```sh There may be times when you are given a file that does not have an extension or the incorrect extension has been applied to add confusion and misdirection. ! [TOC] . Rating: 5.0 # crcket > Category: Forensics > Description: ``` DarkArmy's openers bagging as many runs as possible for our team. A tag already exists with the provided branch name. chunk IDAT at offset 0x20008, length 65524 And at the start of our file, we did have this : chunk IDAT at offset 0x00057, length 65445 There were several corrupted IDAT chunks so we wrote a script to bruteforce the missing bytes of each chunk. To verify correcteness or attempt to repair corrupted PNGs you can use pngcheck. But to search for other encodings, see the documentation for the -e flag. The rest is specified inside the XML files. You may have to grep for a pattern, decode data, or look for anything that stands out and can be used to find the flag. PDF is an extremely complicated document file format, with enough tricks and hiding places to write about for years. By default, it only checks headers of the file for better performance. ffmpeg -i gives initial analysis of the file content. It's possible, but it would entail identifying every possible byte sequence that might have been . What we thought was: the LENGTH section indicates how many bytes should have been in the chunk in the first place so we compared that value with the actual length of the corrupted image DATA section. Prouvez-lui le contraire en investiguant. |Hexa Values|Ascii Translation| No errors detected in mystery_solved_v1.png (9 chunks, 96.3% compression). It is also extensible using plugins for extracting various types of artifact. This is what is referred to as binary-to-text encoding, a popular trope in CTF challenges. |-|-| checksums, and decompressing the image data); it can optionally dump almost all of the chunk-level information in the image in human-readable form. It was probably transmitted in text mode. Didier Stevens has written good introductory material about the format. To use the tool, simply do the following: This project is licensed under the MIT License - see the LICENSE.md file for details. If one thing doesnt work then you move on to the next until you find something that does work. Most CTF challenges are contained in a zip, 7z, rar, tar or tgz file, but only in a forensics challenge will the archive container file be a part of the challenge itself. Example of searching for the PNG magic bytes in a PNG file: The advantage of hexdump is not that it is the best hex-editor (it's not), but that you can pipe output of other commands directly into hexdump, and/or pipe its output to grep, or format its output using format strings. It was easy to understand we had to repair a PNG file, but first, we checked what we had in our hands. Image file formats are complex and can be abused in many ways that make for interesting analysis puzzles involving metadata fields, lossy and lossless compression, checksums, steganography, or visual data encoding schemes. For more information, please see our A PNG image always starts with those 4 bytes: We count the length of the first IDAT chunk starting from 0x5B, and need to add another extra 4 bytes for the checksum. ! ``` Cookie Notice You signed in with another tab or window. Any challenge to examine and process a hidden piece of information out of static data files (as opposed to executable programs or remote servers) could be considered a Forensics challenge (unless it involves cryptography, in which case it probably belongs in the Crypto category). There are several reasons due to which the PNG file becomes corrupted. Some of the useful commands to know are strings to search for all plain-text strings in the file, grep to search for particular strings, bgrep to search for non-text data patterns, and hexdump. the "cover text"), is extraordinarily rare in the real world (made effectively obsolete by strong cryptography), but is another popular trope in CTF forensics challenges. Long story short, heres what we did next: PS: I know that some of you was wondering how wonderful our script wasso have a good headache after it ;-). title: picoCTF 2019 - [Forensic] c0rrupted (250 points) This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository. Embedded device filesystems are a unique category of their own. A directory named _dog.jpg.extracted has been created with the file automatically unzipped. |**Values (hex)** | **Purpose**| Having the PNG magic number doesn't mean it is a well formed PNG file. xxd allows you to take a file and dump it in a hexadecimal (hex) format. Many hex-editors also offer the ability to copy bytes and paste them as a new file, so you don't need to study the offsets. A popular CTF challenge is to provide a PCAP file representing some network traffic and challenge the player to recover/reconstitute a transferred file or transmitted secret. So I corrected it with `bless` hexa editor. "house.png", 2 