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NVD / CVE Program· CNA NLnet Labs

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

Noroxi assessment

Priority: Monitor (action score 22/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 0.17% (FIRST EPSS).
  • Someone on the same network segment can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
  • Fixed version: NLnet Labs Unbound 1.25.2; azl3 unbound 1.25.1-1 on Azure Linux 3.0 1.25.2-1.

Low priority for now. Upgrade to NLnet Labs Unbound 1.25.2; azl3 unbound 1.25.1-1 on Azure Linux 3.0 1.25.2-1 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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.

MediumCVSS 5.7 · v4.0—No exploit Fix available
Published
Jul 22, 2026
Updated
Oct 24, 2026
EPSS
0.1% · 6th percentile

Report tools

JSON

Action score

22

Monitor

Low priority for now.

CVSS
22 / 40 · 5.7 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.1%

CISA SSVC decision

Exploitation
none
Automatable
no
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · NLnet Labs
5.7
NVD has not scored this yet; the score shown is the CNA’s.

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

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

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Follow CVE-2026-50252

When you'll hear from us

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  • when exploit code appears or is weaponized
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  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
nlnetlabsunbound

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • nlnetlabs unbound1.4.22 and later · before 1.25.2

Versions reported by the vendor

Affected version ranges reported by the assigning authority (NLnet Labs). Independent of NVD's CPE analysis and usually ahead of it.

  • NLnet Labs Unbound

    • 1.4.22 and later · before 1.25.2affected · semver

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
AlmaLinux:10python3-unboundbefore 1.24.2-7.el10_2.51.24.2-7.el10_2.5
AlmaLinux:10unboundbefore 1.24.2-7.el10_2.51.24.2-7.el10_2.5
AlmaLinux:10unbound-anchorbefore 1.24.2-7.el10_2.51.24.2-7.el10_2.5
AlmaLinux:10unbound-develbefore 1.24.2-7.el10_2.51.24.2-7.el10_2.5
AlmaLinux:10unbound-dracutbefore 1.24.2-7.el10_2.51.24.2-7.el10_2.5
AlmaLinux:10unbound-libsbefore 1.24.2-7.el10_2.51.24.2-7.el10_2.5
AlmaLinux:10unbound-utilsbefore 1.24.2-7.el10_2.51.24.2-7.el10_2.5
AlmaLinux:8python3-unboundbefore 1.16.2-5.14.el8_10.21.16.2-5.14.el8_10.2
AlmaLinux:8unboundbefore 1.16.2-5.14.el8_10.21.16.2-5.14.el8_10.2
AlmaLinux:8unbound-develbefore 1.16.2-5.14.el8_10.21.16.2-5.14.el8_10.2
AlmaLinux:8unbound-libsbefore 1.16.2-5.14.el8_10.21.16.2-5.14.el8_10.2
AlmaLinux:9python3-unboundbefore 1.24.2-3.el9_8.61.24.2-3.el9_8.6
AlmaLinux:9unboundbefore 1.24.2-3.el9_8.61.24.2-3.el9_8.6
AlmaLinux:9unbound-develbefore 1.24.2-3.el9_8.61.24.2-3.el9_8.6
AlmaLinux:9unbound-dracutbefore 1.24.2-3.el9_8.61.24.2-3.el9_8.6
AlmaLinux:9unbound-libsbefore 1.24.2-3.el9_8.61.24.2-3.el9_8.6
Alpine:v3.22unboundfrom 1.4.22 · before 1.25.2-r01.25.2-r0
Debian:12unboundfrom 0—
Debian:13unboundbefore 1.26.1-0+deb13u11.26.1-0+deb13u1
Debian:14unboundbefore 1.25.2-11.25.2-1
openSUSE:Leap 16.0unboundbefore 1.25.2-160000.1.11.25.2-160000.1.1
openSUSE:Tumbleweedunboundbefore 1.25.2-1.11.25.2-1.1
Red Hat:enterprise_linux:10.2python3-unboundbefore 0:1.24.2-7.el10_2.50:1.24.2-7.el10_2.5
Red Hat:enterprise_linux:10.2python3-unbound-debuginfobefore 0:1.24.2-7.el10_2.50:1.24.2-7.el10_2.5

+44

Same product

nlnetlabs: all records

Other highest-scoring records for the same primary product.

