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Fencer Vulns
critical 9.3

CVE-2026-50252 json

Possible cache poisoning attack by mapping source port population per thread

published 2026-07-22 3 mo ago modified 2026-10-24 15 d ago also EUVD-2026-47676

cvss
9.3
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 6%
public exploits
0
none known

Summary

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.

Patch / fix

  • NLnet Labs Unbound 1.25.2
  • unbound Debian 13 1.26.1-0+deb13u1
  • unbound Debian 14 1.25.2-1

debian advisories DEBIAN-CVE-2026-50252

  • unbound Ubuntu 22.04 LTS 1.13.1-1ubuntu5.17
  • unbound Ubuntu 24.04 LTS 1.19.2-1ubuntu3.12
  • unbound Ubuntu 26.04 LTS 1.24.2-1ubuntu2.4
  • unbound Ubuntu Pro 14.04 LTS 1.4.22-1ubuntu4.14.04.3+esm5
  • unbound Ubuntu Pro 16.04 LTS 1.5.8-1ubuntu1.1+esm5
  • unbound Ubuntu Pro 18.04 LTS 1.6.7-1ubuntu2.6+esm6
  • unbound Ubuntu Pro 20.04 LTS 1.9.4-2ubuntu1.11+esm3

ubuntu advisories UBUNTU-CVE-2026-50252

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • unbound RHEL 8 1.16.2-5.14.el8_10.2
  • unbound-libs RHEL 8 1.16.2-5.14.el8_10.2
  • python3-unbound RHEL 8 1.16.2-5.14.el8_10.2
  • unbound RHEL 10.2 1.24.2-7.el10_2.5
  • unbound-libs RHEL 10.2 1.24.2-7.el10_2.5
  • unbound-utils RHEL 10.2 1.24.2-7.el10_2.5
  • unbound-anchor RHEL 10.2 1.24.2-7.el10_2.5
  • unbound-dracut RHEL 10.2 1.24.2-7.el10_2.5

red hat advisories RHSA-2026:69120RHSA-2026:68291RHSA-2026:68292RHSA-2026:43588

  • unbound SLES for SAP 15 SP6, SLES 15 SP6-LTSS, SLE Module for Package Hub 15 SP7 +1 more 1.25.2-150600.23.19.1
  • unbound SLES for SAP 15 SP5, SLES for SAP 15 SP4, SLES 15 SP5-LTSS +6 more 1.25.2-150100.10.28.1
  • unbound Linux Micro 6.0 1.25.2-1.1
  • unbound Linux Micro 6.1 1.25.2-slfo.1.1_1.1
  • unbound SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 1.25.2-160000.1.1

suse advisories SUSE-SU-2026:3885-1SUSE-SU-2026:3884-1SUSE-SU-2026:23360-1SUSE-SU-2026:23349-1SUSE-SU-2026:23050-1SUSE-SU-2026:23215-1

  • unbound openSUSE Leap 16.0 1.25.2-160000.1.1
  • unbound openSUSE Tumbleweed 1.25.2-1.1

opensuse advisories openSUSE-SU-2026:21550-1openSUSE-SU-2026:11380-1

  • unbound AlmaLinux 8 1.16.2-5.14.el8_10.2
  • unbound-libs AlmaLinux 8 1.16.2-5.14.el8_10.2
  • python3-unbound AlmaLinux 8 1.16.2-5.14.el8_10.2
  • unbound AlmaLinux 10 1.24.2-7.el10_2.5
  • unbound-libs AlmaLinux 10 1.24.2-7.el10_2.5
  • unbound-utils AlmaLinux 10 1.24.2-7.el10_2.5
  • unbound-anchor AlmaLinux 10 1.24.2-7.el10_2.5
  • unbound-dracut AlmaLinux 10 1.24.2-7.el10_2.5

almalinux advisories ALSA-2026:69120ALSA-2026:68291ALSA-2026:68292

  • unbound Rocky Linux 8 1.16.2-5.14.el8_10.2
  • unbound Rocky Linux 10 1.24.2-7.el10_2.5
  • unbound Rocky Linux 9 1.24.2-3.el9_8.6

