An IPv6 FlowSpec validation bypass in the CZ.NIC BIRD Internet Routing Daemon allows an established BGP peer to inject FlowSpec rules matching destinations outside the address space authorized by the unicast-route validation procedure. With validation enabled on the receiving flow6 channel, BIRD fails to reject a destination prefix component with a nonzero offset, contrary to RFC 8956 section 5. It validates a reconstructed prefix with zero-filled skipped bits, although the original FlowSpec component can match a broader set of IPv6 addresses. Given a suitable unicast route from the peer, the rule can pass validation and, where export policy permits, be propagated to downstream peers. Downstream enforcement may cause unauthorized traffic filtering or redirection.
An established BGP peer must be able to send IPv6 FlowSpec to a receiving flow6 channel configured with "validate yes". A suitable unicast route from that peer must be present in the base table. Propagation additionally requires an export policy permitting the rule. Data-plane impact depends on downstream support for, acceptance of, and enforcement of the FlowSpec match and action.
Apply upstream commit 7cf75587b6f38b87849fe192bf5dfd124b7702bc or a verified branch-specific backport. The fix requires the original IPv6 destination-prefix component to have offset zero before it can pass FlowSpec validation. The first fixed release has not yet been verified.