Degradation of resolution service when 'discard-timeout' and 'serve-expired-client-timeout' are combined in unusual configuration
Summary
| CVE | CVE-2026-56444 |
|---|---|
| State | PUBLISHED |
| Assigner | NLnet Labs |
| Source Priority | CVE Program / NVD first with legacy fallback |
| Published | 2026-07-22 14:17:22 UTC |
| Updated | 2026-07-22 20:33:11 UTC |
| Description | In NLnet Labs Unbound 1.20.0 up to and including 1.25.1, when Unbound is configured with 'serve-expired: yes' and 'serve-expired-client-timeout > discard-timeout > 0' (contrary to the suggested values), the discard-timeout branch during the serve expired logic drops an aged client reply without performing the correct accounting for the number of reply addresses for the query. Other identical branches outside of serve expired perform the correct decrement. Since the counter is never decremented in such scenario, it can reach the maximum limit and new clients for duplicate in-flight queries are silently dropped resulting in degradation of resolution service. A malicious actor can exploit the vulnerability by querying the resolver for a client-controlled slow-on-demand authoritative zone that can drive the counter past the threshold. Shipped defaults for 'serve-expired-client-timeout: 1800' and 'discard-timeout: 1900' make the branch unreachable. |
Risk And Classification
Primary CVSS: v3.1 5.9 MEDIUM from [email protected]
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
Problem Types: CWE-772 | CWE-772 CWE-772: Missing Release of Resource after Effective Lifetime
| Version | Source | Type | Score | Severity | Vector |
|---|---|---|---|---|---|
| 3.1 | [email protected] | Secondary | 5.9 | MEDIUM | CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H |
| 3.1 | CNA | CVSS | 5.9 | MEDIUM | CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H |
CVSS v3.1 Breakdown
Attack Vector
NetworkAttack Complexity
HighPrivileges Required
NoneUser Interaction
NoneScope
UnchangedConfidentiality
NoneIntegrity
NoneAvailability
HighCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
Vendor Declared Affected Products
| Source | Vendor | Product | Version | Platforms |
|---|---|---|---|---|
| CNA | NLnet Labs | Unbound | affected 1.20.0 1.25.2 semver | Not specified |
References
| Reference | Source | Link | Tags |
|---|---|---|---|
| www.nlnetlabs.nl/downloads/unbound/CVE-2026-56444.txt | [email protected] | www.nlnetlabs.nl | |
| CVE Program record | CVE.ORG | www.cve.org | canonical |
| NVD vulnerability detail | NVD | nvd.nist.gov | canonical, analysis |
Vendor Comments And Credit
Discovery Credit
CNA: Qifan Zhang (Palo Alto Networks) (en)
CNA: Xin Wang (Northwestern Polytechnical University) (en)
CNA: Jiapeng Li (Northwestern Polytechnical University) (en)
CNA: Jiajia Liu (Northwestern Polytechnical University) (en)
CNA: Haruki Oyama (Waseda University) (en)
Additional Advisory Data
| Source | Time | Event |
|---|---|---|
| CNA | 2026-05-02T00:00:00.000Z | Issue reported by Qifan Zhang |
| CNA | 2026-05-26T00:00:00.000Z | NLnet Labs shares patch with Qifan Zhang |
| CNA | 2026-05-27T00:00:00.000Z | Qifan Zhang verifies patch |
| CNA | 2026-06-04T00:00:00.000Z | Issue reported by Haruki Oyama |
| CNA | 2026-06-06T00:00:00.000Z | NLnet Labs shares patch with Haruki Oyama |
| CNA | 2026-06-06T00:00:00.000Z | Haruki Oyama verifies patch |
| CNA | 2026-06-25T00:00:00.000Z | Issue reported by Xin Wang |
| CNA | 2026-06-26T00:00:00.000Z | NLnet Labs shares patch with Xin Wang |
| CNA | 2026-06-26T00:00:00.000Z | Xin Wang verifies patch |
| CNA | 2026-07-22T00:00:00.000Z | Fixes released with version 1.25.2 |
Solutions
CNA: This issue is fixed starting with version 1.25.2
There are currently no legacy QID mappings associated with this CVE.