CVE-2026-50252
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedIn 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceIn Unbound 1.4.22-1.25.1, SO_REUSEPORT causes UDP source ports to be partitioned into disjoint subsets per worker thread. Since the kernel's load balancing deterministically assigns queries to threads based on source IP/port, the source port in outgoing queries reveals which thread handled the original request. This undermines source port entropy and enables DNS cache poisoning attacks.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data>= 1.4.22, < 1.25.2CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Adjacent
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Changed
- Confidentiality
- None
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:H/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Check Unbound versionRun 'unbound -V' or 'unbound-control status' to retrieve the installed version numberAffected if Version is >= 1.4.22 and < 1.25.2
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Locate unbound.confFind the Unbound configuration file, typically at /etc/unbound/unbound.conf or /etc/unbound.confAffected if Configuration file exists and the installation is vulnerable based on version check
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Check SO_REUSEPORT settingSearch the configuration file for 'so-reuseport:' directive. If present, note its value; if absent, the default 'yes' appliesAffected if so-reuseport is not explicitly set to 'no' (default is enabled)
You are affected if Unbound version is between 1.4.22 and 1.25.1 inclusive and SO_REUSEPORT is enabled (default) in your configuration.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped1.25.2
Disable so-reuseport in unbound.conf or upgrade to a patched version when available. This removes the per-thread port partitioning that leaks thread identity.
1.25.2
- 1. Backup your current Unbound configuration (typically in /etc/unbound/ or /etc/unbound/unbound.conf)
- 2. Check your current Unbound version by running: unbound -V
- 3. Upgrade Unbound to version 1.25.2 or later using your package manager (e.g., apt-get install unbound=1.25.2, yum install unbound-1.25.2, or compile from source)
- 4. After upgrade, verify the new version: unbound -V
- 5. Restart the Unbound service to load the new version: systemctl restart unbound or service unbound restart
- 6. Verify the service is running: systemctl status unbound
- 7. Confirm the fix is applied by checking that SO_REUSEPORT behavior no longer leaks thread information in source port selection
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation8.0 h
- Testing4.0 h
- Review / QA2.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2026-50252 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-50252 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
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