What is a COM comment in a JPEG file, and how can it be used for steganography?

A COM comment is a JPEG marker identified by the bytes 0xFF 0xFE, followed by a length field and arbitrary data. Steganographers can insert a custom COM comment (e.g., 0xFFFF 0x0006 0x11111111) before the DHT marker, and the image will display normally. This technique leverages the JPEG format's tolerance for ignored markers, as detailed in Steganography Techniques - Hiding Payloads Using JPEG File Format. Detection tools like JPEGsnoop can later reveal such hidden annotations.