Unbound 1.7.0-1.25.1 DNSCrypt Config Buffer Underflow
CVE-2026-55990 Published on July 22, 2026
Packet of death for a DNSCrypt misconfigured Unbound
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').
Timeline
Issue reported by Qifan Zhang
NLnet Labs shares patch 23 days later.
Qifan Zhang verifies patch 3 days later.
Fixes released with version 1.25.2 39 days later.
Weakness Type
Use of Uninitialized Variable
The code uses a variable that has not been initialized, leading to unpredictable or unintended results. In some languages such as C and C++, stack variables are not initialized by default. They generally contain junk data with the contents of stack memory before the function was invoked. An attacker can sometimes control or read these contents. In other languages or conditions, a variable that is not explicitly initialized can be given a default value that has security implications, depending on the logic of the program. The presence of an uninitialized variable can sometimes indicate a typographic error in the code.
Products Associated with CVE-2026-55990
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Affected Versions
NLnet Labs Unbound:- Version 1.7.0 and below 1.25.2 is affected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.