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CVE-2026-55991 is a remote denial-of-service vulnerability in NLnet Labs Unbound affecting versions 1.22.0 through 1.25.1. The flaw is in the DNS-over-QUIC error-handling path when interacting with libngtcp2 under stream flow-control pressure. If ngtcp2_conn_writev_stream() returns NGTCP2_ERR_STREAM_DATA_BLOCKED, Unbound incorrectly 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 integer is implicitly converted to 0xFFFFFFFFFFFFFFFF. During subsequent connection-close processing, libngtcp2 attempts to serialize that value as a QUIC variable-length integer, but the value exceeds the 62-bit maximum supported by the encoding. In builds where assertions are enabled, this causes an assertion failure in libngtcp2 and aborts the entire Unbound process. The issue can be triggered remotely and deterministically by a single unauthenticated DNS-over-QUIC client that advertises extremely restrictive stream flow-control parameters and sends one DNS query without reading the stream.
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