Insufficient validation of the RRSIG.Labels field combined with premature cache writes during RFC 8198 aggressive NSEC processing leads to cache poisoning that permits a malicious actor controlling a single delegated zone to poison arbitrary sibling zones under NSEC-signed parent domains. Unbound 1.25.2 includes fixes to add lower-bound check in RRSIG labels, move cache write stricly after validation and adding bailiwick check when updating wildcards.

Hi there,

(The official announcement can be found at: https://community.nlnetlabs.nl/t/unbound-1-25-2-released/3430)

Several vulnerabilities were found in Unbound. The overview of the vulnerabilities with a brief description is:

CVE-2026-32665 - severity: HIGH Remote DNS-over-QUIC denial of service due to quic-size budget bypass

CVE-2026-40691 - severity: HIGH Packet of death for DNSCrypt over TCP

CVE-2026-44690 - severity: HIGH Cross-zone wildcard cache poisoning via RRSIG.labels manipulation

CVE-2026-55973 - severity: HIGH 'dns-error-reporting: yes' leads to stack buffer overflow

CVE-2026-14586 - severity: MEDIUM CVE-2026-44621 - severity: MEDIUM Libunbound applications configured with 'unwanted-reply-threshold' could eventually be abruptly terminated

CVE-2026-50045 - severity: MEDIUM 'max-global-quota' reset by DNSSEC validation restarts

CVE-2026-50046 - severity: MEDIUM CVE-2026-50243 - severity: MEDIUM 'response-ip'/'rpz' can rewrite BOGUS answers instead of returning SERVFAIL

CVE-2026-50248 - severity: MEDIUM BOGUS configured primary hostname accepted for XFR in auth/rpz zones

CVE-2026-50251 - severity: MEDIUM Attacker supplied 0.0.0.0/:: glue triggers defensive full-cache flush

CVE-2026-50252 - severity: MEDIUM Possible cache poisoning attack by mapping source port population per thread

CVE-2026-52863 - severity: MEDIUM Memory corruption could lead to crash and denial of service

CVE-2026-55717 - severity: MEDIUM CVE-2026-55990 - severity: MEDIUM Packet of death for a DNSCrypt misconfigured Unbound

CVE-2026-55991 - severity: MEDIUM Remote DNS-over-QUIC (DoQ) flow-control assertion failure in libngtcp2

CVE-2026-56416 - severity: MEDIUM CVE-2026-56444 - severity: MEDIUM Degradation of resolution service when 'discard-timeout' and 'serve-expired-client-timeout' are combined in unusual configuration

CVE-2026-41637 - severity: LOW CVE-2026-42955 - severity: LOW CVE-2026-44687 - severity: LOW Off-by-one error in 'harden-below-nxdomain' logic can shadow a stub/forward zone by a legitimate parent's NXDOMAIN

CVE-2026-46582 - severity: LOW A wildcard replay, as another piece of data, triggers poisoning in the serve expired reply path

CVE-2026-54478 - severity: LOW DNS Cookie bypass when combined with proxy-protocol use

CVE-2026-55708 - severity: LOW More information about the vulnerabilities can be found at: https://nlnetlabs.nl/projects/unbound/security-advisories/

Best regards, -- Yorgos, on behalf of the Unbound team.

This email is signed. Keys of the NLnet Labs people are published on https://www.nlnetlabs.nl/people/

Certain DNSSEC aspects of the DNS protocol (in RFC 4033, 4034, 4035, 6840, and related RFCs) allow remote attackers to cause a denial of service (CPU consumption) via one or more DNSSEC responses, aka the "KeyTrap" issue. One of the concerns is that, when there is a zone with many DNSKEY and RRSIG records, the protocol specification implies that an algorithm must evaluate all combinations of DNSKEY and RRSIG records.

