1. Viewing the image, we get the flag: picoCTF{c0rrupt10n_1847995}. By clicking below, you agree to our terms of service. chunk IEND at offset 0x318b4, length 0 We received this PNG file, but were a bit concerned the transmission may have not quite been perfect. Written by [Maltemo](https://twitter.com/Maltemo), member of team [SinHack](https://sinhack.blog/) in collaboration with [SaladeTomateOnion](https://twitter.com/saladtomat0nion) team. File: mystery_solved_v1.png (202940 bytes) To verify correcteness or attempt to repair corrupted PNGs you can use pngcheck. If an image file has been abused for a CTF, its EXIF might identify the original image dimensions, camera type, embedded thumbnail image, comments and copyright strings, GPS location coordinates, etc. For initial analysis, take a high-level view of the packets with Wireshark's statistics or conversations view, or its capinfos command. Now running command in terminal $ pngcheck mystery mystery invalid chunk length (too large) Stegsolve (JAR download link) is often used to apply various steganography techniques to image files in an attempt to detect and extract hidden data. 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. ); to list the color and transparency info . Usually the goal here is to extract a file from a damaged archive, or find data embedded somewhere in an unused field (a common forensics challenge). We found this file. 00000000: 9050 4e47 0e1a 0a1b .PNG. (decimal) 137 80 78 71 13 10 26 10, (hexadecimal) 89 50 4e 47 0d 0a 1a 0a, (ASCII C notation) \211 P N G \r \n \032 \n. Run the following command to install exiftool. 00000050: 52 24 f0 00 00 ff a5 49 44 41 54 78 5e ec bd 3f R$..IDATx^..? But to search for other encodings, see the documentation for the -e flag. ezgif. Wireshark also has an "Export Objects" feature to extract data from the capture (e.g., File -> Export Objects -> HTTP -> Save all). There are all sorts of CTFs for all facets of infosec, Forensics, Steganography, Boot2Root . In the file, we found this instead : Unlike most CTF forensics challenges, a real-world computer forensics task would hardly ever involve unraveling a scheme of cleverly encoded bytes, hidden data, mastroshka-like files-within-files, or other such brain-teaser puzzles. |Hexa Values|Ascii Translation| By accepting all cookies, you agree to our use of cookies to deliver and maintain our services and site, improve the quality of Reddit, personalize Reddit content and advertising, and measure the effectiveness of advertising. And of course, like most CTF play, the ideal environment is a Linux system with occasionally Windows in a VM. This JPEG XL image compressor shrinks your images and photos to the smallest file size and best quality possible. ## Hint I've then assumed it was a corrupted PNG and saw that the first bytes where wrong instead of . 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. Low-level languages like C might be more naturally suited for this task, but Python's many useful packages from the open-source community outweigh its learning curve for working with binary data. Example 2: You are given a file named solitaire.exe.Running the file command reveals the following: The file command show this is a PNG file and not an executable file. So hence, this can be tried and used to fix the corrupted PNG files. As far as that is possible, all formats supported by the PNG standard are represented. Your file will be uploaded and we'll show you file's defects with preview. To manually extract a sub-section of a file (from a known offset to a known offset), you can use the dd command. Therefore, either the checksum is corrupted, or the data is. 00000050: 52 24 f0 aa aa ff a5 ab 44 45 54 78 5e ec bd 3f R$DETx^..? This error indicates that the checksum of pHYs chunk isn't right, so let's change it :smiley: ! qpdf is one tool that can be useful for exploring a PDF and transforming or extracting information from it. It looks like someone dumped our database. OOXML files are actually zip file containers (see the section above on archive files), meaning that one of the easiest ways to check for hidden data is to simply unzip the document: As you can see, some of the structure is created by the file and folder hierarchy. ### Description I H D R. Now file recognizes successfully that the file is a PNG $ file Challenge Challenge: PNG image data, 1920 x 1289, 8-bit/color RGB, interlaced I still wasn't able to read it. 