CVE-2026-42959: CWE-824: Access of Uninitialized Pointer in NLnet Labs Unbound
NLnet Labs Unbound up to and including version 1.25.0 has a denial of service vulnerability in the DNSSEC validator that can lead to a crash given malicious upstream replies. When Unbound constructs chase-reply messages for validation, the code uses the wrong counter to calculate write offsets for ADDITIONAL section rrsets. DNAME duplication could increase the ANSWER section count and authority filtering could decrease the AUTHORITY section count and create an uninitialized array slot. Combining these two, the validator later dereferences this uninitialized pointer, causing an immediate process crash. An adversary controlling a DNSSEC-signed domain can trigger this bug with a single query by configuring a DNAME chain with unsigned CNAMEs and a response containing unsigned AUTHORITY records alongside signed ADDITIONAL glue records. Unbound 1.25.1 contains a patch with a fix to use the proper counters to calculate the write offsets.
AI Analysis
Technical Summary
CVE-2026-42959 is a denial of service vulnerability in NLnet Labs Unbound DNSSEC validator present in versions up to 1.25.0. The vulnerability is due to the use of an incorrect counter when calculating write offsets for ADDITIONAL section rrsets in chase-reply messages. Specifically, DNAME duplication can increase the ANSWER section count while authority filtering decreases the AUTHORITY section count, creating an uninitialized array slot. The validator later dereferences this uninitialized pointer, causing an immediate crash. An attacker controlling a DNSSEC-signed domain can exploit this by sending a single query with a crafted DNAME chain containing unsigned CNAMEs and a response with unsigned AUTHORITY records alongside signed ADDITIONAL glue records. The issue is fixed in Unbound 1.25.1 by using the correct counters for write offset calculation.
Potential Impact
Successful exploitation causes an immediate crash of the Unbound DNSSEC validator process, resulting in a denial of service. This can disrupt DNS resolution services relying on Unbound, potentially impacting availability. There is no indication of code execution or data disclosure from this vulnerability.
Mitigation Recommendations
A fix is available in Unbound version 1.25.1, which corrects the write offset counter usage. Users should upgrade to version 1.25.1 or later to remediate this vulnerability. Red Hat has issued security advisories and updates for affected Red Hat Enterprise Linux 10 variants that include this fix. Applying these vendor-provided updates is recommended. Patch status is confirmed by vendor advisories linked in the references.
CVE-2026-42959: CWE-824: Access of Uninitialized Pointer in NLnet Labs Unbound
Description
NLnet Labs Unbound up to and including version 1.25.0 has a denial of service vulnerability in the DNSSEC validator that can lead to a crash given malicious upstream replies. When Unbound constructs chase-reply messages for validation, the code uses the wrong counter to calculate write offsets for ADDITIONAL section rrsets. DNAME duplication could increase the ANSWER section count and authority filtering could decrease the AUTHORITY section count and create an uninitialized array slot. Combining these two, the validator later dereferences this uninitialized pointer, causing an immediate process crash. An adversary controlling a DNSSEC-signed domain can trigger this bug with a single query by configuring a DNAME chain with unsigned CNAMEs and a response containing unsigned AUTHORITY records alongside signed ADDITIONAL glue records. Unbound 1.25.1 contains a patch with a fix to use the proper counters to calculate the write offsets.
CVSS v4.0
Score 8.7high
Affected software
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
CVE-2026-42959 is a denial of service vulnerability in NLnet Labs Unbound DNSSEC validator present in versions up to 1.25.0. The vulnerability is due to the use of an incorrect counter when calculating write offsets for ADDITIONAL section rrsets in chase-reply messages. Specifically, DNAME duplication can increase the ANSWER section count while authority filtering decreases the AUTHORITY section count, creating an uninitialized array slot. The validator later dereferences this uninitialized pointer, causing an immediate crash. An attacker controlling a DNSSEC-signed domain can exploit this by sending a single query with a crafted DNAME chain containing unsigned CNAMEs and a response with unsigned AUTHORITY records alongside signed ADDITIONAL glue records. The issue is fixed in Unbound 1.25.1 by using the correct counters for write offset calculation.
Potential Impact
Successful exploitation causes an immediate crash of the Unbound DNSSEC validator process, resulting in a denial of service. This can disrupt DNS resolution services relying on Unbound, potentially impacting availability. There is no indication of code execution or data disclosure from this vulnerability.
Mitigation Recommendations
A fix is available in Unbound version 1.25.1, which corrects the write offset counter usage. Users should upgrade to version 1.25.1 or later to remediate this vulnerability. Red Hat has issued security advisories and updates for affected Red Hat Enterprise Linux 10 variants that include this fix. Applying these vendor-provided updates is recommended. Patch status is confirmed by vendor advisories linked in the references.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- NLnet Labs
- Date Reserved
- 2026-05-07T10:07:51.848Z
- Cvss Version
- 4.0
- State
- PUBLISHED
- Remediation Level
- null
- Vendor Advisory Urls
- [{"url":"https://access.redhat.com/security/cve/CVE-2026-42959","vendor":"Red Hat"},{"url":"https://access.redhat.com/errata/RHSA-2026:23231","vendor":"Red Hat"},{"url":"https://access.redhat.com/errata/RHSA-2026:24365","vendor":"Red Hat"},{"url":"https://access.redhat.com/errata/RHSA-2026:24369","vendor":"Red Hat"},{"url":"https://access.redhat.com/errata/RHSA-2026:19752","vendor":"Red Hat"}]
Threat ID: 6a0d86fdba1db4736270ee6c
Added to database: 05/20/2026, 10:03:41 UTC
Last enriched: 07/15/2026, 09:18:10 UTC
Last updated: 07/31/2026, 19:22:58 UTC
Views: 126
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