A vulnerability named 'Non-Responsive Delegation Attack' (NRDelegation Attack) has been discovered in various DNS resolving software. The NRDelegation Attack works by having a malicious delegation with a considerable number of non responsive nameservers. The attack starts by querying a resolver for a record that relies on those unresponsive nameservers. The attack can cause a resolver to spend a lot of time/resources resolving records under a malicious delegation point where a considerable number of unresponsive NS records reside. It can trigger high CPU usage in some resolver implementations that continually look in the cache for resolved NS records in that delegation. This can lead to degraded performance and eventually denial of service in orchestrated attacks. Unbound does not suffer from high CPU usage, but resources are still needed for resolving the malicious delegation. Unbound will keep trying to resolve the record until hard limits are reached. Based on the nature of the attack and the replies, different limits could be reached. From version 1.16.3 on, Unbound introduces fixes for better performance when under load, by cutting opportunistic queries for nameserver discovery and DNSKEY prefetching and limiting the number of times a delegation point can issue a cache lookup for missing records.

NLnet Labs Unbound version 1.18.0 up to and including version 1.19.1 contain a vulnerability that can cause denial of service by a certain code path that can lead to an infinite loop. Unbound 1.18.0 introduced a feature that removes EDE records from responses with size higher than the client's advertised buffer size. Before removing all the EDE records however, it would try to see if trimming the extra text fields on those records would result in an acceptable size while still retaining the EDE codes. Due to an unchecked condition, the code that trims the text of the EDE records could loop indefinitely. This happens when Unbound would reply with attached EDE information on a positive reply and the client's buffer size is smaller than the needed space to include EDE records. The vulnerability can only be triggered when the 'ede: yes' option is used; non default configuration. From version 1.19.2 on, the code is fixed to avoid looping indefinitely.

DISPUTED Unbound before 1.9.5 allows an assertion failure and denial of service in dnamepktcopy via an invalid packet. NOTE: The vendor disputes that this is a vulnerability. Although the code may be vulnerable, a running Unbound installation cannot be remotely or locally exploited.

NLnet Labs Unbound, up to and including version 1.12.0, and NLnet Labs NSD, up to and including version 4.3.3, contain a local vulnerability that would allow for a local symlink attack. When writing the PID file, Unbound and NSD create the file if it is not there, or open an existing file for writing. In case the file was already present, they would follow symlinks if the file happened to be a symlink instead of a regular file. An additional chown of the file would then take place after it was written, making the user Unbound/NSD is supposed to run as the new owner of the file. If an attacker has local access to the user Unbound/NSD runs as, she could create a symlink in place of the PID file pointing to a file that she would like to erase. If then Unbound/NSD is killed and the PID file is not cleared, upon restarting with root privileges, Unbound/NSD will rewrite any file pointed at by the symlink. This is a local vulnerability that could create a Denial of Service of the system Unbound/NSD is running on. It requires an attacker having access to the limited permission user Unbound/NSD runs as and point through the symlink to a critical file on the system.

NLnet Labs Unbound, up to and including version 1.16.1 is vulnerable to a novel type of the "ghost domain names" attack. The vulnerability works by targeting an Unbound instance. Unbound is queried for a subdomain of a rogue domain name. The rogue nameserver returns delegation information for the subdomain that updates Unbound's delegation cache. This action can be repeated before expiry of the delegation information by querying Unbound for a second level subdomain which the rogue nameserver provides new delegation information. Since Unbound is a child-centric resolver, the ever-updating child delegation information can keep a rogue domain name resolvable long after revocation. From version 1.16.2 on, Unbound checks the validity of parent delegation records before using cached delegation information.

Unbound before 1.4.4 does not send responses for signed zones after mishandling an unspecified query, which allows remote attackers to cause a denial of service (DNSSEC outage) via a crafted query.

NLnet Labs Unbound, up to and including version 1.16.1, is vulnerable to a novel type of the "ghost domain names" attack. The vulnerability works by targeting an Unbound instance. Unbound is queried for a rogue domain name when the cached delegation information is about to expire. The rogue nameserver delays the response so that the cached delegation information is expired. Upon receiving the delayed answer containing the delegation information, Unbound overwrites the now expired entries. This action can be repeated when the delegation information is about to expire making the rogue delegation information ever-updating. From version 1.16.2 on, Unbound stores the start time for a query and uses that to decide if the cached delegation information can be overwritten.

The resolver in Unbound before 1.4.11 overwrites cached server names and TTL values in NS records during the processing of a response to an A record query, which allows remote attackers to trigger continued resolvability of revoked domain names via a "ghost domain names" attack.