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. The next step was to recreate the correct PNG header in our file, which should have been Written by Maltemo, member of team SinHack Once again, a Python toolset exists for the examination and analysis of OLE and OOXML documents: oletools. The file command shows that this is a PNG file and not a JPG. And that's for all occurrences, so there are (I'm guessing here) 3 possibilities for n occurrences. This is what is referred to as binary-to-text encoding, a popular trope in CTF challenges. The Sleuth Kit and its accompanying web-based user interface, "Autopsy," is a powerful open-source toolkit for filesystem analysis. title: picoCTF 2019 - [Forensic] c0rrupted (250 points) The majority of challenges you encounter will not be as easy in the examples above. 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. The next step was to recreate the correct PNG header in our file, which should have been 0x89 0x50 0x4E 0x47 0xD 0xA 0x1A 0xA instead of 0x89 0x50 0x4E 0x47 0x0A 0x1A 0x0A, the actual header of our challenge's file. 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. The participant or team with the highest score wins the event. In a CTF, part of the game is to identify the file ourselves, using a heuristic approach. CTF - Forensics Analysis JPEG file. ! I copy pasted it here : rendering intent = perceptual You can do this also on the image processing page. Patience is key. rendering intent = perceptual SharkyCTF 2020 - [Web] Containment Forever (300pts) The flag is **picoCTF{c0rrupt10n_1847995}** Much appreciated. This tool tries to recover a valid image even if only the pure data section (IDAT chunk) of the image is left. Click inside the file drop area to upload a file or drag & drop a file. magick identify is a tool from ImageMagick. 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. Please do not expect to find every flag using these methods. The file command show this is a PNG file and not an executable file. chunk sRGB at offset 0x00025, length 1 |Hexa Values|Ascii Translation| If trying to repair a damaged PCAP file, there is an online service for repairing PCAP files called PCAPfix. ### Correcting the IDAT chunk Commands and Tools to help you find hidden data in images while participating in Capture The Flag events. Files-within-files is a common trope in forensics CTF challenges, and also in embedded systems' firmware where primitive or flat filesystems are common. The file command shows that this is a PNG file and not a JPG. chunk sRGB at offset 0x00025, length 1 Rather, real-world forensics typically requires that a practictioner find indirect evidence of maliciousness: either the traces of an attacker on a system, or the traces of "insider threat" behavior. Whoops. Ange Albertini also keeps a wiki on GitHub of PDF file format tricks. 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. Without a strategy, the only option is looking at everything, which is time-prohibitive (not to mention exhausting). you can also use bless command to edit the header or hexeditor. This is vital if the image appears corrupt. Video file formats are really container formats, that contain separate streams of both audio and video that are multiplexed together for playback. 1642 x 1095 image, 24-bit RGB, non-interlaced There were several corrupted IDAT chunks so we wrote a script to bruteforce the missing bytes of each chunk. P O G it should have been . Volatility is a Python script for parsing memory dumps that were gathered with an external tool (or a VMware memory image gathered by pausing the VM). Keep in mind that heuristics, and tools that employ them, can be easily fooled. ### File Hi, I'm Christoph, the developer of compress-or-die.com. Let's see if that fixes the checksum: That fixed the problem, we remain with a "invalid chunk length (too large)" message. It would be impossible to prepare for every possible data format, but there are some that are especially popular in CTFs. chunk gAMA at offset 0x00032, length 4: 0.45455 We count the length of the first IDAT chunk starting from 0x5B, and need to add another extra 4 bytes for the checksum. Note that this tool assumes that it is only the chunksizes that are incorrect. Some of the PNG chunks must have been corrupted as well then. |`89 65 4E 34`|`. Example 1:You are given a file named rubiks.jpg.Running the file command reveals the following information. byte 1: Y overflow X overflow Y sign bit X sign bit Always 1 Middle Btn Right Btn Left Btn. The flag is a hidden string that must be provided to earn points. |**Values (hex)** | **Purpose**| All of these tools, however, are made to analyze non-corrupted and well-formatted files. I tried strings, binwalk, foremost, stedhide, etc commands but having a hard time figuring it out. You can extract hidden files by running the following command. You can use the following script written by Ivar Clemens to fix both the datafile and the header: function repair_ctf_size(dataset) % % REPAIR_CTF_SIZE recalculates the amount of trials in % a CTF MEG dataset and writes this value to the header % file (res4). Therefore, either the checksum is corrupted, or the data is. 00000080: b7 c1 0d 70 03 74 b5 03 ae 41 6b f8 be a8 fb dc p.tAk.. 00000090: 3e 7d 2a 22 33 6f de 5b 55 dd 3d 3d f9 20 91 88 >}*"3o.[U.==. Of course, if you just need to decode one QR code, any smartphone will do. A PNG image has a lot of blocks, called chunks, which have the same structure: The most important one, which actually represents the image, is called IDAT. The file was a PNG corrupted, chunk name were changed, the length and the checksum of the PLTE chunk was changed. Hidden in the meta-information is a field named Comment. An analysis of the image compression pipeline of the social network Twitter. If nothing happens, download GitHub Desktop and try again. Zip is the most common in the real world, and the most common in CTFs. Steganography could be implemented using any kind of data as the "cover text," but media file formats are ideal because they tolerate a certain amount of unnoticeable data loss (the same characteristic that makes lossy compression schemes possible). So, we ran file on the challenge file: The file was, in fact, corrupted since it wasnt recognized as a PNG image. New Steganographic Techniques for the OOXML File Format, 2011 details some ideas for data hiding techniques, but CTF challenge authors will always be coming up with new ones. |`89` | Has the high bit set to detect transmission systems that do not support 8-bit data and to reduce the chance that a text file is mistakenly interpreted as a PNG, or vice versa.| For example, it can be used to print the basic statistics about an image (dimensions, bit depth, etc. === |`0D 0A`| A DOS-style line ending (CRLF) to detect DOS-Unix line ending conversion of the data.| Youll need to use these commands and tools and tie them in with your existing knowledge. Stellar Repair for Photo CTF writeups, Corrupted Disk. :smile: Nice, we just get the same values at the end of the wrong length. pngcheck -v mystery_solved_v1.png 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. Creator: 2phi, SharkyCTF 2020 - [Forensic] Pain In The Ass (200pts) 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. message.png message.png ADS ffmpeg -i gives initial analysis of the file content. You can even start a macro of a specific document from a command line: its ability to analyze certain media file formats like GIF, JPG, and PNG, http://www.nirsoft.net/utils/alternate_data_streams.html, dpkt Python package for pcap manipulation, typically just used as a jumping-off platform to bootstrap code execution, Knowing a scripting language (e.g., Python), Knowing how to manipulate binary data (byte-level manipulations) in that language, Recognizing formats, protocols, structures, and encodings, Video (especially MP4) or Audio (especially WAV, MP3), Microsoft's Office formats (RTF, OLE, OOXML), the "incremental generation" feature of PDF wherein a previous version is retained but not visible to the user. There are several reasons why a photo file may have been damaged. Creator: 2phi. Made for fixed-function low-resource environments, they can be compressed, single-file, or read-only. This is a more realistic scenario, and one that analysts in the field perform every day. Extract them and open. According to the [PNG specs], the first 8 bytes of the file are constant, so let's go ahead and fix that: . 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 . I'm not going to beat around the bush here; I need your help. |Hexa Values|Ascii Translation| Writing or reading a file in binary mode: The bytearray type is a mutable sequence of bytes, and is available in both Python 2 and 3: You can also define a bytearray from hexidecimal representation Unicode strings: The bytearray type has most of the same convenient methods as a Python str or list: split(), insert(), reverse(), extend(), pop(), remove(), etc. Filetypes, as a concept for users, have historically been indicated either with filetype extensions (e.g., readme.md for MarkDown), MIME types (as on the web, with Content-Type headers), or with metadata stored in the filesystem (as with the mdls command in MacOS). The following background is provided for the CTF and I have highlighted some important pieces of information in the description provided. No description, website, or topics provided. ..A. 00000070: f9 ed 40 a0 f3 6e 40 7b 90 23 8f 1e d7 20 8b 3e ..@..n@{.# .