wai/CHANGELOG.md

@@ -6,6 +6,12 @@ wire format may change and such changes are recorded here.
 
 ## Unreleased
 
+### A probe no longer panics on two adjacent start codes
+
+- `wai.video.avs3` and `wai.audio.avs3`: a payload with a start code prefix directly after another (`00 00 01 00 00 01`)
+  made the unit split slice backwards and panic, so a hostile payload could stop whatever probed it. It is now the empty
+  unit format-probes §8.2 reads, and the payload is refused. A test now gives every probe 30,000 payloads of hostile bytes.
+
 ### A third party's hop has a signed record: what was sent, what came back, and what the provider bills
 
 - **The third-party hop record** ([extensions/route-record.md](extensions/route-record.md) §6.2, §7.2 step 9),

wai/wai-rs/src/format_probe.rs

@@ -164,6 +164,41 @@ mod tests {
         assert_eq!(probe(CAP_IMAGE_PNG, &[]), Err(Refusal::NoProbe));
     }
 
+    /// A payload is whatever arrives: no probe may panic on it, whatever its bytes.
+    #[test]
+    fn no_bytes_make_a_probe_panic() {
+        let mut x = 0x9e37_79b9_7f4a_7c15u64;
+        let mut next = move || {
+            x ^= x << 13;
+            x ^= x >> 7;
+            x ^= x << 17;
+            x
+        };
+        // What the probes' framing is made of, so that noise reaches past their first checks.
+        const EDGE: [u8; 14] = [0, 0, 0, 1, 2, 3, 0x40, 0x42, 0x44, 0xb0, 0xb3, 0xff, 0x10, 0x7c];
+        const STARTS: [&[u8]; 8] = [
+            b"", b"\x00\x00\x01", b"\x00\x00\x01\x00\x00\x01", b"\x00\x00\x01\xb0", b"\x00\x00\x01\x40\x01",
+            b"\x00\x00\x01\x7c", b"\xff\x10\xff\x50", b"\xc0\x01\x10",
+        ];
+        for round in 0..30_000u32 {
+            let mut b = STARTS[round as usize % STARTS.len()].to_vec();
+            for _ in 0..next() as usize % 128 {
+                let r = next();
+                b.push(if r & 1 == 0 { EDGE[(r >> 8) as usize % EDGE.len()] } else { (r >> 16) as u8 });
+            }
+            for &(c, _) in PROBES {
+                let _ = probe(c, &b);
+            }
+        }
+    }
+
+    #[test]
+    fn adjacent_start_codes_are_refused_and_do_not_panic() {
+        for c in [CAP_VIDEO_AVS3, CAP_AUDIO_AVS3, CAP_VIDEO_HEVC, CAP_VIDEO_VVC, CAP_VIDEO_LCEVC] {
+            assert!(probe(c, b"\x00\x00\x01\x00\x00\x01\xb0\x00\x00\x01").is_err(), "{c}");
+        }
+    }
+
     #[test]
     fn codes_are_distinct_and_read_back() {
         for r in Refusal::ALL {

wai/wai-rs/src/format_probe/avs3.rs

@@ -37,7 +37,9 @@ pub fn units(b: &[u8]) -> Result<Vec<(u8, &[u8])>, Refusal> {
     let mut out = Vec::new();
     for (k, &s) in starts.iter().enumerate() {
         let end = if k + 1 < starts.len() { starts[k + 1] - 3 } else { b.len() };
-        out.push((b[s], &b[s + 1..end]));
+        // A prefix directly after a prefix: that unit's start code is the next prefix's first
+        // byte, and it holds nothing.
+        out.push((b[s], b.get(s + 1..end).unwrap_or(&[])));
     }
     Ok(out)
 }