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CVE-2026-56099 is an out-of-bounds read in OpenBSD's MPLS error-handling path, specifically in mpls_do_error() in sys/netmpls/mpls_input.c. The function parses an incoming MPLS label stack into a fixed-size local array of 16 struct shim_hdr entries. If an attacker sends a crafted MPLS frame containing 16 labels with no Bottom-of-Stack (BoS) bit set, the parsing logic allows nstk to reach the maximum depth of 16. Subsequent code paths then compute a copy length of (nstk + 1) * sizeof(*shim), resulting in an attempted copy of 17 shim headers from a 16-entry local array. This causes a 4-byte over-read of adjacent kernel stack memory. The leaked data is then reflected back to the sender in the generated ICMP/MPLS error response. The vulnerable path is remotely reachable through mpls_input() -> mpls_do_error() on systems with MPLS enabled.
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