Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain
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AccessReach recursor with ECS enabled
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ExploitTrigger scope-zero answer path
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ExecutionCause out-of-bounds read in findScopeZero
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ImpactCrash or leak recursor memory
Vulnerability Assessment
| Exploitation Conditions | Exploitation is tied to PowerDNS Recursor's ECS handling: the changelog states the scope-zero code path is 'only relevant when running with use-incoming-edns-subnet enabled and a scope zero answer is sent to the client,' so the use-incoming-edns-subnet option must be enabled (not the default) and a scope-zero answer must be produced to reach findScopeZero. Limiting factors: deployments without EDNS Client Subnet enabled are not on this code path and are effectively unaffected, and the out-of-bounds read most likely results in a crash or limited memory exposure rather than control-flow hijack. Affected versions are those prior to 5.3.12 / 5.4.8; patched releases were published 21 October 2026. |
| Impact | No CVSS vector, EPSS score, KEV listing, or POC is provided for CVE-2026-85559, so quantitative exploitation signals are entirely absent and risk must be inferred from the vendor advisory and changelog alone. PowerDNS classifies Advisory 2026-12 as containing three High-severity CVEs with the remainder Medium or Low; the input does not state which bucket 85559 falls in, but an out-of-bounds READ in findScopeZero most plausibly yields a crash (denial of service) or limited memory disclosure rather than code execution, consistent with a Medium/Low rating. Critically, the 5.3.13 changelog notes that the scope-zero code path 'is only relevant when running with use-incoming-edns-subnet enabled and a scope zero answer is sent to the client,' indicating this is a configuration-dependent flaw rather than a default-configuration network-exploitable bug. This is a genuine fix-worthy issue for operators running ECS, but on current evidence it is a moderate-priority patch-management item, not an emergency: there is no public exploit identified and no sign of active exploitation. Verify the exact severity and impact against the full PowerDNS advisory 2026-12, which was not fully available in the provided data. |
| Exploit ScenarioDerived | An operator runs PowerDNS Recursor with use-incoming-edns-subnet enabled. A client (or a response flowing through the scope-zero handling path) triggers the findScopeZero routine on data that drives an out-of-bounds read, causing the recursor to read beyond buffer bounds and most likely crash, degrading DNS resolution for all served clients. No public exploit code has been identified at time of analysis. |
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Alert meAnalysisAIv1 – Oct 21, 2026
Out-of-bounds read in PowerDNS Recursor's findScopeZero routine affects releases prior to 5.3.12 and 5.4.8 and is reachable by unauthenticated remote clients only when the non-default use-incoming-edns-subnet option is enabled and a scope-zero answer is produced for a query. Triggering the flaw yields limited information exposure and a plausible denial of service via process crash, reflected in the assessed CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:L vector, where the high attack complexity stems from the narrow configuration and answer-shape prerequisites rather than attacker-side effort. No public exploit code, EPSS score, or CISA KEV listing was identified at time of analysis, so exploitation has not been observed and quantitative risk signals are absent; this CVE is one of six issues bundled in PowerDNS Security Advisory 2026-12 and is among the lower-severity members of that set.
RemediationAI
Vendor-released patch: upgrade to PowerDNS Recursor 5.4.8 (5.4 train) or 5.3.12 (5.3 train), both published 21 October 2026; administrators on 5.3 should consider 5.3.13 instead, which contains the same SA-2026-12 fixes plus a fix for a performance regression introduced with SA-2026-08 that specifically affects use-incoming-edns-subnet deployments sending scope-zero answers. Signed tarballs are at https://downloads.powerdns.com/releases/pdns-recursor-5.4.8.tar.xz and https://downloads.powerdns.com/releases/pdns-recursor-5.3.12.tar.xz, with packages in the PowerDNS repository at https://repo.powerdns.com/; because SA-2026-12 bundles six CVEs including three High-severity issues, treat the upgrade as urgent even though this particular CVE is low-impact. Where upgrading must be deferred, the most direct compensating control is to disable use-incoming-edns-subnet, which removes the recursor from the findScopeZero path entirely at the cost of losing ECS-based answer tailoring and potentially reducing cache-hit efficiency and geo-accurate responses for clients. Additionally, restrict recursion service to trusted client networks via allow-from/ACL configuration so that arbitrary internet hosts cannot supply crafted queries, understanding that this narrows but does not eliminate exposure when the recursor serves a broad client base. Monitor for recursor crashes or unexpected restarts as a signal of attempted triggering, and consult https://doc.powerdns.com/recursor/security-advisories/powerdns-advisory-2026-12.html for any vendor-supplied detection or configuration guidance.
