NLnet Labs Unbound up to 1.25.1 Varint serialization ngtcp2_put_uvarintlen error_code input validation

Summaryinfo

A vulnerability described as critical has been identified in NLnet Labs Unbound up to 1.25.1. This vulnerability affects the function ngtcp2_put_uvarintlen of the component Varint serialization. Such manipulation of the argument error_code leads to input validation. This vulnerability is uniquely identified as CVE-2026-55991. The attack can be launched remotely. No exploit exists.

Detailsinfo

A vulnerability was found in NLnet Labs Unbound up to 1.25.1 and classified as critical. Affected by this issue is the function ngtcp2_put_uvarintlen of the component Varint serialization. The manipulation of the argument error_code with an unknown input leads to a input validation vulnerability. Using CWE to declare the problem leads to CWE-20. The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly. Impacted is availability. CVE summarizes:

In NLnet Labs Unbound 1.22.0 up to and including 1.25.1, a remote unauthenticated client can trigger a libngtcp2 assertion (if compiled with assertions on) and terminate the entire Unbound process using a single DNS-over-QUIC (DoQ) connection and one normal DNS query. This is caused by an erroneous error value passed to libngtcp2. When 'ngtcp2_conn_writev_stream()' returns 'NGTCP2_ERR_STREAM_DATA_BLOCKED', Unbound continues to call 'ngtcp2_ccerr_set_application_error()' with a '-1' error value. The 'int' literal '-1' is implicitly converted to the function's 'uint64_t error_code' parameter as '0xFFFFFFFFFFFFFFFF'. The follow-on 'ngtcp2_conn_write_connection_close()' serialises that value as a QUIC variable-length integer; because '2^64-1' exceeds the 62-bit varint ceiling, 'ngtcp2_put_uvarintlen()' fails 'assert(n < 4611686018427387904ULL)' and the whole resolver process aborts. A remote, unauthenticated DoQ client can trigger this deterministically with a single QUIC connection by advertising 'initial_max_stream_data_bidi_local = 1' in its transport parameters and sending one DoQ query without ever reading the stream.

The advisory is shared for download at nlnetlabs.nl. This vulnerability is handled as CVE-2026-55991 since 06/22/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. There are known technical details, but no exploit is available.

Upgrading to version 1.25.2 eliminates this vulnerability.

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Productinfo

Vendor

• NLnet Labs

Name

• Unbound

Version

• 1.25.0
• 1.25.1

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CPE 2.2info

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