VulnerabilityPublished 16 Sep 2026

CVE-2026-85501: NLnet Labs Unbound · Unthrottled Resource Allocation

Affects NLnet Labs UnboundSource CVE Database V5

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Not known to be exploited · CVSS 5.3 Medium : upgrade to 1.26.1 when convenient.

Data as of 31 Oct 2026, 08:20 UTC

CVSS 3.1 base score

5.3Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.4% chance of exploitation in 30 days – higher than 41% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 1.26.1

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: Low

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 16 Sep 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 30 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade NLnet Labs Unbound to 1.26.1.

Affected software

Product	Affected	Fixed in
NLnet LabsUnbound	Affected <1.26.1	Fixed in 1.26.1

AI-suggested

If you cannot patch yet, consider limiting exposure to untrusted DNS zones and monitoring resolver performance for signs of resource exhaustion related to DNSSEC validation.

Description

CVE-2026-85501 affects NLnet Labs Unbound DNS resolver up to version 1.26.0 and involves allocation of resources without limits or throttling, leading to potential service degradation. The vulnerability arises from algorithmic complexity attacks on DNSSEC validation, including multiple attack vectors such as TagTrap, DelegationTrap, NsecTrap, and AdditionalTrap. These exploit excessive or malformed DNSSEC-related records to cause high computational overhead and resource exhaustion. The CVSS score is 5.3, indicating a medium severity impact primarily on availability. No known exploits are reported in the wild.

Impact

The impact is a degradation of service (availability) due to excessive computational overhead and resource exhaustion during DNSSEC validation. There is no confidentiality or integrity impact reported. The vulnerability can be triggered remotely by malicious DNS zones crafted to exploit these algorithmic complexity weaknesses.

Summary

CVE-2026-85501 describes a set of algorithmic complexity vulnerabilities in NLnet Labs Unbound DNS resolver versions prior to 1.26.1. These vulnerabilities allow attackers to cause resource exhaustion by exploiting DNSSEC validation mechanisms. Specifically, TagTrap exploits the triple matching mechanism of Zone, Algorithm, and KeyTag with numerous mismatched DNSKEY, RRSIG, and DS records. DelegationTrap causes iterative chain-of-trust validation overhead for deeply nested domains. NsecTrap forces validation of excessive invalid NSEC records. AdditionalTrap exploits the default behavior of validating the ADDITIONAL section, increasing resource consumption. These issues can degrade service availability by overwhelming the resolver's validation processes.

CWE-770Allocation of Resources Without Limits or Throttling (opens in a new tab)
Base weaknessLikelihood of exploit: HighCWE Top 25 (2025): #25
The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.
Requirements
Clearly specify the minimum and maximum expectations for capabilities, and dictate which behaviors are acceptable when resource allocation reaches limits.
Architecture and Design
Limit the amount of resources that are accessible to unprivileged users. Set per-user limits for resources. Allow the system administrator to define these limits. Be careful to avoid CWE-410.
Architecture and Design
Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended.
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
OWASP ASVS 4.0.3
V8.1.4 (opens in a new tab)
V13.4.1 (opens in a new tab)
V11.1.4 (opens in a new tab)
Attack patterns
XML Ping of the Death (opens in a new tab)
Exponential Data Expansion (opens in a new tab)
Serialized Data Parameter Blowup (opens in a new tab)
Serialized Data with Nested Payloads (opens in a new tab)
Oversized Serialized Data Payloads (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

nlnetlabs.nl/downloads/unbound/CVE-2026-85501.txt (opens in a new tab)

Community reviews

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 17 Oct 2026, 23:26 UTC

WeaknessesCWE-770

Technical details

References

Community reviews

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