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CVE-2026-81634— Possible heap buffer overflow during DNSSEC canonicalization

Quick assessment

Affected
NLnet Labs Unbound
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

In NLnet Labs Unbound up to and including 1.26.0, a 255 length query name with a large TCP response can lead to a heap buffer overflow during the RRSet canonicalisation routine. This is caused by missing to add the first owner name into the buffer length check

CVSS 7.5 · High
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I. Basic Information for CVE-2026-81634

Vulnerability Information
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Vulnerability Title
Possible heap buffer overflow during DNSSEC canonicalization
Source: CVE Program / CVE List V5
Vulnerability Description
In NLnet Labs Unbound up to and including 1.26.0, a 255 length query name with a large TCP response can lead to a heap buffer overflow during the RRSet canonicalisation routine. This is caused by missing to add the first owner name into the buffer length check. A malicious actor operating a malicious name server or tampering with an incoming response to Unbound (canonicalisation happens before DNSSEC validation), can trigger the vulnerability.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
堆缓冲区溢出
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
NLnet Labs Unbound 0 ~ 1.26.1 -

II. Public POCs for CVE-2026-81634

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III. Intelligence Information for CVE-2026-81634

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Other References for CVE-2026-81634 (1)

Same Patch Batch · NLnet Labs · 2026-09-16 · 9 CVEs total

CVE-2026-81642 9.1 CRITICAL Heap buffer overflow and possible Remote Code Execution when digesting DNSKEY
CVE-2026-82717 8.4 HIGH CNAME synthesis could lead to heap corruption
CVE-2026-78227 6.5 MEDIUM Use-after-free in DoQ stream output buffer on reset re-transmission
CVE-2026-82720 5.9 MEDIUM Use-after-free in DoH stream cleanup code path
CVE-2026-85501 5.3 MEDIUM Retrap: Novel Vulnerabilities to launch Algorithmic Complexity Attacks on DNSSEC
CVE-2026-80225 5.3 MEDIUM Possible degradation of service from continuous queries on the same TCP/DoT connection
CVE-2026-77955 4.4 MEDIUM Possible ZONEMD verification bypass window
CVE-2026-77860 3.7 LOW 'serve-expired' can bypass Unbound 'wait-limit'

IV. Related Vulnerabilities

Same product: Unbound
Same vendor: NLnet Labs
Same weakness: CWE-122

V. Comments for CVE-2026-81634

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