Affected ProductsAI
All PowerDNS Recursor deployments running a release earlier than 5.3.12 on the 5.3 train, or earlier than 5.4.8 on the 5.4 train, are affected, though only those that enable use-incoming-edns-subnet (not a default setting) and actually produce scope-zero answers to clients traverse the vulnerable findScopeZero code; deployments with ECS inbound processing disabled are effectively off this code path. No CPE identifiers were supplied with the CVE data, so version-range matching must come from the vendor changelogs and advisory. PowerDNS also notes a revised Open Source End of Life policy (older trains supported one year past the following major release), so administrators on retired trains should confirm support status at https://docs.powerdns.com/recursor/appendices/EOL.html before assuming a fixed build is available. Full details are in the PowerDNS Security Advisory 2026-12 at https://doc.powerdns.com/recursor/security-advisories/powerdns-advisory-2026-12.html, with per-branch change entries at https://doc.powerdns.com/recursor/changelog/5.3.html#change-5.3.12 and https://doc.powerdns.com/recursor/changelog/5.4.html#change-5.4.8.
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Technical ContextAI · show details
PowerDNS Recursor is a caching, validating DNS resolver; the affected code path handles EDNS Client Subnet (ECS), an extension that carries a client subnet in the OPT pseudo-record's option data so authoritative servers can return geographically tailored answers. When ECS is in play, a resolver may produce a 'scope zero' answer, meaning the answer is valid for all client subnets, and the recursor caches and later re-serves such entries (changelog 5.4.6 explicitly changed the recursor to 'store scope zero replies in packetcache and modify on retrieval,' indicating this path was recently reworked, with a follow-up performance regression fix in 5.3.13 tied to use-incoming-edns-subnet plus scope-zero answers). The findScopeZero routine locates and processes that scope-zero ECS data, and it reads past the bounds of the relevant buffer while doing so; the input data assigns no CWE identifier, but the described defect is an out-of-bounds read (CWE-125 class) arising from insufficient validation of option/label length data before indexing. Because the read is out-of-bounds rather than an out-of-bounds write, the realistic consequences are process termination or limited disclosure of adjacent memory contents, not control-flow hijack. Fixes landed as pull request 18197 in 5.4.8 and pull request 18198 in 5.3.12, both released 21 October 2026.
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Severity by source
Reachable over the network via DNS but only with non-default use-incoming-edns-subnet enabled and specific scope-zero conditions (AC:H); OOB read gives limited disclosure (C:L) or crash (A:L).
Description PRE-NVD
Executive ViewAI
PowerDNS Recursor, the DNS resolution service many organizations rely on for internal and external name resolution, can be crashed by a remote unauthenticated query - but only on deployments that have deliberately enabled the optional EDNS Client Subnet feature and that return a specific answer shape. The realistic impact is a short loss of DNS resolution for applications depending on that resolver, plus a narrow information-leak risk, rather than a data breach; teams on releases before 5.3.12 or 5.4.8 that need the feature should treat this primarily as an availability concern. Exploitation has not been observed and the prerequisites are narrow, placing this among the lower-severity items in PowerDNS Security Advisory 2026-12.
No vendor-released patch is confirmed as available at time of analysis (the advisory identifies 5.3.12 and 5.4.8 as the corrected releases - validate and deploy once obtainable). Interim: disable the non-default use-incoming-edns-subnet option wherever it is not required and restart the recursor, restrict recursive query access to trusted client networks so untrusted remote clients cannot reach the findScopeZero path, and monitor recursor process restarts or crash loops to detect and contain a denial-of-service attempt.
Threat Intelligence Overview
CVE.org
How this decision was reached
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Attack Technique
A buffer overflow occurs when a program writes more data to a memory buffer than it was allocated to hold, causing the excess data to spill into adjacent memory regions.
Denial of Service attacks render applications or systems unavailable by overwhelming resources or triggering failure conditions.
GitHub PoC
Exploit-DB