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CVE-2026-40691— Packet of death for DNSCrypt over TCP

Quick assessment

Affected
NLnet Labs Unbound
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

NLnet Labs Unbound是荷兰NLnet Labs组织的域名系统解析服务器。 NLnet Labs Unbound 1.9.0版本至1.25.1版本存在缓冲区错误漏洞，该漏洞源于对回复长度限制不当，导致堆分配写入越界，单个恶意加密查询可能造成解析器崩溃并导致拒绝服务。

CVSS 7.5 · High EPSS 0.46% · P38

Possible ATT&CK Techniques 1 AI

T1498 · Network Denial of Service

Affected Version Matrix 1

VendorProduct Version RangeStatus
NLnet Labs Unbound 1.9.0< 1.25.2 affected
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I. Basic Information for CVE-2026-40691

Vulnerability Information
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Vulnerability Title
Packet of death for DNSCrypt over TCP
Source: CVE Program / CVE List V5
Vulnerability Description
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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
堆缓冲区溢出
Source: CVE Program / CVE List V5
Vulnerability Title
NLnet Labs Unbound 缓冲区错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
NLnet Labs Unbound是荷兰NLnet Labs组织的域名系统解析服务器。 NLnet Labs Unbound 1.9.0版本至1.25.1版本存在缓冲区错误漏洞，该漏洞源于对回复长度限制不当，导致堆分配写入越界，单个恶意加密查询可能造成解析器崩溃并导致拒绝服务。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
NLnet Labs Unbound 1.9.0 ~ 1.25.2 -

II. Public POCs for CVE-2026-40691

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III. Intelligence Information for CVE-2026-40691

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Other References for CVE-2026-40691 (1)

Same Patch Batch · NLnet Labs · 2026-07-22 · 24 CVEs total

CVE-2026-55973 7.5 HIGH 'dns-error-reporting: yes' leads to stack buffer overflow
CVE-2026-32665 7.5 HIGH Remote DNS-over-QUIC denial of service due to `quic-size` budget bypass
CVE-2026-44690 7.5 HIGH Cross-zone wildcard cache poisoning via RRSIG.labels manipulation
CVE-2026-50248 6.5 MEDIUM BOGUS configured primary hostname accepted for XFR in auth/rpz zones
CVE-2026-52863 5.9 MEDIUM Memory corruption could lead to crash and denial of service
CVE-2026-55717 5.9 MEDIUM 'serve-expired-client-timeout' and 'response-ip' CNAME redirect could lead to a crash
CVE-2026-55991 5.9 MEDIUM Remote DNS-over-QUIC (DoQ) flow-control assertion failure in libngtcp2
CVE-2026-50046 5.9 MEDIUM Possible heap use-after-free in an error path when a DoT forwarded query is jostled out
CVE-2026-55990 5.9 MEDIUM Packet of death for a DNSCrypt misconfigured Unbound
CVE-2026-56444 5.9 MEDIUM Degradation of resolution service when 'discard-timeout' and 'serve-expired-client-timeout
CVE-2026-44621 5.9 MEDIUM Libunbound applications configured with 'unwanted-reply-threshold' could eventually be abr
CVE-2026-14586 5.9 MEDIUM Assertion in libngtcp2 when under pressure in high concurrency DNS-over-QUIC environments
CVE-2026-50251 5.3 MEDIUM Attacker supplied '0.0.0.0'/'::' glue triggers defensive full-cache flush
CVE-2026-50045 5.3 MEDIUM 'max-global-quota' reset by DNSSEC validation restarts
CVE-2026-56416 4.8 MEDIUM Possible heap buffer overflow when validator canonicalizes RDATA that contains domain name
CVE-2026-46582 3.7 LOW A wildcard replay, as another piece of data, triggers poisoning in the serve expired reply
CVE-2026-44687 3.7 LOW Off-by-one error in 'harden-below-nxdomain' logic can shadow a stub/forward zone by a legi
CVE-2026-41637 3.7 LOW Degradation of resolution service from improperly accounted client-terminated DNS-over-QUI
CVE-2026-54478 3.7 LOW DNS Cookie bypass when combined with proxy-protocol use
CVE-2026-42955 3.7 LOW Extra fix for CVE-2026-40622 to also clamp the TTL of A/AAAA records disallowing a one-tim

Showing top 20 of 24 CVEs. View all on vendor page &rarr; →

IV. Related Vulnerabilities

Same product: Unbound
Same vendor: NLnet Labs
Same weakness: CWE-122

V. Comments for CVE-2026-40691

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