CVE-2026-55990
Received Received - Intake

Memory Corruption in Unbound DNS Server

Publication date: 2026-07-22

Last updated on: 2026-07-22

Assigner: NLnet Labs

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').

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Meta Information

Published
2026-07-22
Last Modified
2026-07-22
Generated
2026-07-22
AI Q&A
2026-07-22
EPSS Evaluated
N/A
NVD
CVE-2026-55990
EUVD
EUVD-2026-47682

Affected Vendors & Products

Showing 3 associated CPEs
Vendor Product Version / Range
nlnet_labs unbound to 1.25.1 (inc)
nlnet_labs unbound From 1.7.0 (inc) to 1.25.1 (inc)
nlnet_labs unbound 1.25.2

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Exploitability

CWE
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KEV
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CWE ID Description
CWE-457 The code uses a variable that has not been initialized, leading to unpredictable or unintended results.

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Executive Summary

This vulnerability affects Unbound DNS software versions 1.7.0 through 1.25.1 when compiled with DNSCrypt support. It occurs when the 'dnscrypt:' clause has more 'dnscrypt-provider-cert:' files than matching 'dnscrypt-secret-key:' files. Unbound fills unmatched slots with '0xdb' bytes from libsodium's allocator. An attacker can send a UDP datagram of at least 68 bytes starting with '0xdb' to the DNSCrypt port, causing a garbage dereference that crashes the server.

Detection Guidance

Check Unbound configuration for mismatched 'dnscrypt-provider-cert' and 'dnscrypt-secret-key' files. Monitor for crashes when receiving UDP datagrams starting with '0xdb' on the DNSCrypt port. Review logs for garbage dereference errors.

Impact Analysis

This vulnerability allows an unauthenticated attacker to crash the Unbound DNS server by sending a specially crafted UDP datagram. This results in a denial of service (DoS) where the DNS service becomes unavailable, disrupting network operations and internet access for users relying on the server.

Compliance Impact

This vulnerability does not directly affect compliance with GDPR or HIPAA as it is a denial-of-service issue in Unbound DNS software. However, service disruption caused by crashes could impact availability requirements under these regulations if DNS resolution is critical for compliance.

Mitigation Strategies

Upgrade Unbound to version 1.25.2 or later. If upgrading is not possible, manually verify that 'dnscrypt-provider-cert' and 'dnscrypt-secret-key' files are correctly paired. Disable DNSCrypt support if unused by recompiling without '--enable-dnscrypt'.

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