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

What is CVE-2026-50252?

CVE-2026-50252 is a medium-severity vulnerability in Nlnetlabs Unbound, published on 22 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

In NLnet Labs Unbound 1.4.22 up to and including 1.25.1, UDP source port is randomized and intended to serve as a secret value that increases the entropy of DNS transactions. When resolver load balancing policies depend on the source port while their outcome is revealed this secrecy is undermined. The vulnerability arises when the load balancing policy is consistent with respect to the incoming source UDP port and IP address while heavily depending on the incoming source UDP port as a randomization source. When the SO_REUSEPORT configuration option is enabled ('so-reuseport: yes') in Unbound (by default), it meets these conditions, making it vulnerable for DNS cache poisoning attacks. Upon startup, Unbound randomly partitions the available UDP source port space into disjoint subsets of (almost) equal size, assigning each subset to a specific worker thread. When an incoming DNS query is received, the kernel’s SO_REUSEPORT load balancing mechanism deterministically assigns the query to a socket associated with a particular thread. All outgoing DNS queries generated during the resolution of that request use source ports selected exclusively from the port subset assigned to the corresponding thread. Since these port subsets are disjoint across threads, the source port observed in a resolver’s outgoing query to an authoritative name server serves as a reliable indicator of the worker thread that processed the original client query. A malicious actor can acquire the mapping between incoming UDP source ports (for a given fixed source IP address) and Unbound worker threads and leverage it to conduct DNS cache poisoning attacks by effectively lowering the random port population per thread.

Weakness: CWE-349

Which versions are affected by CVE-2026-50252?

It affects 1 product.

Products include Nlnetlabs Unbound.

How do I fix CVE-2026-50252?

Upgrade to a fixed version.

Is CVE-2026-50252 being exploited?

There is no known exploitation of CVE-2026-50252 in the wild and no public exploit code.

Other vulnerabilities in Nlnetlabs Unbound

• CVE-2026-81642 Heap buffer overflow and possible Remote Code Execution when digesting DNSKEY Critical 9.1
• CVE-2026-33278 Possible arbitrary code execution during DNSSEC validation Critical 9.1
• CVE-2026-42944 Heap overflow with multiple NSID, COOKIE, PADDING EDNS options High 8.7
• CVE-2026-42959 Crash during DNSSEC validation of malicious content High 8.7
• CVE-2026-82717 CNAME synthesis could lead to heap corruption High 8.4

All Nlnetlabs Unbound vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as ALPINE-CVE-2026-50252, ALSA-2026:68291, ALSA-2026:68292, ALSA-2026:69120, AZL-92982, CLSA-2026-1785241157 and 18 more.