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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

A remote attacker can make vulnerable Unbound DNS resolvers consume excessive CPU by inducing processing of unusually large DNS record sets. Coordinated abuse may slow or stop DNS resolution, disrupting applications and users that depend on the resolver. The issue affects Unbound through 1.25.0; version 1.25.1 contains the vendor’s fix.

Executive priority

Treat remediation as high priority for production DNS infrastructure because resolver exhaustion can interrupt many dependent services simultaneously. Patch exposed and shared resolvers first. There is no supplied evidence of active exploitation, so this does not currently warrant an emergency designation based on exploitation status alone.

Technical view

Unbound’s DNS name-compression counter was not incremented after certain compression-tree lookup failures. Large RRsets whose records share no suffix above the DNS root could therefore trigger unbounded compression work while Unbound prepared downstream replies. The result is remotely induced CPU exhaustion and denial of service. This complements the earlier CVE-2024-8508 correction.

Likely exposure

Exposure is greatest where Unbound through 1.25.0 performs recursive resolution and can query attacker-controlled DNS zones. Shared, externally reachable, or business-critical resolvers present greater operational impact. The supplied structured affected-version field is inconsistent, but the narrative and vendor information explicitly identify versions through 1.25.0.

Exploitation context

The CVSS 3.1 score is 7.5: network-accessible, low complexity, requiring neither privileges nor user interaction, with availability impact only. An attacker must influence upstream DNS content using a malicious zone containing very large RRsets. The CVE is not listed in KEV, and the supplied sources do not confirm exploitation in the wild.

Researcher notes

The flaw is CWE-1050/CWE-407 resource exhaustion caused by an incomplete compression bound. The correction increments the compression counter regardless of compression-tree lookup outcome. Availability is the stated impact; no confidentiality or integrity impact is identified. The bundle’s affected array contains an anomalous version value of “0,” so version conclusions should follow the explicit advisory narrative.

Mitigation direction

  • Upgrade NLnet Labs Unbound to version 1.25.1 or later.
  • For packaged distributions, verify and apply the vendor update containing the backported fix.
  • Prioritize shared, externally reachable, and business-critical recursive resolvers.
  • Monitor vendor advisories if immediate upgrading is operationally blocked.

Validation and detection

  • Inventory deployed Unbound versions and identify installations at 1.25.0 or earlier.
  • Confirm version 1.25.1 or a documented vendor backport is installed.
  • Verify package advisories or changelogs explicitly reference CVE-2026-44390.
  • Review resolver CPU saturation, latency, timeouts, and service interruptions for abnormal patterns.
Prepared
Confidence
high
Sources
4

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

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cwe · low confidence lookup

CWE-1050: Exact CWE lookup

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Open ATT&CK lookup
cwe · low confidence lookup

CWE-407: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

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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
9Source 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