# CVE-2026-50252: NLnet Labs Unbound DNS cache poisoning via predictable UDP source ports

Source: https://junglewise.ai/threats/cve-2026-50252-nlnet-labs-unbound-dns-cache-poisoning-via-predictable-udp-source  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-22. Updated: 2026-07-22  
License: CC BY 4.0 with a link back

CVE-2026-50252 · Severity: info · CVSS 5.7 · Published 2026-07-22

Unbound is a widely used DNS resolver that translates human-readable domain names into IP addresses. A vulnerability in how it handles network traffic allows an attacker to predict the security codes used to verify DNS responses. This could allow a malicious actor to redirect users to fraudulent websites by poisoning the resolver's memory with incorrect data.

Technologies: [NLnet Labs Unbound](https://junglewise.ai/threats/technologies/unbound). Vendors: [NLnet Labs](https://junglewise.ai/threats/vendors/nlnet-labs).

## Quick answers

**Is CVE-2026-50252 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-50252 being exploited in the wild.

**How severe is CVE-2026-50252?** Informational severity, with a CVSS score of 5.7, published 22 July 2026.

**Which products and versions are affected?** NLnet Labs Unbound 1.4.22 up to and including 1.25.1.

**Is there a fix for CVE-2026-50252?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-50252 attacked?** It is attacked from the adjacent network, in a hosted service.

**Am I affected by CVE-2026-50252?** Check whether you run NLnet Labs Unbound at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

Unbound (versions 1.4.22 through 1.25.1) is vulnerable to DNS cache poisoning when 'so-reuseport' is enabled (the default setting). The vulnerability stems from Unbound partitioning the UDP source port space into disjoint subsets assigned to specific worker threads. Because the kernel's SO_REUSEPORT mechanism deterministically assigns incoming queries to threads based on the source port/IP, an attacker can identify which thread is processing a query. This allows the attacker to predict the limited range of outgoing source ports used by that thread, significantly reducing the entropy of the DNS transaction and facilitating cache poisoning. The issue is resolved in version 1.25.2 by implementing a shared ports structure across threads.

## Affected products

- NLnet Labs Unbound 1.4.22 up to and including 1.25.1

## Timeline

- 2026-07-22: advisory: NLnet Labs released security advisory and version 1.25.2
- 2026-07-22: disclosed

## References

- [https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-50252.txt](https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-50252.txt)

## Related threats

- [NLnet Labs Unbound use-after-free in DNS-over-QUIC stream cleanup](https://junglewise.ai/threats/cve-2026-97206-nlnet-labs-unbound-1-22-0-up-to-and-including-version-1-26-1)
- [NLnet Labs Unbound DNS decompression CPU exhaustion](https://junglewise.ai/threats/cve-2026-96405-nlnet-labs-unbound-up-to-and-including-version-1-26-1-limits)
- [NLnet Labs Unbound DNS validator bypass in DNSSEC validation](https://junglewise.ai/threats/cve-2026-50106-when-nlnet-labs-unbound-up-to-and-including-version-1-26-1-is)
- [NLnet Labs Unbound cache side-channel in upstream source port probe](https://junglewise.ai/threats/cve-2026-109125-in-nlnet-labs-unbound-up-to-and-including-version-1-26-1-an)
- [NLnet Labs Unbound DNS complexity attacks via ReTrap](https://junglewise.ai/threats/cve-2026-85501-nlnet-labs-unbound-dns-complexity-attacks-via-retrap)