CVE-2026-32665
Received Received - Intake

Memory Accounting Bypass in Unbound DNS

Publication date: 2026-07-22

Last updated on: 2026-07-22

Assigner: NLnet Labs

Description
In NLnet Labs Unbound 1.22.0 up to and including 1.25.1, when downstream DNS-over-QUIC (DoQ) is enabled, the first two bidirectional streams on a new QUIC connection (stream_id 0 and 4) bypass the per-stream 'quic-size' gate entirely, and large input buffers are allocated later, after only the 2-byte length prefix has been received from the initial streams. As a result, a remote client can make Unbound exceed the configured 'quic-size' limit with low-cost input. Using only one connection and two streams, each sending a declared 65535-byte length prefix and then holding the streams open, a client can already trivially make Unbound roughly allocate double that amount. This is a remote availability issue / memory-accounting bypass in the downstream DoQ implementation that leads to denial of service for new DoQ clients. This vulnerability needs Unbound to be compiled with DoQ support ('--with-libngtcp2') and the 'quic-port' to be configured for the listening interfaces.

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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-2026-32665
EUVD
EUVD-2026-47663

Affected Vendors & Products

Showing 2 associated CPEs
Vendor Product Version / Range
nlnet_labs unbound to 1.25.1 (inc)
nlnet_labs unbound 1.25.2

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-1284 The product receives input that is expected to specify a quantity (such as size or length), but it does not validate or incorrectly validates that the quantity has the required properties.

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Executive Summary

CVE-2026-32665 is a memory accounting bypass in NLnet Labs Unbound DNS software versions 1.22.0 to 1.25.1 when DNS-over-QUIC (DoQ) is enabled. It allows remote clients to bypass the 'quic-size' memory limit by sending large length prefixes on the first two QUIC streams, causing Unbound to allocate double the intended buffer size with minimal effort.

Detection Guidance

To detect this vulnerability, check if Unbound is running with DoQ support and a configured 'quic-port'. Use commands like 'unbound -V' to verify version and features, and 'ss -tulnp | grep quic' to check for active QUIC listeners. Monitor memory usage for unexpected spikes.

Impact Analysis

This vulnerability can lead to a denial of service for new DoQ clients by exhausting memory resources on the Unbound server. Attackers can remotely trigger excessive memory allocation with low-cost input, potentially crashing the service or degrading performance for legitimate users.

Compliance Impact

This vulnerability primarily impacts availability by enabling denial-of-service attacks through memory exhaustion. It does not directly affect data confidentiality or integrity. Compliance standards like GDPR and HIPAA focus on protecting personal and health data, respectively, but this issue could indirectly impact compliance by disrupting DNS services critical for secure communications and logging.

Mitigation Strategies

Immediately upgrade Unbound to version 1.25.2 or apply a manual patch to versions 1.25.1. Disable DoQ support by recompiling without '--with-libngtcp2' or removing 'quic-port' from configuration if not needed.

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