icecast: cap fallback capture buffer growth

internal/ingest/adapters/icecast/source.go

@@ -312,12 +312,25 @@ func (s *Source) decodeWithPreference(ctx context.Context, stream io.Reader, met
 	}
 }
 
+// maxCaptureBytes caps the amount of stream data buffered while the native
+// decoder is deciding whether it can handle the format. Without a cap, a
+// decoder that reads extensively before returning ErrUnsupported could grow
+// this buffer unboundedly on a corrupt or adversarial stream.
+const maxCaptureBytes = 1 << 20 // 1 MiB
+
+// errCaptureLimitExceeded is returned by capturingReader when the buffer cap
+// is hit. The caller should treat it like ErrUnsupported and fall back.
+var errCaptureLimitExceeded = errors.New("capture buffer limit exceeded")
+
 type capturingReader struct {
 	r   io.Reader
 	buf bytes.Buffer
 }
 
 func (r *capturingReader) Read(p []byte) (int, error) {
+	if r.buf.Len() >= maxCaptureBytes {
+		return 0, errCaptureLimitExceeded
+	}
 	n, err := r.r.Read(p)
 	if n > 0 {
 		_, _ = r.buf.Write(p[:n])