Insufficient archive validation can cause out-of-bounds reads in archives containing Rc/Arc

Summary

Shared pointer validation keyed already-validated pointees by their address and type, but did not include pointer metadata. For unsized pointees, an archive could therefore contain multiple `Rc`, `Arc`, or weak pointers that shared a data address but used different metadata, such as different slice lengths.

Once the first pointer had been validated, later pointers to the same address skipped pointee validation. The safe checked `rkyv::access` API could consequently return a slice with a forged length, allowing safe indexing to read out of bounds. The same validation bypass was reachable through checked deserialization with `rkyv::from_bytes`.

Version 0.8.17 includes pointer metadata in shared pointer validation and rejects conflicting metadata. The 0.7 series is also affected but is no longer supported by upstream. Users who process untrusted archives should upgrade to 0.8.17 or later.