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CVE-2026-50252

Possible cache poisoning attack by mapping source port population per thread

No known exploitation. EPSS puts it in the 6th percentile. Fixed in 1.25.2.

CVSS 3.19.3CriticalEPSS percentile6th0.2% chance in 30 daysCISA KEVNot listedNo confirmed exploitation
No public exploit knownFix1.25.2Upgrade to this build

Published Updated Sources: CVE.org, NVD

NVD CVE.org

What to do

Find your product and version, then upgrade to the build on the right.

  • NLnet Labs UnboundAffected: ≥ 1.4.22, < 1.25.2Upgrade to: 1.25.2Stated by: CVE record
  • Nlnetlabs UnboundAffected: ≥ 1.4.22, < 1.25.2Upgrade to: 1.25.2Stated by: NVD

Versions as the sources state them. Full version matrix after sign-in

Steps

Written by AI from the record
  1. Check your Unbound version and confirm whether you have UDP SO_REUSEPORT enabled (commonly via so-reuseport: yes in the Unbound configuration).
  2. If Unbound is 1.25.1 or older AND SO_REUSEPORT is enabled, plan an upgrade immediately.
  3. Upgrade Unbound to version 1.25.2 (this is the fix).
  4. After upgrading, restart Unbound and re-verify the running version and the so-reuseport setting.
Vendor advisory Usually a quick update
Check it yourselfA free OpenVAS check exists for this CVE. Run it against your hosts.Is your organisation exposed?Request an assessment to establish which checks fit your systems.

What it is

From the CVE record

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.

In plain language

Written by AI from the record

CVE-2026-50252 is an Unbound DNS resolver issue that can let an attacker make DNS cache poisoning more feasible in some setups; if you run Unbound and can be reached with DNS traffic, you should update to 1.25.2.

In unbound, CVE-2026-50252 can enable cache poisoning by undermining intended UDP source-port randomness when SO_REUSEPORT is enabled, letting an attacker infer which worker thread handled a query and then reduce the effective random port space used for upstream DNS responses.

If you're affected

  • DNS cache poisoning risk
  • Traffic redirection/misdirection
  • Service instability from attacks
  • Potential fraud via poisoned DNS

Exploitation

Where each signal puts this CVE on the scale from published to confirmed exploited.

EPSS6th
CISA KEV

Not in the catalog. CISA has not confirmed exploitation.

Public exploits

No public exploit or proof of concept found in the sources we track.

EPSS

0.2% chance of exploitation activity in the next 30 days, which ranks it in the 6th percentile of scored CVEs.

Exploit Prediction Scoring System, FIRST.org. A probability, not a confirmation.

Lifecycle

9 events over 7 days, from the signal feeds we watch.

  1. OpenVAS check added
  2. Patch availablerecord updated
  3. Patch availablepatch available, published, weakness classified, record updated, record updated

Affected products

Technical detail

CVSS 3.1 vector

Open in the CVSS calculator

CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:H/A:H

Scored 9.3 by NVD.

How it is reached

  • Attack Vector AdjacentRequires access to the local network (e.g. same Wi-Fi, Bluetooth)
  • Attack Complexity LowNo special conditions — the attack can be reliably reproduced
  • Privileges Required NoneNo authentication required — anyone can exploit this
  • User Interaction NoneNo user interaction needed — fully automated exploitation

Scope

  • Scope ChangedThe exploit can affect other components (e.g. sandbox escape, host compromise from VM)

Impact if exploited

  • Confidentiality NoneNo confidentiality impact
  • Integrity HighTotal loss of integrity — attacker can modify any data in the component
  • Availability HighTotal denial of service — the component is completely unavailable

Weaknesses

Sources

References in the record

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We'll flag the next CVE, public exploit or patch for Unbound, not every advisory.