CVE-2026-40691

In Unbound 1.9.0 up to and including 1.25.1, when a DNSCrypt query is received over TCP, the routine that encrypts the reply in place fails to bound the reply length against the destination buffer size. The size clamp that protects the UDP path is not applied on the TCP path, so a reply larger than 65504 bytes is shifted forward by 48 bytes inside a buffer of capacity equal to 'msg-buffer-size', writing past the end of the heap allocation. A single malicious encrypted query crashes the resolver and lead to denial of service. This vulnerability needs Unbound to be compiled with DNSCrypt support ('--enable-dnscrypt') and the 'dnscrypt:' clause to be configured and enabled for the listening interfaces.

CVSS v3 7.5 HIGH

7.5^/10

CVSS v3 : HIGH

Vector :

Exploitability : 3.9 / Impact : 3.6

Attack Vector NETWORK

Attack Complexity LOW

Privileges Required NONE

User Interaction NONE

Confidentiality Impact NONE

Integrity Impact NONE

Availability Impact HIGH

Scope UNCHANGED

References

Link	Resource
https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-40691.txt	Vendor Advisory

Configurations

Configuration 1 (hide)

cpe:2.3:a:nlnetlabs:unbound:*:*:*:*:*:*:*:*

History

24 Jul 2026, 13:57

Type	Values Removed	Values Added
References	~~() https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-40691.txt -~~	() https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-40691.txt - Vendor Advisory
First Time		Nlnetlabs Nlnetlabs unbound
CPE		cpe:2.3:a:nlnetlabs:unbound::::::::

22 Jul 2026, 14:17

Type	Values Removed	Values Added
New CVE

Information

Published : 2026-07-22 14:17

Updated : 2026-07-24 13:57

NVD link : CVE-2026-40691

Mitre link : CVE-2026-40691

CVE.ORG link : CVE-2026-40691

JSON object : View

Products Affected

nlnetlabs

unbound

CWE

CWE-122

Heap-based Buffer Overflow

CWE-787

Out-of-bounds Write

{"id": "CVE-2026-40691", "cveTags": [], "metrics": {"ssvcV203": [{"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "ssvcData": {"id": "CVE-2026-40691", "role": "CISA Coordinator", "options": [{"exploitation": "none"}, {"automatable": "yes"}, {"technicalImpact": "partial"}], "version": "2.0.3", "timestamp": "2026-07-22T14:30:33.327603Z"}}], "cvssMetricV31": [{"type": "Secondary", "source": "sep@nlnetlabs.nl", "cvssData": {"scope": "UNCHANGED", "version": "3.1", "baseScore": 7.5, "attackVector": "NETWORK", "baseSeverity": "HIGH", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H", "integrityImpact": "NONE", "userInteraction": "NONE", "attackComplexity": "LOW", "availabilityImpact": "HIGH", "privilegesRequired": "NONE", "confidentialityImpact": "NONE"}, "impactScore": 3.6, "exploitabilityScore": 3.9}]}, "affected": [{"source": "sep@nlnetlabs.nl", "affectedData": [{"vendor": "NLnet Labs", "product": "Unbound", "versions": [{"status": "affected", "version": "1.9.0", "lessThan": "1.25.2", "versionType": "semver"}], "defaultStatus": "unaffected"}]}], "published": "2026-07-22T14:17:18.437", "references": [{"url": "https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-40691.txt", "tags": ["Vendor Advisory"], "source": "sep@nlnetlabs.nl"}], "vulnStatus": "Analyzed", "weaknesses": [{"type": "Secondary", "source": "sep@nlnetlabs.nl", "description": [{"lang": "en", "value": "CWE-122"}, {"lang": "en", "value": "CWE-787"}]}], "descriptions": [{"lang": "en", "value": "In Unbound 1.9.0 up to and including 1.25.1, when a DNSCrypt query is received over TCP, the routine that encrypts the reply in place fails to bound the reply length against the destination buffer size. The size clamp that protects the UDP path is not applied on the TCP path, so a reply larger than 65504 bytes is shifted forward by 48 bytes inside a buffer of capacity equal to 'msg-buffer-size', writing past the end of the heap allocation. A single malicious encrypted query crashes the resolver and lead to denial of service. This vulnerability needs Unbound to be compiled with DNSCrypt support ('--enable-dnscrypt') and the 'dnscrypt:' clause to be configured and enabled for the listening interfaces."}], "lastModified": "2026-07-24T13:57:32.377", "configurations": [{"nodes": [{"negate": false, "cpeMatch": [{"criteria": "cpe:2.3:a:nlnetlabs:unbound:*:*:*:*:*:*:*:*", "vulnerable": true, "matchCriteriaId": "2ED63F1A-AE65-4D9D-BC7E-1E674484BB80", "versionEndExcluding": "1.25.2", "versionStartIncluding": "1.9.0"}], "operator": "OR"}]}], "sourceIdentifier": "sep@nlnetlabs.nl"}