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](/news/steganography-techniques-hiding-payloads-using-jpeg-file-format). Detection tools like JPEGsnoop can later reveal such hidden annotations.