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

> Attackers can poison DNS cache by exploiting predictable source port distribution across worker threads in Unbound resolver.

- **Severity:** Medium 5.7 (`CVSS:4.0/AV:A/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:H/SA:H/E:P/U:Amber`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 22 Jul 2026
- **Updated:** 22 Jul 2026
- **Also known as:** ALPINE-CVE-2026-50252, ALSA-2026:68291, ALSA-2026:68292, ALSA-2026:69120, AZL-92982, CLSA-2026-1785241157, CLSA-2026-1785241895, CLSA-2026-1785409943, CLSA-2026-1785410023, CLSA-2026-1785412633, CLSA-2026-1786008652, DEBIAN-CVE-2026-50252, ECHO-a6ed-1cea-98bc, GHSA-jfgc-mgqq-r573, OESA-2026-3232, RHSA-2026:68291, RHSA-2026:68292, RHSA-2026:69120, RLSA-2026:68291, RLSA-2026:68292, RLSA-2026:69120, ROOT-OS-DEBIAN-11-CVE-2026-50252, ROOT-OS-DEBIAN-12-CVE-2026-50252, UBUNTU-CVE-2026-50252

## 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 (Acceptance of Extraneous Untrusted Data With Trusted Data)

## Which versions are affected by CVE-2026-50252?

It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `python3-unbound` | AlmaLinux:10 | All versions | 1.24.2-7.el10_2.5 |
| `unbound` | AlmaLinux:10 | All versions | 1.24.2-7.el10_2.5 |
| `unbound-anchor` | AlmaLinux:10 | All versions | 1.24.2-7.el10_2.5 |
| `unbound-devel` | AlmaLinux:10 | All versions | 1.24.2-7.el10_2.5 |
| `unbound-dracut` | AlmaLinux:10 | All versions | 1.24.2-7.el10_2.5 |
| `unbound-libs` | AlmaLinux:10 | All versions | 1.24.2-7.el10_2.5 |
| `unbound-utils` | AlmaLinux:10 | All versions | 1.24.2-7.el10_2.5 |
| `python3-unbound` | AlmaLinux:8 | All versions | 1.16.2-5.14.el8_10.2 |
| `unbound` | AlmaLinux:8 | All versions | 1.16.2-5.14.el8_10.2 |
| `unbound-devel` | AlmaLinux:8 | All versions | 1.16.2-5.14.el8_10.2 |
| `unbound-libs` | AlmaLinux:8 | All versions | 1.16.2-5.14.el8_10.2 |
| `python3-unbound` | AlmaLinux:9 | All versions | 1.24.2-3.el9_8.6 |
| `unbound` | AlmaLinux:9 | All versions | 1.24.2-3.el9_8.6 |
| `unbound-devel` | AlmaLinux:9 | All versions | 1.24.2-3.el9_8.6 |
| `unbound-dracut` | AlmaLinux:9 | All versions | 1.24.2-3.el9_8.6 |
| `unbound-libs` | AlmaLinux:9 | All versions | 1.24.2-3.el9_8.6 |
| `python3-unbound` | Red Hat:enterprise_linux:10.2 | All versions | 0:1.24.2-7.el10_2.5 |
| `python3-unbound-debuginfo` | Red Hat:enterprise_linux:10.2 | All versions | 0:1.24.2-7.el10_2.5 |
| `unbound` | Red Hat:enterprise_linux:10.2 | All versions | 0:1.24.2-7.el10_2.5 |
| `unbound-anchor` | Red Hat:enterprise_linux:10.2 | All versions | 0:1.24.2-7.el10_2.5 |
| `unbound-anchor-debuginfo` | Red Hat:enterprise_linux:10.2 | All versions | 0:1.24.2-7.el10_2.5 |
| `unbound-debuginfo` | Red Hat:enterprise_linux:10.2 | All versions | 0:1.24.2-7.el10_2.5 |
| `unbound-debugsource` | Red Hat:enterprise_linux:10.2 | All versions | 0:1.24.2-7.el10_2.5 |
| `unbound-devel` | Red Hat:enterprise_linux:10.2 | All versions | 0:1.24.2-7.el10_2.5 |
| `unbound-dracut` | Red Hat:enterprise_linux:10.2 | All versions | 0:1.24.2-7.el10_2.5 |
| `unbound-libs` | Red Hat:enterprise_linux:10.2 | All versions | 0:1.24.2-7.el10_2.5 |
| `unbound-libs-debuginfo` | Red Hat:enterprise_linux:10.2 | All versions | 0:1.24.2-7.el10_2.5 |
| `unbound-utils` | Red Hat:enterprise_linux:10.2 | All versions | 0:1.24.2-7.el10_2.5 |
| `unbound-utils-debuginfo` | Red Hat:enterprise_linux:10.2 | All versions | 0:1.24.2-7.el10_2.5 |
| `python3-unbound` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.16.2-5.14.el8_10.2 |
| `python3-unbound-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.16.2-5.14.el8_10.2 |
| `unbound` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.16.2-5.14.el8_10.2 |
| `unbound-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.16.2-5.14.el8_10.2 |
| `unbound-debugsource` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.16.2-5.14.el8_10.2 |
| `unbound-devel` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.16.2-5.14.el8_10.2 |
| `unbound-libs` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.16.2-5.14.el8_10.2 |
| `unbound-libs-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.16.2-5.14.el8_10.2 |
| `python3-unbound` | Red Hat:enterprise_linux:9::appstream | All versions | 0:1.24.2-3.el9_8.6 |
| `python3-unbound-debuginfo` | Red Hat:enterprise_linux:9::appstream | All versions | 0:1.24.2-3.el9_8.6 |
| `unbound` | Red Hat:enterprise_linux:9::appstream | All versions | 0:1.24.2-3.el9_8.6 |

Products: 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](https://insights.sbomify.com/CVE-2026-81642/): Heap buffer overflow and possible Remote Code Execution when digesting DNSKEY (critical 9.1)
- [CVE-2026-33278](https://insights.sbomify.com/CVE-2026-33278/): Possible arbitrary code execution during DNSSEC validation (critical 9.1)
- [CVE-2026-42944](https://insights.sbomify.com/CVE-2026-42944/): Heap overflow with multiple NSID, COOKIE, PADDING EDNS options (high 8.7)
- [CVE-2026-42959](https://insights.sbomify.com/CVE-2026-42959/): Crash during DNSSEC validation of malicious content (high 8.7)
- [CVE-2026-82717](https://insights.sbomify.com/CVE-2026-82717/): CNAME synthesis could lead to heap corruption (high 8.4)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-50252 and fix it if it is affected.

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

Affected product: Nlnetlabs Unbound. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-50252/ (Markdown: https://insights.sbomify.com/CVE-2026-50252/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-50252
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-50252
- OSV: https://osv.dev/vulnerability/CVE-2026-50252

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-50252/
Summaries are generated and may contain mistakes; the official description above is the record.