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NLnet Labs Unbound CVE-2026-55717

| EUVDEUVD-2026-47680 MEDIUM
NULL Pointer Dereference (CWE-476)
2026-07-22 NLnet Labs GHSA-9vcf-h83p-jghm
Denial Of Service Null Pointer Dereference Red Hat Suse Unbound Nlnet Labs
5.9
CVSS 3.1 · Vendor: NLnet Labs
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Severity by source

Vendor (NLnet Labs) PRIMARY
5.9 MEDIUM
AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
vuln.today AI
5.9 MEDIUM

AC:H reflects three simultaneous non-default configuration requirements plus a cache-expiration timing dependency; no confidentiality or integrity impact applies.

3.1 AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
4.0 AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
SUSE
MEDIUM
qualitative
Red Hat
5.9 MEDIUM
qualitative

Primary rating from Vendor (NLnet Labs).

CVSS VectorVendor: NLnet Labs

Attack Vector
Network
Attack Complexity
High
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

Lifecycle Timeline

2
Patch available
Jul 22, 2026 - 16:18 EUVD
Analysis Generated
Jul 22, 2026 - 13:56 vuln.today

DescriptionCVE.org

In NLnet Labs Unbound 1.10.0 up to and including 1.25.1, when 'serve-expired: yes' is set together with a 'response-ip: <net> redirect' /'response-ip-data: <net> CNAME <target>' rule (or the RPZ 'rpz-cname-override' equivalent), a remote client who controls any delegated domain can crash the daemon. The serve-expired-client-timeout callback runs a two-pass loop to chase the respip-generated CNAME alias; on the second pass it resets 'alias_rrset' but not 'partial_rep'. Later, this inconsistency leads to a NULL pointer dereference and an eventual crash. A malicious actor can exploit the vulnerability by controlling any zone that replies with an A/AAAA record that falls inside the configured response-ip/rpz subnet. By delaying the answer when the previous record has expired, the vulnerable path of 'serve-expired-client-timeout' is taken leading to denial of service via the server crash.

AnalysisAI

Denial of service in NLnet Labs Unbound 1.10.0 through 1.25.1 allows a remote attacker controlling any delegated DNS zone to crash the resolver daemon via a NULL pointer dereference. The vulnerable path is activated only when both the serve-expired feature and a response-ip or RPZ CNAME redirect rule are configured simultaneously; the serve-expired-client-timeout callback's two-pass CNAME alias loop resets alias_rrset on the second pass without resetting partial_rep, causing an eventual null dereference and process crash. No public exploit code has been identified and this vulnerability is not listed in CISA KEV, but the network-accessible attack surface makes it a meaningful availability risk for any resolver running these co-configured features.

Technical ContextAI

Unbound is an open-source validating, recursive DNS resolver developed by NLnet Labs, widely deployed in enterprise, ISP, and government environments. The CPE string cpe:2.3:a:nlnet_labs:unbound:*:*:*:*:*:*:*:* covers the affected version range 1.10.0 through 1.25.1. The root cause is CWE-476 (NULL Pointer Dereference) in the serve-expired-client-timeout callback. The 'serve-expired' feature permits Unbound to return stale cached responses to clients while the resolver refreshes the record in the background; the 'response-ip' and 'response-ip-data' directives (or their RPZ equivalent, rpz-cname-override) allow operators to transparently redirect DNS responses whose A/AAAA results fall within a specified subnet to a CNAME target. When both features are active and a cache record has expired, the timeout callback initiates a two-pass loop to chase the CNAME alias generated by the response-ip policy; on the second pass the code resets the alias_rrset pointer but leaves partial_rep in an inconsistent state, producing a null pointer dereference that terminates the daemon.

RemediationAI

Upgrade Unbound to the fixed version identified in the vendor advisory at https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55717.txt; an exact patched version number is not confirmed in the available input data and must be verified directly from that advisory before deployment. For operators who cannot patch immediately, two configuration-based workarounds eliminate the vulnerable code path: first, removing or disabling all response-ip, response-ip-data, and RPZ rpz-cname-override redirect rules breaks the CNAME-alias loop that triggers the null dereference - the trade-off is loss of any IP-based DNS policy enforcement currently in use; second, setting serve-expired: no in unbound.conf prevents the serve-expired-client-timeout callback from ever firing - the trade-off is increased resolver latency and higher NXDOMAIN rates for clients during cache misses. Either workaround alone is sufficient; applying both provides additional defense but is not required.

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Same weakness CWE-476 – NULL Pointer Dereference

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Vendor StatusVendor

SUSE

Severity: Moderate
Product Status
openSUSE Tumbleweed Fixed
SUSE Linux Enterprise Desktop 15 SP7 Affected
SUSE Linux Enterprise High Performance Computing 15 SP7 Affected
SUSE Linux Enterprise Micro 5.5 Affected
SUSE Linux Enterprise Module for Basesystem 15 SP7 Affected

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CVE-2026-55717 vulnerability details – vuln.today
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