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CVE-2026-78227— Use-after-free in DoQ stream output buffer on reset re-transmission

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.

NLnet Labs Unbound 1.22.0 up to and including 1.26.1, has a use-after-free vulnerability when compiled for DNS-over-QUIC support with '--with-libngtcp2'. Each DoQ stream owns an output buffer that holds the DNS response. ngtcp2's retransmission buffer keeps a

CVSS 6.5 · Medium
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I. Basic Information for CVE-2026-78227

Vulnerability Information

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Vulnerability Title
Use-after-free in DoQ stream output buffer on reset re-transmission
Source: CVE Program / CVE List V5
Vulnerability Description
NLnet Labs Unbound 1.22.0 up to and including 1.26.1, has a use-after-free vulnerability when compiled for DNS-over-QUIC support with '--with-libngtcp2'. Each DoQ stream owns an output buffer that holds the DNS response. ngtcp2's retransmission buffer keeps a shallow pointer into the output buffer for as long as a STREAM frame may be resent. On a client RESET_STREAM, the output buffer is freed but ngtcp2 still holds the matching retransmission entries. The next PTO timeout makes ngtcp2 re-encode the STREAM frame and copy from the freed buffer. A malicious actor that can query Unbound over DoQ and that withholds ACKs, sends RESET_STREAM, and waits for PTO, reaches this use-after-free with no privilege. This leads to retransmissions against freed memory and eventually an abnormal server exit under a 20-query spray.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:L
Source: CVE Program / CVE List V5
Vulnerability Type
释放后使用
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
NLnet Labs Unbound 1.22.0 ~ 1.26.1 -

II. Public POCs for CVE-2026-78227

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III. Intelligence Information for CVE-2026-78227

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Other References for CVE-2026-78227 (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-2026-81634 7.5 HIGH Possible heap buffer overflow during DNSSEC canonicalization
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-416

V. Comments for CVE-2026-78227

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