GHSA-35rm-7j9c-2f7m: async-tar PAX extension-header desync enables tar entry/content smuggling

Summary async-tar v0.6.0 mis-applies a buffered PAX size extension to an intermediary extension header (a GNU longname L, a GNU longlink K, or a PAX x/g header) instead of to the next *file* entry. POSIX requires a PAX extended-header record set to describe the next file entry, never an intervening extension header. Because poll_next_raw (src/archive.rs) threads the buffered PAX records into the size computation of whatever raw header it reads next — and that header can be an intermediary L — the stream cursor is advanced by an attacker-chosen amount when the L body is consumed. The parser then desyncs relative to a POSIX-correct tar parser (e.g. GNU tar), reading subsequent bytes at the wrong block boundary. An attacker who can influence a tar stream that an async-tar consumer extracts can construct an x → L → file sequence whose entry list and on-disk result differ between async-tar and a reference parser. This enables content/entry smuggling: a file that a GNU-tar-based scanner/validator/AV sees as benign opaque data is extracted by async-tar as a different file with different bytes (e.g. an executable script), and vice versa. Type confusion / improper validation of the specified quantity (size). CWE-20, CWE-843. Severity assessed Medium, consistent with the same defect class in the upstream tar-rs / tokio-tar lineage. Affected code Package: async-tar (crates.io). Affected version: 0.6.0 (latest release) and current main HEAD. Both lack the extension-header guard. src/archive.rs, poll_next_raw (line numbers from the v0.6.0 tag, commit 45814b19295b7398e119c90c57d8c8bf70a798b6): let file_pos = *next; let mut header = current_header.take().unwrap(); // when pax extensions are available, the size should come from there. let mut size = header.entry_size()?; // the size above will be overriden by the pax data if it has a size field. // same for uid and gid, which will be overridden in the header itself. if let Some(pax_extensions_data) = pax_extensions_data {