See Summary of standard chunks in PNG Specification. It seems to stop reading at the PNG IDAT chunk even if there is data beyond it, which is allowed by the spec. PNG compression method 0 (the only compression method presently defined for PNG) specifies deflate/inflate compression with a sliding window of at most 32768 bytes. The compressed datastream is then the concatenation of the contents of the data fields of all the 'fdAT' chunks within a frame. For now we'll assume that pixels are always stored as 3 bytes representing the RGB color channels. The IDAT chunk contains the actual image data which is the output stream of the compression algorithm. A valid PNG image must contain an IHDR chunk, one or more IDAT chunks, and an IEND chunk. PNG:CreationTime may not show up properly when written by exiftool. So when we should wait till we meet IEND chunk before we decode the IDAT chunk. If you have a particular PNG chunk type in mind, you can look here to see what support PyPNG provides for it. For now we’ll assume that pixels are always stored as 3 bytes representing the RGB color channels. The 'fdAT' chunk has the same purpose as an 'IDAT' chunk. It has the same structure as an 'IDAT' chunk, except preceded by a sequence number. IDAT contains the image, which may be split among multiple IDAT chunks. chunk IDAT at offset 0x150008, length 45027 chunk IDAT at offset 0x15aff7, length 138 chunk IEND at offset 0x15b08d, length 0 No errors detected in sctf.png (28 chunks, 36.8% compression). Within the PNG file format (we'll focus on true-color PNG files rather than indexed) the IDAT chunk stores the pixel information. This document is intended to help users who are interested in a particular PNG chunk type. After reading fin1te’s post on “An XSS on Facebook via PNGs & Wonky Content Types“, and idontplaydarts’ post on “Encoding Web Shells in PNG IDAT chunks“, I figured it would be useful to create my own. At least one 'fdAT' chunk is required for each frame. IDAT chunk can be split into multiple chunks. PNG: Chunk by Chunk¶ The PNG specification defines 18 chunk types. See this Exiftool Forum post. There are 4 kinds of critical chunk and 14 kinds of ancillary chunk. It supports Windows XP and higher. How hard could it be, right? PNG file format basics. In order to make much use of it, you will have to be at least somewhat familiar with the internal format of PNG files. It’s in this chunk that we’ll store the PHP shell. It's in this chunk that we'll store the PHP shell. The IDAT Chunk . PNG file format basics. Within the PNG file format (we’ll focus on true-color PNG files rather than indexed) the IDAT chunk stores the pixel information. 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