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CVE-2026-42923— Degradation of service with unbounded NSEC3 hash calculations

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NLnet Labs开源的一个高性能DNS解析器。 NLnet Labs Unbound 1.25.0及之前版本存在安全漏洞，该漏洞源于DNSSEC验证器中查询DS记录负缓存时未考虑NSEC3哈希计算限制，可能导致服务降级，攻击者通过签名高迭代次数的NSEC3记录并查询可消耗哈希计算资源，同时持有全局锁阻塞其他线程。

AI Predicted 7.5 Difficulty: Moderate EPSS 0.34% · P27

Possible ATT&CK Techniques 1 AI

T1498 · Network Denial of Service

Affected Version Matrix 1

VendorProduct Version RangeStatus
NLnet Labs Unbound < 1.25.1 affected
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I. Basic Information for CVE-2026-42923

Vulnerability Information
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Vulnerability Title
Degradation of service with unbounded NSEC3 hash calculations
Source: CVE Program / CVE List V5
Vulnerability Description
NLnet Labs Unbound up to and including version 1.25.0 has a vulnerability in the DNSSEC validator where the code path to consult the negative cache for DS records does not take into account the limit on NSEC3 hash calculations introduced in 1.19.1. This leads to degradation of service during the attack. An adversary that controls a DNSSEC signed zone can exploit this by signing NSEC3 records with acceptably high iterations for child delegations and querying a vulnerable Unbound. Unbound will keep performing the allowed hash calculations on the NSEC3 records and will not limit the work by the mitigation introduced in 1.19.1. As a side effect, a global lock for the negative cache will be held for the duration of the hashing, blocking other threads that need to consult the negative cache. Coordinated attacks could raise the vulnerability to denial of service. Unbound 1.25.1 contains a patch with a fix to bound the vulnerable code path with the existing limit for NSEC3 hash calculations.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/U:Amber
Source: CVE Program / CVE List V5
Vulnerability Type
算法复杂性
Source: CVE Program / CVE List V5
Vulnerability Title
NLnet Labs Unbound 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
NLnet Labs Unbound是NLnet Labs开源的一个高性能DNS解析器。 NLnet Labs Unbound 1.25.0及之前版本存在安全漏洞，该漏洞源于DNSSEC验证器中查询DS记录负缓存时未考虑NSEC3哈希计算限制，可能导致服务降级，攻击者通过签名高迭代次数的NSEC3记录并查询可消耗哈希计算资源，同时持有全局锁阻塞其他线程。
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 ~ 1.25.1 -

II. Public POCs for CVE-2026-42923

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III. Intelligence Information for CVE-2026-42923

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Vendor Advisories for CVE-2026-42923 (1)

Same Patch Batch · NLnet Labs · 2026-05-20 · 11 CVEs total

CVE-2026-33278 9.1 CRITICAL Possible arbitrary code execution during DNSSEC validation
CVE-2026-42534 6.9 MEDIUM Jostle logic bypass degrades resolution performance
CVE-2026-44390 6.9 MEDIUM Unbounded name compression in certain cases causes degradation of service
CVE-2026-41292 6.6 MEDIUM Long list of incoming EDNS options degrades performance
CVE-2026-42959 Crash during DNSSEC validation of malicious content
CVE-2026-42944 Heap overflow with multiple NSID, COOKIE, PADDING EDNS options
CVE-2026-42960 Possible cache poisoning via promiscuous records for the authority section
CVE-2026-32792 Packet of death with DNSCrypt
CVE-2026-40622 Another 'ghost domain names' attack variant
CVE-2026-44608 Use after free and crash under special conditions in RPZ code

IV. Related Vulnerabilities

Same product: Unbound
Same vendor: NLnet Labs
Same weakness: CWE-407

V. Comments for CVE-2026-42923

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