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NLnet Labs Unbound CVE-2026-41292

| EUVDEUVD-2026-31078 MEDIUM
Inefficient Algorithmic Complexity (CWE-407)
2026-05-20 sep@nlnetlabs.nl GHSA-2wvj-gvc7-gfhx
Denial Of Service Unbound
6.6
CVSS 4.0 · Vendor: nlnetlabs
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Severity by source

Vendor (nlnetlabs) PRIMARY
6.6 MEDIUM
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:U/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Red
SUSE
7.5 HIGH
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Red Hat
7.5 HIGH
qualitative

Primary rating from Vendor (nlnetlabs).

CVSS VectorVendor: nlnetlabs

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:U/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Red
Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
X

Lifecycle Timeline

2
Patch available
May 20, 2026 - 11:02 EUVD
Analysis Generated
May 20, 2026 - 10:33 vuln.today

DescriptionCVE.org

NLnet Labs Unbound up to and including version 1.25.0 is vulnerable to a degradation of service attack related to parsing long lists of incoming EDNS options. An adversary sending queries with too many EDNS options can hold Unbound threads hostage while they are parsing and creating internal data structures for the options. Coordinated attacks can result in degradation and/or denial of service. Unbound 1.25.1 contains a patch with a fix to limit acceptable incoming EDNS options (100).

AnalysisAI

Unbound DNS resolver versions up to and including 1.25.0 allow remote unauthenticated attackers to degrade or deny service by sending DNS queries carrying abnormally large numbers of EDNS options, causing resolver threads to become occupied with unbounded parsing and internal data structure allocation. Coordinated multi-source attacks amplify thread exhaustion into full denial of service for legitimate DNS clients. No public exploit identified at time of analysis; vendor-released patch is available in Unbound 1.25.1, which enforces a hard cap of 100 incoming EDNS options.

Technical ContextAI

Unbound is an open-source validating, recursive, and caching DNS resolver maintained by NLnet Labs. EDNS (Extension Mechanisms for DNS, RFC 6891) extends the base DNS protocol by allowing additional option fields to be carried in queries and responses. The vulnerability stems from Unbound's failure to bound the number of EDNS options it will process per incoming query: for each option present, the resolver allocates and populates internal data structures before enforcing any limit. This maps directly to CWE-407 (Inefficient Algorithmic Complexity), where attacker-controlled input size drives unbounded computational work - thread time scales linearly (or worse) with option count. The affected product is NLnet Labs Unbound, all releases up to and including 1.25.0. No CPE string was supplied in the input, but the product is identifiable as cpe:2.3:a:nlnetlabs:unbound with version constraint ≤ 1.25.0.

RemediationAI

The primary and definitive fix is to upgrade to Unbound 1.25.1 or later, which patches the vulnerability by enforcing a hard limit of 100 incoming EDNS options per query, as documented at https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-41292.txt. For deployments where immediate upgrading is not feasible, a compensating control is to deploy an EDNS-aware DNS firewall or proxy upstream of Unbound configured to drop or truncate queries containing more than a threshold number of EDNS options; note that this requires EDNS-inspecting appliances and risks false positives against legitimate high-option queries. Additionally, restricting the resolver's listener to trusted client IP ranges (via Unbound's access-control: directives) eliminates attack surface from untrusted networks, at the cost of excluding open-resolver use cases. Rate-limiting inbound DNS queries per source IP at the firewall layer reduces coordinated flood effectiveness but does not eliminate single-source degradation.

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Same weakness CWE-407 – Inefficient Algorithmic Complexity

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Same technique Denial Of Service

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Vendor StatusVendor

SUSE

Severity: High
Product Status
SUSE Linux Enterprise High Performance Computing 15 SP4-ESPOS Affected
SUSE Linux Enterprise High Performance Computing 15 SP4-LTSS Affected
SUSE Linux Enterprise High Performance Computing 15 SP5-ESPOS Affected
SUSE Linux Enterprise High Performance Computing 15 SP5-LTSS Affected
SUSE Linux Enterprise Micro 5.5 Affected

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CVE-2026-41292 vulnerability details – vuln.today
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