CVE-2026-55990 Packet of death for a DNSCrypt misconfigured Unbound
An unauthenticated attacker can crash the Unbound DNS server by sending a specific UDP packet if DNSCrypt is enabled with mismatched certificate and key configurations.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-55990?
CVE-2026-55990 is a medium-severity vulnerability in Nlnetlabs Unbound, published on 22 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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').
Weakness: CWE-457
Which versions are affected by CVE-2026-55990?
It affects 1 product.
Products include Nlnetlabs Unbound.
How do I fix CVE-2026-55990?
Apply the vendor's patch.
Is CVE-2026-55990 being exploited?
There is no known exploitation of CVE-2026-55990 in the wild and no public exploit code.
Other vulnerabilities in Nlnetlabs Unbound
- CVE-2026-81642 Heap buffer overflow and possible Remote Code Execution when digesting DNSKEY
- CVE-2026-33278 Possible arbitrary code execution during DNSSEC validation
- CVE-2026-42944 Heap overflow with multiple NSID, COOKIE, PADDING EDNS options
- CVE-2026-42959 Crash during DNSSEC validation of malicious content
- CVE-2026-82717 CNAME synthesis could lead to heap corruption
Sources
Also known as AZL-92973, DEBIAN-CVE-2026-55990, ECHO-b416-47ad-fa4d, GHSA-hpr4-88jh-4pvx, UBUNTU-CVE-2026-55990.