CVE-2026-42534

In NLnet Labs Unbound up to and including 1.26.0, a vulnerability in the DNSSEC validator enables denial of service and possible remote code execution via DNSKEY processing. A crafted DNSKEY can overflow the digest buffer.

In Unbound library versions 1.4.22 through 1.25.1, a vulnerability allows DNS cache poisoning attacks. The issue arises from deterministic assignment of UDP port ranges to worker threads when SO_REUSEPORT is enabled, enabling an attacker to predict the source port and reduce DNS transaction entropy.

NLnet Labs Unbound up to version 1.25.0 is vulnerable to cache poisoning via promiscuous records in the authority section. An attacker can inject non-NS records (e.g., MX) along with address records that Unbound caches, leading to cache poisoning. Version 1.25.1 includes a fix that disregards address records from the additional section if they are not explicitly relevant to NS records.

NLnet Labs Unbound from version 1.19.1 to 1.25.0 has a vulnerability in the DNSSEC validator due to a struct-assignment bug during deep copy, leading to a dangling pointer and potential code execution.

NLnet Labs Unbound, 1.22.0 up to and including version 1.26.1, contains a DNS-over-QUIC (DoQ) lifetime bug reachable by a remote DoQ client under multi-threaded operation. A client-triggered QUIC path migration can enter stream/connection cleanup while a mesh reply still retains a pointer to the DoQ stream. Cleanup frees the DoQ stream and when the pending mesh reply later completes it dereferences a stale pointer leading to a crash.

NLnet Labs Unbound up to and including version 1.26.1 limits compression-pointer traversal for each individual domain-name operation but has no aggregate inbound decompression-work budget. Many chained compression pointers in a message result in a lot of CPU usage following them. A malicious actor with recursive-query access and control of an ordinarily delegated authority that serves specially crafted replies can exploit the vulnerability and degrade Unbound's general performance.

In NLnet Labs Unbound up to and including version 1.26.1, when configured with both a root trust anchor and an additional 'trust-anchor:' for an island-of-security zone whose parent is provably insecure under the root chain, a malicious actor who can deliver one forged NXDOMAIN response for a name inside the island, with the NSEC/SOA RRSIG 'SignerName' set to the insecure parent, tricks Unbound's validator to use the root trust anchor instead, and consecutively the insecure parent, thus causing that record to be treated as insecure and denying existence for records in the trust island.

In NLnet Labs Unbound up to and including version 1.26.1, an observable cache-level side-channel can be exposed with 'unwanted-reply-threshold > 0' configurations (not a default configuration). When a response-like UDP datagram reaches the upstream source port currently used by a recursive query but its TXID does not match any pending query, Unbound counts it as an unwanted reply and, upon reaching the threshold, clears the worker's RRset and message caches, allowing an external observer to distinguish whether a candidate port matches the current upstream source port.

NLnet Labs Unbound up to and including 1.26.0 is vulnerable to algorithmic complexity attacks (known as 'ReTrap') that cause service degradation. Attacks include TagTrap (large number of mismatched DNSKEY, RRSIG, DS records), DelegationTrap (costly iterative validation for deeply nested domains), NsecTrap (excessive invalid NSEC records), and AdditionalTrap (default validation of ADDITIONAL section).

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 (e.g., RPZ drop, jostle), a dropped DoH stream brings down the whole DoH session and does not account properly for other streams, leading to use-after-free. An attacker can trigger it with a single DoH connection, but impact is limited as reads are not user controlled; however, a hardened allocator can catch the use-after-free and controllably terminate the process, causing DoS.