CVE-2026-55990: nlnet labs unbound Use of Uninitialized Variable

Vulnerability summary

CVE-2026-55990 is a medium (CVSS 5.9) Use of Uninitialized Variable vulnerability in nlnet labs unbound. EPSS estimates a 0.25% chance of exploitation in the next 30 days, higher than 15% of scored vulnerabilities.

Timeline

1. 22-07-2026
   PublishedVulnerability first disclosed.
2. 22-07-2026
   Last modifiedVulnerability information updated.

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').

• CWE-457Use of Uninitialized VariableThe code uses a variable that has not been initialized, leading to unpredictable or unintended results.