CVE-2026-50252

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.

In NLnet Labs Unbound up to and including 1.26.0, a degradation of service vulnerability is present in the TCP/DoT reading procedure where there is no limit on consecutive reads. A malicious actor that can stream and sustain a rate of distinct uncached names over the TCP/DoT connection monopolizes a single worker's entire event loop for as long as its writes stay ahead of the drain.

NLnet Labs Unbound 1.22.0 up to and including 1.26.1 has a use-after-free vulnerability when compiled for DNS-over-QUIC support with '--with-libngtcp2'. Each DoQ stream owns an output buffer holding the DNS response, and ngtcp2's retransmission buffer keeps a shallow pointer into it. On client RESET_STREAM, the output buffer is freed but ngtcp2 still holds matching retransmission entries, leading to use-after-free on next PTO. An unprivileged attacker can trigger this by querying over DoQ, withholding ACKs, sending RESET_STREAM, and waiting for PTO, causing abnormal server exit.

In NLnet Labs Unbound 1.13.2 up to and including 1.26.1, in zones with ZONEMD enabled (zonemd-check: yes) located below a trust anchor, there is an attack window where tampered zone contents are served (or stored to disk) prior to the ZONEMD integrity check. This is caused by the needed DS/DNSKEY asynchronous resolution before the ZONEMD check completes. If a zonefile is written to disk while the ZONEMD check failed, the tampered data are reloaded on startup and available until ZONEMD verification concludes again.

In NLnetLabs Unbound 1.20.0 up to and including 1.26.0, a vulnerability on the 'serve-expired' code path can cause a double decrement on the 'wait-limit' counter per client IP essentially bypassing one of the counter measures that was introduced for DNSBomb (CVE-2024-33655). A malicious actor can exploit this by controlling an authoritative zone with short TTL, so cached entries expire quickly. Each 'slow' query, one the attacker's authoritative never answers, is followed by one query for an expired cached name, which is answered immediately via the 'serve-expired' path and decrements the counter twice. This second query was named 'pump'. By alternating slow queries and pumps, the attacker keeps the per-client counter at or below the configured 'wait-limit' indefinitely, and can hold an arbitrary number of pending queries from a single source IP, up to the global mesh quota (num-queries-per-thread); eventually bypassing one of the counter measures introduced for DNSBomb (CVE-2024-33655). This vulnerability is present on the 'serve-expired' code path.

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.

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.

In Unbound 1.20.0 through 1.25.1, with 'serve-expired: yes' and 'serve-expired-client-timeout > discard-timeout > 0', the discard-timeout logic fails to properly decrement a counter, leading to new clients being dropped and service degradation. An attacker can exploit this by querying a slow authoritative zone.