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CVE-2026-109125

MEDIUM 5.3

Published 2026-10-21 · Last modified 2026-10-21

In NLnet Labs Unbound up to and including version 1.26.1, an observable cache-level side-channel can be exposed with 'unwanted-reply-threshold > 0' configurations (not a default configuration). If a response-like UDP datagram reaches the upstream source port currently used by a recursive query, but its TXID does not match any pending query, Unbound counts the packet as an unwanted reply. When the accumulated count reaches the configured threshold, Unbound clears the worker's RRset cache and message cache. This cache clear action does not destroy the current upstream UDP pending query. Therefore, an external observer can distinguish whether a candidate port matches the current upstream source port by checking whether an existing cached item has been cleared. This is a cache side-channel vulnerability that can be used to probe the upstream source port. Although the attack is not feasible on default configurations and the suggested value (10 million) makes it an ureliable attack on a public resolver, in scenarios where the attacker can keep a concurrent query window alive, the attacker can first filter the source port and then perform birthday-style guessing against the TXID, making cache poisoning a feasible attack path.

NO EXPLOITATION SIGNALS

No known exploitation, public exploit, or elevated probability at this time. Track for changes.

Exploitation likelihood

0.2%chance of exploitation in 30 days · 7th percentile

○ In CISA KEV ○ Catalogued PoC

Impact if exploited

5.3CVSS 3.1 · MEDIUM

  • ConfidentialityNone
  • IntegrityHigh
  • AvailabilityNone

What an attacker needs

  • ⚠Access: Must sit on the same / adjacent network
  • ✓Privileges: No account or privileges required
  • ✓User interaction: No user interaction needed
  • ⚠Complexity: Needs a race window or specific setup

✓ lowers the bar for an attacker · ⚠ raises it

Proof of concept

No public PoC is currently catalogued.

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Data snapshot:

Affected

Vendors Nlnet Labs

Products Unbound

Weakness (CWE)

  • CWE-205: : Observable Behavioral Discrepancy

CVSS vector

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

References and research data (JSON)

All CVSS metrics

  • MEDIUM 5.3 v3.1 · CNA Primary
    CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N
  • MEDIUM 5.3 v3.1 · NVD Secondary
    CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

References

Advisories

Sources: NVD · CVE.org · EPSS