Vulnerability · Published Jul 22, 2026 · Assigned by NLnet Labs
CVE-2026-55990Packet of death for a DNSCrypt misconfigured Unbound
Medium 5.9
In plain English
A medium-severity security flaw in Unbound from NLnet Labs. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 16% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
In NLnet Labs Unbound 1.7.0 up to and including 1.25.1, when the 'dnscrypt:' clause lists more 'dnscrypt-provider-cert:' files than there are matching 'dnscrypt-secret-key:' files, Unbound fills only the matched prefix and leaves the tail slots at the '0xdb' fill that libsodium's allocator writes into every allocation. Unbound would then iterate over the number of cert files, not the actual slots, so it walks into a slot with garbage data filled with '0xdb' bytes. Any unauthenticated client that sends one UDP datagram of ≥ 68 bytes whose first 8 bytes are '0xdb' to 'dnscrypt-port' will use that garbage entry which leads to a garbage dereference killing the server. This is a silent faulty configuration that goes unnoticed until triggered with the right client query. Unbound needs to be compiled with DNSCrypt support ('--enable-dnscrypt').
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | unbound | no fix listed | DEBIAN-CVE-2026-55990 |
| Debian:13 | unbound | 1.26.1-0+deb13u1 | DEBIAN-CVE-2026-55990 |
| Debian:14 | unbound | 1.25.2-1 | DEBIAN-CVE-2026-55990 |
| Ubuntu:22.04:LTS | unbound | no fix listed | UBUNTU-CVE-2026-55990 |
| Ubuntu:24.04:LTS | unbound | no fix listed | UBUNTU-CVE-2026-55990 |
| Ubuntu:26.04:LTS | unbound | no fix listed | UBUNTU-CVE-2026-55990 |
| Ubuntu:26.10 | unbound | no fix listed | UBUNTU-CVE-2026-55990 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| NLnet Labs | Unbound | 1.7.0 – before 1.25.2 |
Weakness: CWE-457 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
References
- https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55990.txt vendor-advisory