0"house02.png" . I have been asked by a few folks what tools I use for CTF's. What I use all depends on what the CTF is. I can't open this file. Since all three of \r\n, \r and \n are translated into \n, you cannot know what code it originally was. I copy pasted it here : Now, we'll discuss more specific categories of forensics challenges, and the recommended tools for analyzing challenges in each category. There is also an online service called PacketTotal where you can submit PCAP files up to 50MB, and graphically display some timelines of connections, and SSL metadata on the secure connections. You may also try zsteg. The file within the zip file is named hidden_text.txt. |-|-| The string THIS IS A HIDDEN FLAG is displayed at the end of the file. Learn why such statements are most of the time meaningless, understand the technical background, and find out which tool you should use as of today. We found this file. But malicious VBA macros are rarely complicated, since VBA is typically just used as a jumping-off platform to bootstrap code execution. Run the following command to install binwalk. Wireshark also has an "Export Objects" feature to extract data from the capture (e.g., File -> Export Objects -> HTTP -> Save all). Fixing the corruption problems Usual tips to uncorrupt a PNG Use an hexadecimal editor like bless,hexeditor,nano with a specific option or many more. Statement of the challenge chunk pHYs at offset 0x00042, length 9: 2852132389x5669 pixels/meter This is a tool I created intended to be used in forensics challenges for CTFs where you are given a corrupted PNG file. With the aforementioned assumption in our mind, we checked if any chunk had an unexpected checksum: pngcheck helped us doing this. Bad news ahead: by opening the image we were greeted by a fantastic 960x600 black image. We use -n 7 for strings of length 7+, and -t x to view- their position in the file. For example, it can be used to print the basic statistics about an image (dimensions, bit depth, etc. In this file, I found and IEND and multiple IDAT chunks name in the hexa values, so at this moment I already knew it was a corrupted PNG picture. Therefore, either the checksum is corrupted, or the data is. Broadly speaking, there are two generations of Office file format: the OLE formats (file extensions like RTF, DOC, XLS, PPT), and the "Office Open XML" formats (file extensions that include DOCX, XLSX, PPTX). In a CTF, you might find a challenge that provides a memory dump image, and tasks you with locating and extracting a secret or a file from within it. ## TL;DR For debugging and detect CRC problem, you can use : pngcheck -v [filename] For solving forensics CTF challenges, the three most useful abilities are probably: The first and second you can learn and practice outside of a CTF, but the third may only come from experience. Challenges incorporate several hacking skills such as web exploitation, reverse engineering, cryptography, and steganography. Some of the PNG chunks must have been corrupted as well then. |`0A`| **A Unix-style line ending (LF) to detect Unix-DOS line ending conversion. Try fixing the file header Then, the challenge says "you will have to dig deeper", so I analyzed the new image that I obtain but was not able to analyze it further. PNG files, in particular, are popular in CTF challenges, probably for their lossless compression suitable for hiding non-visual data in the image. It's worth a look. File is CORRUPTED. Reddit and its partners use cookies and similar technologies to provide you with a better experience. CTF Image Steganography Checklist. Reading a file into a bytearray for processing: What follows is a high-level overview of some of the common concepts in forensics CTF challenges, and some recommended tools for performing common tasks. From the wikipedia [PNG format page](https://en.wikipedia.org/wiki/Portable_Network_Graphics#File_header), everything is explained. Hopefully with this document, you can at least get a good headstart. I noticed that it was not correct ! We can read `0xffa5 bytes`. In some cases, it is possible to fix and recover the corrupt jpeg/jpg, gif, tiff, bmp, png, raw (JPEG, GIF89a, GIF87a, BMP, TIFF, PNG and RAW) file. Plus it will highlight file transfers and show you any "suspicious" activity. Subscribe to our weekly newsletter for the coolest infosec updates: https://weekly.infosecwriteups.com/, Cybersecurity Enthusiast | Cloud Security & Information Protection @ Boeing | Trying to pass on knowledge to others | www.thecyberblog.com. It looks like someone dumped our database. Jeopardy-style capture the flag events are centered around challenges that participants must solve to retrieve the flag. You signed in with another tab or window. Be careful to **select only the data chunk and not the checksum (CRC)** with it ! Are you sure you want to create this branch? ## Hint You may not be looking for a file in the visible filesystem at all, but rather a hidden