supplied '0.0.0.0'/'::' glue triggers defensive full-cache flush
Source: CVE Program / CVE List V5
Vulnerability Description
In NLnet Labs Unbound up to and including version 1.25.1, when 'unwanted-reply-threshold' is enabled (set to any value greater than zero), glue records of 0.0.0.0/::0 can short-circuit Unbound, on systems that can direct such traffic, by issuing DNS queries and receiving seemingly unwanted replies since the remote IP does not match the original source IP of 0.0.0.0/::0. This behavior keeps on looping for the glue records and pushing the counter to the configured 'unwanted-reply-threshold' that triggers a defensive cache clear. A malicious actor who controls a delegation that returns in-bailiwick glue of 0.0.0.0/::0 can drive the counter to the limit of 'unwanted-reply-threshold' to the threshold and trigger a cache clean of the message and rrset caches; at will, indefinitely, without sending a single spoofed packet. The iterator uses the 0.0.0.0/::0 glue, and a system that can route this (e.g., Linux kernel routes the datagram over loopback), Unbound's own listener answers from 127.0.0.1. Because of the mismatch of 0.0.0.0 and 127.0.0.1, in this example, Unbound accounts the reply as an unwanted (probably spoofed) answer. The counter resets to zero on every cache flush, so the attack loops forever.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
Source: CVE Program / CVE List V5
Vulnerability Type
不完整的黑名单
Source: CVE Program / CVE List V5
Vulnerability Title
NLnet Labs Unbound 输入验证错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
NLnet Labs Unbound是荷兰NLnet Labs组织的域名系统解析服务器。 NLnet Labs Unbound 1.25.1及之前版本存在输入验证错误漏洞，该漏洞源于当启用“unwanted-reply-threshold”时，0.0.0.0/::0的粘合记录可导致Unbound短路，通过发送DNS查询并接收看似不需要的回复，循环触发缓存清除，可能导致拒绝服务。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
NLnet Labs Unbound 0 ~ 1.25.2 -

II. Public POCs for CVE-2026-50251

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III. Intelligence Information for CVE-2026-50251

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Vendor Advisories for CVE-2026-50251 (1)

Same Patch Batch · NLnet Labs · 2026-07-22 · 24 CVEs total

CVE-2026-55973 7.5 HIGH 'dns-error-reporting: yes' leads to stack buffer overflow
CVE-2026-32665 7.5 HIGH Remote DNS-over-QUIC denial of service due to `quic-size` budget bypass
CVE-2026-40691 7.5 HIGH Packet of death for DNSCrypt over TCP
CVE-2026-44690 7.5 HIGH Cross-zone wildcard cache poisoning via RRSIG.labels manipulation
CVE-2026-50248 6.5 MEDIUM BOGUS configured primary hostname accepted for XFR in auth/rpz zones
CVE-2026-52863 5.9 MEDIUM Memory corruption could lead to crash and denial of service
CVE-2026-55717 5.9 MEDIUM 'serve-expired-client-timeout' and 'response-ip' CNAME redirect could lead to a crash
CVE-2026-55991 5.9 MEDIUM Remote DNS-over-QUIC (DoQ) flow-control assertion failure in libngtcp2
CVE-2026-50046 5.9 MEDIUM Possible heap use-after-free in an error path when a DoT forwarded query is jostled out
CVE-2026-55990 5.9 MEDIUM Packet of death for a DNSCrypt misconfigured Unbound
CVE-2026-56444 5.9 MEDIUM Degradation of resolution service when 'discard-timeout' and 'serve-expired-client-timeout
CVE-2026-44621 5.9 MEDIUM Libunbound applications configured with 'unwanted-reply-threshold' could eventually be abr
CVE-2026-14586 5.9 MEDIUM Assertion in libngtcp2 when under pressure in high concurrency DNS-over-QUIC environments
CVE-2026-50045 5.3 MEDIUM 'max-global-quota' reset by DNSSEC validation restarts
CVE-2026-56416 4.8 MEDIUM Possible heap buffer overflow when validator canonicalizes RDATA that contains domain name
CVE-2026-46582 3.7 LOW A wildcard replay, as another piece of data, triggers poisoning in the serve expired reply
CVE-2026-44687 3.7 LOW Off-by-one error in 'harden-below-nxdomain' logic can shadow a stub/forward zone by a legi
CVE-2026-41637 3.7 LOW Degradation of resolution service from improperly accounted client-terminated DNS-over-QUI
CVE-2026-54478 3.7 LOW DNS Cookie bypass when combined with proxy-protocol use
CVE-2026-42955 3.7 LOW Extra fix for CVE-2026-40622 to also clamp the TTL of A/AAAA records disallowing a one-tim

Showing top 20 of 24 CVEs. View all on vendor page &rarr; →

IV. Related Vulnerabilities

Same product: Unbound
Same vendor: NLnet Labs
Same weakness: CWE-184

V. Comments for CVE-2026-50251

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