Buffer Overflow Vulnerability in Unbound by NLnet Labs
CVE-2026-55973

7.5HIGH

Key Information:

Vendor

Nlnet Labs

Status
Unbound
Vendor
CVE Published:
22 July 2026

What is CVE-2026-55973?

A buffer overflow vulnerability exists in NLnet Labs Unbound versions 1.23.0 through 1.25.1. When the 'dns-error-reporting' feature is enabled, improper handling of the EDNS Report-Channel option can lead to a crafted DNS response exploiting an inherent flaw. Specifically, the extraction of the agent domain from the upstream response occurs without proper validation, allowing an attacker to manipulate query names. When Unbound processes this malformed input, it incorrectly calculates label lengths, which results in writing over a stack variable, presenting a risk of daemon termination. This vulnerability highlights the importance of secure response handling in DNS services.

Affected Version(s)

Unbound 1.23.0 < 1.25.2

References

https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55973...
vendor-advisory

CVSS V3.1

Score:
7.5
Severity:
HIGH
Confidentiality:
None
Integrity:
None
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
None
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

Credit

Qifan Zhang (Palo Alto Networks)
.