Bug 2535052 (CVE-2026-78227)

Summary: CVE-2026-78227 unbound: Unbound: Denial of Service via use-after-free in DoQ stream output buffer
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: akhatavk, aos-team-art-private, asdas, dpaolell, jdelft, jupierce, lgarciaa, mbiarnes, ppalepu, ppostler, prdhamdh, rhel-process-autobot, sghai, sidsharm, suppawar, vlaad, watson-tool-maintainers
Target Milestone: ---Keywords: Security
Target Release: ---   
Hardware: All   
OS: Linux   
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A flaw was found in Unbound. This vulnerability occurs in the DNS-over-QUIC (DoQ) stream output buffer when a client sends a RESET_STREAM, causing the output buffer to be freed while retransmission entries still point to it. A remote attacker, by querying Unbound over DoQ, withholding acknowledgments, and sending a RESET_STREAM, can trigger this flaw. This leads to retransmissions against freed memory, ultimately resulting in a denial of service due to an abnormal server exit.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
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Bug Depends On: 2537316, 2537317    
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Description OSIDB Bzimport 2026-09-16 09:02:06 UTC 
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.