Bug 2535060 (CVE-2026-82720)

Summary: CVE-2026-82720 unbound: Unbound: Denial of Service via use-after-free in DoH stream cleanup
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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, sghai, sidsharm, suppawar, vlaad
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in Unbound. When compiled with DNS-over-HTTPs (DoH) support, a use-after-free vulnerability exists in the DoH stream cleanup code. A remote malicious actor can exploit this by sending a specially crafted DoH connection under specific failure conditions, such as an RPZ drop query or heavy traffic. This can lead to a denial of service (DoS) by causing the Unbound process to terminate.
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Description OSIDB Bzimport 2026-09-16 09:07:58 UTC 
NLnet Labs Unbound 1.12.0 up to and including 1.26.0 has a use-after-free vulnerability when compiled for DNS-over-HTTPs support with '--with-libnghttp2'. During failure code paths (i.e., RPZ drop query, jostle due to heavy traffic), a dropped DoH stream brings down the whole DoH session and does not account properly for other DoH streams in the same session. This leads to use-after-free in those code paths. If the prerequisites are satisfied (possible RPZ drop or heavy client traffic), a malicious actor can trigger the vulnerability with a single DoH connection and the appropriate traffic. Impact is limited as the reads are not user controlled and the use-after-free leads to early returns. However, a hardened allocator can catch the use-after-free and controllably terminate the process resulting to denial of service.