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