volume, unallocated space (disk space that is not a part of any partition), a deleted file, or a non-file filesystem structure like an http://www.nirsoft.net/utils/alternate_data_streams.html. There was a problem preparing your codespace, please try again. For images of embedded devices, you're better off analyzing them with firmware-mod-kit or binwalk. You can download the recovered file after repairing it. There will be images associated with each command and tool. Not bad. At first, I analyzed the png file using binwalk command and was able to extract the base 64 string which converted as another file image (base64 to image/file conversion). Running the file command reveals the following: mrkmety@kali:~$ file solitaire.exe solitaire.exe: PNG image data, 640 x 449, 8-bit/color RGBA, non-interlaced. If you were prepared with tools for analyzing the following, you would be prepared for the majority of Forensics challenges: Some of the harder CTF challenges pride themselves on requiring players to analyze an especially obscure format for which no publicly available tools exist. When an image is downloaded as text through FTP (ASCII Mode), each 0x0D 0x0A bytes tuple (\r\n) is truncated to 0x0A. If you are writing a custom image file format parser, import the Python Image Library (PIL) aka Pillow. * Use an hexadecimal editor like `bless`,`hexeditor`,`nano` with a specific option or many more. |-|-| The hardest part of CTF really is reading the flag. We intercepted this image, but it must have gotten corrupted during the transmission. 1642 x 1095 image, 24-bit RGB, non-interlaced Why we see the red compression artifacts so well and what we can do about them. chunk IDAT at offset 0x30008, length 6304 Files-within-files is a common trope in forensics CTF challenges, and also in embedded systems' firmware where primitive or flat filesystems are common. Also, the creator of the challenge give you a hint with the two last letters. If you like this post, consider a small donation. pngcheck -v mystery_solved_v1.png ctf. Your first step should be to take a look with the mediainfo tool (or exiftool) and identify the content type and look at its metadata. chunk IHDR at offset 0x0000c, length 13 The following background is provided for the CTF and I have highlighted some important pieces of information in the description provided. Dig deeper to find what was hidden! corrupt.png.fix: PNG image data, 500 x 408, 8-bit/color RGBA, non-interlaced pngcheck -v corrupt.png.fix File: corrupt.png.fix (469363 . On October 14th and 15th 2022 we participated in the Reply Cyber Security Challenge 2022. [](https://proxy.duckduckgo.com/iu/?u=https%3A%2F%2Fmedia.tenor.com%2Fimages%2F4641449478493d8645990c3794ea7429%2Ftenor.gif&f=1&nofb=1) * For more in depth knowledge about how works chunks in PNG, I strongly recommend you two read my other write-ups that explains a lot of things : Web pages For each test-set there is an html-page containing the PNG images. There are plugins for extracting SQL databases, Chrome history, Firefox history and much more. **| The definition of pHYs is: Pixels per unit, X axis: 4 bytes (unsigned . |-|-| To verify the correctness or attempt to repair corrupted PNGs you can use, You can try to repair corrupted PNGs using online tools like, https://online.officerecovery.com/pixrecovery/. $ pngcheck mystery mystery CRC error in chunk pHYs (computed 38d82c82, expected 495224f0) This tells us the calculated CRC value from the data field, and the current CRC (expected). For EXT3 and EXT4 filesystems, you can attempt to find deleted files with extundelete. Before going further with the challenge details, Id like to quickly summarize how a PNG file actually is. At first you may not have any leads, and need to explore the challenge file at a high-level for a clue toward what to look at next. It enables you to extract frames from animated GIFs or even individual pixels from a JPG it has native support for most major image file formats. Usually they end with a simple: "It generates smaller pictures, so it's got to be better.". - Jongware. zlib: deflated, 32K window, fast compression You can find the length value of what you select in the right bottom corner: Also, if a file contains another file embedded somewhere inside it, the file command is only going to identify the containing filetype. ezgif. Let's see if that fixes the checksum: That fixed the problem, we remain with a "invalid chunk length (too large)" message. Like image file formats, audio and video file trickery is a common theme in CTF forensics challenges not because hacking or data hiding ever happens this way in the real world, but just because audio and video is fun. Gimp is also good for confirming whether something really is an image file: for instance, when you believe you have recovered image data from a display buffer in a memory dump or elsewhere, but you lack the image file header that specifies pixel format, image height and width and so on.