CVE-2026-55990 Details
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').
A denial-of-service vulnerability has been identified in NLnet Labs Unbound versions 1.7.0 prior to 1.25.1. The issue arises when the 'dnscrypt:' clause specifies more 'dnscrypt-provider-cert:' files than there are corresponding 'dnscrypt-secret-key:' files. In such cases, Unbound only populates the matched entries and leaves the remaining slots filled with '0xdb' bytes, a default allocation from libsodium's allocator. Unbound then processes the number of certificate files instead of the actual filled slots, leading to the use of a garbage entry. This allows any unauthenticated client to send a UDP datagram of 68 bytes or more, with the first eight bytes being '0xdb', to the 'dnscrypt-port'. This exploitation causes a dereference of the garbage data, resulting in a server crash. The vulnerability remains undetected until triggered by a suitable client query. Additionally, Unbound must be compiled with DNSCrypt support ('--enable-dnscrypt') for the vulnerability to be exploitable.
Users can upgrade to Unbound version 1.25.2, which includes a fix for this vulnerability. Alternatively, for those using Unbound 1.25.1, a patch is available that can be applied manually. After applying the patch, Unbound can be recompiled and installed.
Metrics
CVSS 4.0 Severity and Vector Strings:
No CVSS 4.0 data is available for this CVE.
CVSS 3.x Severity and Vector Strings:
No data available for CVSS Version 2.0 on this CVE.
CISA-ADP
Assessed Jul 22, 2026References to Advisories, Solutions, and Tools
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| URL | Source(s) | Tag(s) |
|---|---|---|
| https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55990.txt | [email protected] | Vendor Advisory |
Weakness Enumeration
| CWE-ID | CWE Name | Source |
|---|---|---|
| CWE-457 | Use of Uninitialized Variable | [email protected] |
Affected Products
| Product | Versions |
|---|---|
| nlnetlabs unbound | >= 1.7.0, < 1.25.2 |
CPE
Remediation
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Change History
3 change records found show changes
| Date | Action | Recorded By |
|---|---|---|
| Jul 24, 2026 | Initial Analysis | [email protected] |
| Jul 22, 2026 | CVE Modified | CISA-ADP |
| Jul 22, 2026 | New CVE Received | [email protected] |