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CVE Record

CVE-2026-44390: Unbounded name compression in certain cases causes degradation of service

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.

HighCVSS 7.5Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

Unbound DNS resolvers up to and including 1.25.0 can be forced into expensive name-compression work when processing maliciously large DNS record sets. A coordinated attack could degrade resolver performance or cause denial of service. The issue affects availability, not data confidentiality or integrity.

Executive priority

Treat as high priority for DNS infrastructure. This can impair service availability without compromising data, but DNS resolver outages can cascade into application incidents across the business.

Technical view

The flaw is an unbounded operation in Unbound name compression for replies containing very large RRsets whose records do not share a suffix above the root. A prior compression counter limit missed this code path. Unbound 1.25.1 fixes it by incrementing the compression counter regardless of compression tree lookup outcome.

Likely exposure

Organizations running NLnet Labs Unbound as recursive or validating DNS infrastructure are the main exposure. Public, partner-facing, or large internal resolvers carry higher business impact because resolver saturation can disrupt dependent applications and users.

Exploitation context

The sources describe network-reachable, unauthenticated exploitation through queries that cause Unbound to retrieve specially crafted malicious zone content. KEV is false, and the provided sources do not cite active exploitation in the wild.

Researcher notes

This complements CVE-2024-8508. Focus analysis on the compression tree lookup failure path and whether downstream vendor builds include the 1.25.1 counter fix. Do not assume exploit activity beyond the source evidence.

Mitigation direction

  • Upgrade Unbound to 1.25.1 or a vendor package containing the fix.
  • Apply Red Hat security updates where Unbound is supplied by Red Hat.
  • Prioritize resolvers reachable from untrusted or broad client networks.
  • Check NLnet Labs and OS vendor guidance for deployment-specific mitigations.
  • Monitor resolver CPU, latency, and timeout rates during rollout.

Validation and detection

  • Inventory all Unbound instances and record deployed versions.
  • Confirm any backported package explicitly addresses CVE-2026-44390.
  • Identify resolvers accepting queries from public or broad internal networks.
  • Review resolver telemetry for sustained CPU spikes or degraded DNS response times.
  • Verify change management records show patched resolver builds in production.
Prepared
Confidence
high
Sources
6

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

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cwe · low confidence lookup

CWE-1050: Exact CWE lookup

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cwe · low confidence lookup

CWE-407: Exact CWE lookup

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cve · low confidence lookup

CVE-2026-44390 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

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Vulnerability profileCVE Program record
Severity
High
CVSS
7.5 (3.1)
Known Exploited
No
Published

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

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

2CVSS vectors
9Timeline events
2ADP providers
6Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: yesTechnical Impact: partial

CVSS vector scores

2 official scores

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
7.5CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H3.93.6redhat-SADP
6.9CVSS 4.0MediumCVSS: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——NLnet Labs

Vulnerability scoring details

Base CVSS 4.0 score

6.9Medium
CVSS 4.0 vector shape for CVE-2026-44390Attack VectorAttack ComplexityAttack RequirementsPrivileges RequiredUser InteractionVS ConfidentialityVS IntegrityVS AvailabilitySS ConfidentialitySS IntegritySS Availability

Vector: 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

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Attack Requirements
NonePresent
Privileges Required
NoneLowHigh
User Interaction
NonePassiveActive
VS Confidentiality
HighLowNone
VS Integrity
HighLowNone
VS Availability
HighLowNone
SS Confidentiality
HighLowNone
SS Integrity
HighLowNone
SS Availability
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. Source timelineNLnet Labs

    Issue reported by Qifan Zhang

  2. Source timelineNLnet Labs

    NLnet Labs shares patch

  3. Source timelineNLnet Labs

    Qifan Zhang verifies patch

  4. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  5. Source timelineNLnet Labs

    Fixes released with version 1.25.1

  6. ADP timelineredhat-SADP

    Made public.

  7. CVE publishedCVE Program

    The CVE record was published.

  8. ADP timelineredhat-SADP

    Reported to Red Hat.

  9. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc
redhat-SADPunbound: Unbound: Denial of Service due to excessive resource consumption with large DNS Resource Record Sets
other:Red Hat severity ratingcvssV3_1
  • 2026-05-20T10:01:12.753Z: Reported to Red Hat.
  • 2026-05-20T09:21:24.590Z: Made public.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
NLnet LabsUnbound0unaffected
Weakness

CWE details

CWE-1050CWE-407

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-1050 · source CWE mapping

Excessive Platform Resource Consumption within a Loop

Excessive Platform Resource Consumption within a Loop represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs
CWE-407 · source CWE mapping

Inefficient Algorithmic Complexity

Inefficient Algorithmic Complexity represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs