CVE-2026-81634: CWE-122: Heap-based Buffer Overflow in NLnet Labs Unbound
NLnet Labs Unbound up to and including version 1.26.0 contains a heap-based buffer overflow vulnerability triggered by a 255 length query name combined with a large TCP response during the RRSet canonicalisation routine. This occurs because the first owner name is not included in the buffer length check. An attacker controlling a malicious name server or tampering with DNS responses can exploit this before DNSSEC validation.
AI Analysis
Technical Summary
CVE-2026-81634 is a heap-based buffer overflow vulnerability in NLnet Labs Unbound DNS resolver software versions prior to 1.26.1. The flaw arises during the RRSet canonicalisation process when handling a 255 length query name with a large TCP response. The vulnerability is caused by a missing buffer length check for the first owner name, allowing an attacker who controls a malicious name server or can tamper with incoming DNS responses to trigger a heap overflow before DNSSEC validation occurs.
Potential Impact
Successful exploitation can cause a heap-based buffer overflow leading to potential denial of service or other impacts on availability. The vulnerability does not directly affect confidentiality or integrity according to the CVSS vector. No known exploits are reported in the wild at this time.
Mitigation Recommendations
A fix is available in Unbound version 1.26.1. Users should upgrade to version 1.26.1 or later to remediate this vulnerability. No other mitigation guidance is provided by the vendor advisory.
CVE-2026-81634: CWE-122: Heap-based Buffer Overflow in NLnet Labs Unbound
Description
NLnet Labs Unbound up to and including version 1.26.0 contains a heap-based buffer overflow vulnerability triggered by a 255 length query name combined with a large TCP response during the RRSet canonicalisation routine. This occurs because the first owner name is not included in the buffer length check. An attacker controlling a malicious name server or tampering with DNS responses can exploit this before DNSSEC validation.
CVSS v3.1
Score 7.5high
Affected software
NLnet Labs
Unbound
pkg:github/nlnetlabs/unboundRun on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
CVE-2026-81634 is a heap-based buffer overflow vulnerability in NLnet Labs Unbound DNS resolver software versions prior to 1.26.1. The flaw arises during the RRSet canonicalisation process when handling a 255 length query name with a large TCP response. The vulnerability is caused by a missing buffer length check for the first owner name, allowing an attacker who controls a malicious name server or can tamper with incoming DNS responses to trigger a heap overflow before DNSSEC validation occurs.
Potential Impact
Successful exploitation can cause a heap-based buffer overflow leading to potential denial of service or other impacts on availability. The vulnerability does not directly affect confidentiality or integrity according to the CVSS vector. No known exploits are reported in the wild at this time.
Mitigation Recommendations
A fix is available in Unbound version 1.26.1. Users should upgrade to version 1.26.1 or later to remediate this vulnerability. No other mitigation guidance is provided by the vendor advisory.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- NLnet Labs
- Date Reserved
- 2026-09-07T14:06:21.967Z
- Cvss Version
- 3.1
- State
- PUBLISHED
Threat ID: 6aaa578955bf5e2cf540b1fa
Added to database: 09/16/2026, 08:47:05 UTC
Last enriched: 09/16/2026, 09:01:39 UTC
Last updated: 09/16/2026, 09:06:42 UTC
Views: 4
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