Unbounded name compression in certain cases causes degradation of service

Summary

CVE	CVE-2026-44390
State	PUBLISHED
Assigner	NLnet Labs
Source Priority	CVE Program / NVD first with legacy fallback
Published	2026-05-20 10:16:28 UTC
Updated	2026-05-20 22:52:27 UTC
Description	NLnet Labs Unbound up to and including version 1.25.0 has a vulnerability when handling replies with very large RRsets that Unbound needs to perform name compression for. Malicious upstream responses with very large RRsets with records that don't share a suffix above the root can cause Unbound to spend a considerable time applying name compression to downstream replies. This can lead to degraded performance and eventually denial of service in well orchestrated attacks. An adversary can exploit the vulnerability by querying Unbound for the specially crafted contents of a malicious zone with very large RRsets. Before Unbound replies to the query it will try to apply name compression which was an unbounded operation that could lock the CPU until the whole packet was complete. A compression limit was introduced in 1.21.1 for this but it didn't account for the case where records would not share any suffix above the root. That causes Unbound to go in a different code path because of the compression tree lookup failure and eventually not increment the compression counter for those operations. Unbound 1.25.1 contains a patch with a fix that increments the compression counter regardless of the compression tree lookup. This is a complement fix to CVE-2024-8508.

Risk And Classification

Primary CVSS: v4.0 6.9 MEDIUM from [email protected]

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:X/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

EPSS: 0.000570000 probability, percentile 0.177890000 (date 2026-05-27)

Problem Types: CWE-407 | CWE-407 CWE-407: Inefficient Algorithmic Complexity

Version	Source	Type	Score	Severity	Vector
4.0	[email protected]	Secondary	6.9	MEDIUM	`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:X/C...`
4.0	CNA	CVSS	6.9	MEDIUM	`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/U:Amber`
3.1	[email protected]	Primary	5.3	MEDIUM	`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L`

CVSS v4.0 Breakdown

Attack Vector

Network

Attack Complexity

Low

Attack Requirements

None

Privileges Required

None

User Interaction

None

Confidentiality

None

Integrity

None

Availability

Low

Sub Conf.

None

Sub Integrity

None

Sub Availability

None

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:X/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

CVSS v3.1 Breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

Low

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

NVD Known Affected Configurations (CPE 2.3)

Type	Vendor	Product	Version	Update	Edition	Language
Application	Nlnetlabs	Unbound	All	All	All	All

Vendor Declared Affected Products

Source	Vendor	Product	Version	Platforms
CNA	NLnet Labs	Unbound	affected 1.25.1 semver	Not specified

References

Reference	Source	Link	Tags
www.nlnetlabs.nl/downloads/unbound/CVE-2026-44390.txt	[email protected]	www.nlnetlabs.nl	Mitigation, Vendor Advisory
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)

Additional Advisory Data

Source	Time	Event
CNA	2026-04-26T00:00:00.000Z	Issue reported by Qifan Zhang
CNA	2026-04-28T00:00:00.000Z	NLnet Labs shares patch
CNA	2026-04-29T00:00:00.000Z	Qifan Zhang verifies patch
CNA	2026-05-20T00:00:00.000Z	Fixes released with version 1.25.1

Solutions

CNA: This issue is fixed starting with version 1.25.1

There are currently no legacy QID mappings associated with this CVE.