CVE-2026-40691
published 2026-07-22CVE-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…
PriorityP342high7.5CVSS 3.1
AVNACLPRNUINSUCNINAH
EPSS
0.46%
37.5th percentile
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.
Affected
2 ranges
| Vendor | Product | Version range | Fixed in |
|---|---|---|---|
| nlnet_labs | unbound | >= 1.9.0 < 1.25.2 | 1.25.2 |
| nlnetlabs | unbound | >= 1.9.0 < 1.25.2 | 1.25.2 |
CVSS provenance
nvdv3.17.5HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
vendor_redhat7.5HIGH
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Red Hat
unbound: Unbound: Denial of Service via crafted DNSCrypt query
vendor_redhat·2026-07-22·CVSS 7.5
CVE-2026-40691 [HIGH] unbound: Unbound: Denial of Service via crafted DNSCrypt query
unbound: Unbound: Denial of Service via crafted DNSCrypt query
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.
A flaw was found in Unboun
GHSA
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 siz
ghsa_unreviewed·2026-07-22
CVE-2026-40691 [HIGH] CWE-122 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 siz
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.
No detection rules found.
No public exploits indexed.
Bugzilla
CVE-2026-40691 unbound: Unbound: Denial of Service via crafted DNSCrypt query [fedora-all]
bugzilla·2026-07-22·CVSS 7.5
CVE-2026-40691 [HIGH] CVE-2026-40691 unbound: Unbound: Denial of Service via crafted DNSCrypt query [fedora-all]
CVE-2026-40691 unbound: Unbound: Denial of Service via crafted DNSCrypt query [fedora-all]
Disclaimer: Community trackers are created by Red Hat Product Security team on a best effort basis. Package maintainers are required to ascertain if the flaw indeed affects their package, before starting the update process.
A denial of service vulnerability has been discovered in Unbound when compiled with DNSCrypt support ('--enable-dnscrypt').
A single bad DNSCrypt query over TCP could underflow Unbound's DNSCrypt packet reading procedure that may lead to heap overflow and eventual server crash and denial of service.
Unbound 1.25.2 includes fixes to select the proper buffer TCP buffer for DNSCrypt, to properly check against the buffer's capacity and to clear the buffer before writing the dnscryp
Bugzilla
CVE-2026-40691 unbound: Unbound: Denial of Service via crafted DNSCrypt query
bugzilla·2026-07-20·CVSS 7.5
CVE-2026-40691 [HIGH] CVE-2026-40691 unbound: Unbound: Denial of Service via crafted DNSCrypt query
CVE-2026-40691 unbound: Unbound: Denial of Service via crafted DNSCrypt query
A denial of service vulnerability has been discovered in Unbound when compiled with DNSCrypt support ('--enable-dnscrypt').
A single bad DNSCrypt query over TCP could underflow Unbound's DNSCrypt packet reading procedure that may lead to heap overflow and eventual server crash and denial of service.
Unbound 1.25.2 includes fixes to select the proper buffer TCP buffer for DNSCrypt, to properly check against the buffer's capacity and to clear the buffer before writing the dnscrypt contents otherwise UDP queries can hit assertions.
2026-07-22
Published