Unbound before 1.3.4 does not properly verify signatures for NSEC3 records, which allows remote attackers to cause secure delegations to be downgraded via DNS spoofing or other DNS-related attacks in conjunction with crafted delegation responses.

validator/valnsec3.c in Unbound before 1.4.13p2 does not properly perform proof processing for NSEC3-signed zones, which allows remote DNS servers to cause a denial of service (daemon crash) via a malformed response that lacks expected NSEC3 records, a different vulnerability than CVE-2011-4528.

Unbound before 1.4.13p2 attempts to free unallocated memory during processing of duplicate CNAME records in a signed zone, which allows remote DNS servers to cause a denial of service (daemon crash) via a crafted response.

daemon/worker.c in Unbound 1.x before 1.4.10, when debugging functionality and the interface-automatic option are enabled, allows remote attackers to cause a denial of service (assertion failure and daemon exit) via a crafted DNS request that triggers improper error handling.

iterator.c in NLnet Labs Unbound before 1.5.1 does not limit delegation chaining, which allows remote attackers to cause a denial of service (memory and CPU consumption) via a large or infinite number of referrals.

DISPUTED Unbound before 1.9.5 allows an assertion failure via a compressed name in dnamepktcopy. NOTE: The vendor disputes that this is a vulnerability. Although the code may be vulnerable, a running Unbound installation cannot be remotely or locally exploited.

DISPUTED Unbound before 1.9.5 allows an out-of-bounds write via a compressed name in rdatacopy. NOTE: The vendor disputes that this is a vulnerability. Although the code may be vulnerable, a running Unbound installation cannot be remotely or locally exploited.

DISPUTED Unbound before 1.9.5 allows an integer overflow in a size calculation in dnscrypt/dnscrypt.c. NOTE: The vendor disputes that this is a vulnerability. Although the code may be vulnerable, a running Unbound installation cannot be remotely or locally exploited.

DISPUTED Unbound before 1.9.5 allows an integer overflow in sldnsstr2wirednamebuforigin, leading to an out-of-bounds write. NOTE: The vendor disputes that this is a vulnerability. Although the code may be vulnerable, a running Unbound installation cannot be remotely or locally exploited.

DISPUTED Unbound before 1.9.5 allows an out-of-bounds write in sldnsbgettokenpar. NOTE: The vendor disputes that this is a vulnerability. Although the code may be vulnerable, a running Unbound installation cannot be remotely or locally exploited.

DISPUTED Unbound before 1.9.5 allows an integer overflow in the regional allocator via regionalalloc. NOTE: The vendor disputes that this is a vulnerability. Although the code may be vulnerable, a running Unbound installation cannot be remotely or locally exploited.

DISPUTED Unbound before 1.9.5 allows an integer overflow in a size calculation in respip/respip.c. NOTE: The vendor disputes that this is a vulnerability. Although the code may be vulnerable, a running Unbound installation cannot be remotely or locally exploited.

DISPUTED Unbound before 1.9.5 allows an assertion failure and denial of service in synthcname. NOTE: The vendor disputes that this is a vulnerability. Although the code may be vulnerable, a running Unbound installation cannot be remotely or locally exploited.

DISPUTED Unbound before 1.9.5 allows an infinite loop via a compressed name in dnamepktcopy. NOTE: The vendor disputes that this is a vulnerability. Although the code may be vulnerable, a running Unbound installation cannot be remotely or locally exploited.

DISPUTED Unbound before 1.9.5 allows an integer overflow in the regional allocator via the ALIGNUP macro. NOTE: The vendor disputes that this is a vulnerability. Although the code may be vulnerable, a running Unbound installation cannot be remotely or locally exploited.

Unbound 1.6.4 through 1.9.4 contain a vulnerability in the ipsec module that can cause shell code execution after receiving a specially crafted answer. This issue can only be triggered if unbound was compiled with --enable-ipsecmod support, and ipsecmod is enabled and used in the configuration.

Unbound before 1.10.1 has Insufficient Control of Network Message Volume, aka an "NXNSAttack" issue. This is triggered by random subdomains in the NSDNAME in NS records.

Unbound before 1.10.1 has an infinite loop via malformed DNS answers received from upstream servers.