rocky linux advisories RLSA-2026:69120RLSA-2026:68291RLSA-2026:68292

  • unbound Alpine 3.24, Alpine 3.23, Alpine 3.22 1.25.2-r0

alpine advisories ALPINE-CVE-2026-50252

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • unbound Azure Linux 3.0 1.25.2-1

azure linux advisories AZL-92982

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-50252 updated cvss 6.5

unbound: Unbound: DNS cache poisoning via UDP source port predictability

  • Red Hat Enterprise Linux 7 unbound, unbound-libs, unbound-python Red Hat OpenShift Container Platform 4 openshift/ose-rhel-coreos-8 Red Hat OpenShift Container Platform 5 openshift/ose-rhel-coreos-9
    affected: workaround, no fix available
  • Red Hat Enterprise Linux AppStream (v. 8) unbound 1.16.2-5.14.el8_10.2, unbound-libs 1.16.2-5.14.el8_10.2, python3-unbound 1.16.2-5.14.el8_10.2
    fixed: vendor fix RHSA-2026:69120
  • Red Hat Enterprise Linux AppStream (v. 9) unbound 1.24.2-3.el9_8.6, unbound-libs 1.24.2-3.el9_8.6, unbound-dracut 1.24.2-3.el9_8.6, python3-unbound 1.24.2-3.el9_8.6
    fixed: vendor fix RHSA-2026:68292
  • Red Hat Enterprise Linux AppStream (v. 10) unbound 1.24.2-7.el10_2.5, unbound-libs 1.24.2-7.el10_2.5, unbound-utils 1.24.2-7.el10_2.5, unbound-anchor 1.24.2-7.el10_2.5, unbound-dracut 1.24.2-7.el10_2.5, python3-unbound 1.24.2-7.el10_2.5
    fixed: vendor fix RHSA-2026:68291
  • Red Hat Enterprise Linux CodeReady Linux Builder (v. 9) unbound-devel 1.24.2-3.el9_8.6, unbound-debuginfo 1.24.2-3.el9_8.6, unbound-debugsource 1.24.2-3.el9_8.6, unbound-libs-debuginfo 1.24.2-3.el9_8.6, python3-unbound-debuginfo 1.24.2-3.el9_8.6
    fixed: vendor fix RHSA-2026:68292
  • Red Hat Enterprise Linux CodeReady Linux Builder (v. 10) unbound-devel 1.24.2-7.el10_2.5, unbound-debuginfo 1.24.2-7.el10_2.5, unbound-debugsource 1.24.2-7.el10_2.5, unbound-libs-debuginfo 1.24.2-7.el10_2.5, unbound-utils-debuginfo 1.24.2-7.el10_2.5, unbound-anchor-debuginfo 1.24.2-7.el10_2.5, +1 more
    fixed: vendor fix RHSA-2026:68291
  • Red Hat Hardened Images unbound 1.25.2-0.1.hum1, unbound-libs 1.25.2-0.1.hum1, unbound-munin 1.25.2-0.1.hum1, unbound-utils 1.25.2-0.1.hum1, unbound-anchor 1.25.2-0.1.hum1, unbound-dracut 1.25.2-0.1.hum1, +1 more
    fixed: vendor fix RHSA-2026:43588
  • Red Hat OpenShift Container Platform 4.22 rhcos 4.22.9.8.202611032353
    fixed: vendor fix RHSA-2026:95346
show 1 more
  • Red Hat Enterprise Linux 6 unbound, unbound-libs, unbound-python
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
3 mo ago

Exploit prediction

0.17% chance of exploitation in the next 30 days, higher than 6% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 4.0 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
CNA 4.0 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/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Amber
NVD 3.1 9.3 CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:H/A:H

Affected products

  • NLnet Labs Unbound

Classification

Weakness
  • CWE-349 Acceptance of Extraneous Untrusted Data With Trusted Data

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.