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CVE-2026-55991 is a denial-of-service vulnerability in NLnet Labs Unbound affecting versions 1.22.0 through 1.25.1. The flaw is reachable through the DNS-over-QUIC (DoQ) code path and stems from incorrect error handling when libngtcp2 reports a stream flow-control condition. Specifically, when ngtcp2_conn_writev_stream() returns NGTCP2_ERR_STREAM_DATA_BLOCKED, Unbound continues by calling ngtcp2_ccerr_set_application_error() with an error value of -1. Because the callee expects a uint64_t error_code, the signed value is implicitly converted to 0xFFFFFFFFFFFFFFFF. During subsequent connection-close processing, libngtcp2 attempts to serialize that value as a QUIC variable-length integer, which exceeds the 62-bit encoding limit and triggers an assertion failure. If assertions are enabled in the build, the entire Unbound resolver process aborts. The issue can be triggered deterministically by a remote unauthenticated client using a single QUIC connection that advertises extremely restrictive stream flow-control parameters and then sends one DoQ query without reading the stream.
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