Interlacd PNG are encoded in a way that the users feel the the image is loaded faster. 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. 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 IDAT chunk contains the actual image data, which is the output stream of the compression algorithm. If you have a particular PNG chunk type in mind, you can look here to see what support PyPNG provides for it. The IDAT chunk contains the actual image data which is the output stream of the compression algorithm. Such splitting increases filesize slightly, but makes it possible to generate a PNG in a streaming manner. See this Exiftool Forum post. Within the PNG file format (we’ll focus on true-color PNG files rather than indexed) the IDAT chunk stores the pixel information. It seems to stop reading at the PNG IDAT chunk even if there is data beyond it, which is allowed by the spec. It supports Windows XP and higher. See Summary of standard chunks in PNG Specification. If you're curious about the filtering and compression on PNG images check out Filtering and Compression. So when we should wait till we meet IEND chunk before we decode the IDAT chunk. Compression. 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. At least one 'fdAT' chunk is required for each frame. The IDAT Chunk . IDAT chunk can be split into multiple chunks. It has the same structure as an 'IDAT' chunk, except preceded by a sequence number. The compressed datastream is then the concatenation of the contents of the data fields of all the 'fdAT' chunks within a frame. There are 4 kinds of critical chunk and 14 kinds of ancillary chunk. PNG file format basics. IDAT contains the image, which may be split among multiple IDAT chunks. The 'fdAT' chunk has the same purpose as an 'IDAT' 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). PNG file format basics. 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. How hard could it be, right? It's in this chunk that we'll store the PHP shell. This document is intended to help users who are interested in a particular PNG chunk type. The four-byte chunk type field contains the decimal values 73 68 65 84. PNG:CreationTime may not show up properly when written by exiftool. TweakPNG is a low-level utility for examining and modifying PNG image files. 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