  • CVE-2019-25042Unbound before 1.9.5 allows an out-of-bounds write via a compressed name in rdata_copy.
    40Plan
  • CVE-2019-25039Unbound before 1.9.5 allows an integer overflow in a size calculation in respip/respip.c.
    40Plan
  • CVE-2019-25038Unbound before 1.9.5 allows an integer overflow in a size calculation in dnscrypt/dnscrypt.c.
    40Plan

Published before and after this one in the same product

  • ← EarlierCVE-2026-50248Jul 22, 2026 · BOGUS configured primary hostname accepted for XFR in auth/rpz zones
    26Monitor
  • ← EarlierCVE-2026-50251Jul 22, 2026 · Attacker supplied '0.0.0.0'/'::' glue triggers defensive full-cache flush
    21Monitor
  • Later →CVE-2026-52863Jul 22, 2026 · Memory corruption could lead to crash and denial of service
    23Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
NLnet Labs Unbound1.25.2Vendor (CNA)
azl3 unbound 1.25.1-1 on Azure Linux 3.01.25.2-1 · CBL-Mariner ReleasesMicrosoft (MSRC)
almalinux:python3-unbound1.24.2-7.el10_2.5 · AlmaLinux:10Package registry (OSV)
almalinux:unbound1.24.2-7.el10_2.5 · AlmaLinux:10Package registry (OSV)
almalinux:unbound-anchor1.24.2-7.el10_2.5 · AlmaLinux:10Package registry (OSV)
almalinux:unbound-devel1.24.2-7.el10_2.5 · AlmaLinux:10Package registry (OSV)
almalinux:unbound-dracut1.24.2-7.el10_2.5 · AlmaLinux:10Package registry (OSV)
almalinux:unbound-libs1.24.2-7.el10_2.5 · AlmaLinux:10Package registry (OSV)
almalinux:unbound-utils1.24.2-7.el10_2.5 · AlmaLinux:10Package registry (OSV)
alpine:unbound1.25.2-r0 · Alpine:v3.22Package registry (OSV)
debian:unbound1.26.1-0+deb13u1 · Debian:13Package registry (OSV)
opensuse:unbound1.25.2-1.1 · openSUSE:TumbleweedPackage registry (OSV)
red hat:python3-unbound0:1.25.2-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:python3-unbound-debuginfo0:1.24.2-7.el10_2.5 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:unbound0:1.25.2-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-anchor0:1.24.2-7.el10_2.5 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:unbound-anchor0:1.25.2-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-anchor-debuginfo0:1.24.2-7.el10_2.5 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:unbound-debuginfo0:1.24.2-7.el10_2.5 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:unbound-debugsource0:1.24.2-7.el10_2.5 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:unbound-devel0:1.25.2-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-dracut0:1.24.2-7.el10_2.5 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:unbound-dracut0:1.25.2-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-libs0:1.24.2-7.el10_2.5 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:unbound-libs0:1.25.2-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-libs-debuginfo0:1.24.2-7.el10_2.5 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:unbound-munin0:1.25.2-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:unbound-utils0:1.24.2-7.el10_2.5 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:unbound-utils0:1.25.2-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
rocky linux:unbound0:1.24.2-7.el10_2.5 · Rocky Linux:10Package registry (OSV)

+1

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

  • CVE-2026-4296063 days apartPossible cache poisoning via promiscuous records for the authority section
    22Monitor

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Someone on the same network segment can trigger it.
  • No account or password is required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
Integrity
Availability
Attack vector
Adjacent
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
X

Weakness class (CWE)

CWE-349 · Acceptance of Extraneous Untrusted Data With Trusted Data

CVSS vector

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-secondary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records