It seems to stop reading at the PNG IDAT chunk even if there is data beyond it, which is allowed by the spec. See this Exiftool Forum post. It supports Windows XP and higher. If you have a particular PNG chunk type in mind, you can look here to see what support PyPNG provides for it. In order to make much use of it, you will have to be at least somewhat familiar with the internal format of PNG files. The IDAT Chunk . Within the PNG file format (we’ll focus on true-color PNG files rather than indexed) the IDAT chunk stores the pixel information. PNG: Chunk by Chunk¶ The PNG specification defines 18 chunk types. For now we'll assume that pixels are always stored as 3 bytes representing the RGB color channels. The compressed datastream is then the concatenation of the contents of the data fields of all the 'fdAT' chunks within a frame. It’s in this chunk that we’ll store the PHP shell. Interlacd PNG are encoded in a way that the users feel the the image is loaded faster. PNG file format basics. 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. See Summary of standard chunks in PNG Specification. So when we should wait till we meet IEND chunk before we decode the IDAT chunk. IDAT chunk can be split into multiple chunks. TweakPNG is a low-level utility for examining and modifying PNG image files. 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. PNG file format basics. Compression. At least one 'fdAT' chunk is required for each frame. This document is intended to help users who are interested in a particular PNG chunk type. For now we’ll assume that pixels are always stored as 3 bytes representing the RGB color channels. Within the PNG file format (we'll focus on true-color PNG files rather than indexed) the IDAT chunk stores the pixel information. 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. It's in this chunk that we'll store the PHP shell. There are 4 kinds of critical chunk and 14 kinds of ancillary chunk. 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). The 'fdAT' chunk has the same purpose as an 'IDAT' chunk. The four-byte chunk type field contains the decimal values 73 68 65 84. How hard could it be, right? It has the same structure as an 'IDAT' chunk, except preceded by a sequence number. PNG:CreationTime may not show up properly when written by exiftool. 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