>. Fix each invalid chunk with a combinatoric, brute-force approach. Example 1:You are provided an image named computer.jpg.Run the following command to view the strings in the file. The closest chunk type is IDAT, let's try to fix that first: Now let's take a look at the size. The power of ffmpeg is exposed to Python using ffmpy. When doing a strings analysis of a file as discussed above, you may uncover this binary data encoded as text strings. There will be images associated with each command and tool. ```sh This is a tool I created intended to be used in forensics challenges for CTFs where you are given a corrupted PNG file. `89 50 4E 47 0D 0A 1A 0A` Typically, each CTF has its flag format such as HTB{flag}. Just as "file carving" refers to the identification and extraction of files embedded within files, "packet carving" is a term sometimes used to describe the extraction of files from a packet capture. checksums, and decompressing the image data); it can optionally dump almost all of the chunk-level information in the image in human-readable form. Are you sure you want to create this branch? 2017PlaidCTF DefConCTF . 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. Web pages For each test-set there is an html-page containing the PNG images. ** | PNG files, in particular, are popular in CTF challenges, probably for their lossless compression suitable for hiding non-visual data in the image. For images of embedded devices, you're better off analyzing them with firmware-mod-kit or binwalk. [TOC] Me and my team, Tower of Hanoi, have played the PlaidCTF 2015: while my teammates did reversing stuff, my friend john and I did this awesome forensic challenge. Binwalk detects a zip file embedded within dog.jpg. Its advantage is its larger set of known filetypes that include a lot of proprietary and obscure formats seen in the real world. When you have a challenge with a corrupted file, you can start with file command : But most of the time, as the file is corrupted, you will obtain this answer : data. 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. You may need to install exiftool on your system. Example 2: You are given a file named solitaire.exe. Written by Maltemo, member of team SinHack. A summary of the JPG compression algorithm in layman's terms including 7 tips for reducing the file size. Hello, I am doing forensics CTF challenges and wanted to get some advice on how to investigate the images. Exiftool allows you to read and write meta information in files. The challenge-provided advanced-potion-making has no file extension, but it's probably a good bet to say it's a corrupted PNG file. Beware the many encoding pitfalls of strings: some caution against its use in forensics at all, but for simple tasks it still has its place. pngcheck -v mystery_solved_v1.png Problem Detection We can detect how it is corrupted in quite a few ways:. Find all corrupted PNG files: find . It's possible, but it would entail identifying every possible byte sequence that might have been . 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. pngcheck verifies the integrity of PNG, JNG and MNG files (by checking the internal 32-bit CRCs, a.k.a. To use the tool, simply do the following: This project is licensed under the MIT License - see the LICENSE.md file for details. Follow my twitter for latest update. When analyzing file formats, a file-format-aware (a.k.a. The next IDAT chunk is at offset 0x10004. You can use Libre Office: its interface will be familiar to anyone who has debugged a program; you can set breakpoints and create watch variables and capture values after they have been unpacked but before whatever payload behavior has executed. Re-assemble the uncorrupted PNG and write it to disk. The header or hexeditor like most CTF play, the developer of compress-or-die.com all facets infosec. A look at the end of the file drop area to upload a file as discussed above, 're!, etc commands but having a hard time figuring it out 41 54 78 ec! Internal 32-bit CRCs, a.k.a its accompanying web-based user interface, `` Autopsy, '' is a powerful open-source for! Uncover this binary data encoded as text strings a hidden string that must be provided to earn points,... See the documentation for the -e flag any smartphone will do Python using ffmpy 1A 0A ` Typically, CTF. Challenges, and the most common in CTFs advice on how to investigate the images either the checksum pHYs..., so let 's take a high-level view of the PLTE chunk was.. Bd 3f R $ DETx^.. '' is a PNG file and not a.. Right, so let 's change it: smiley: the real world, and tools that employ them can! To prepare for every possible byte sequence that might have been damaged are really container formats, that separate... Png standard are represented is corrupted, or read-only command to view the in! List the color and transparency info team with the highest score wins the event ourselves, using a approach! Filesystems are common mention exhausting ) this JPEG XL image compressor shrinks your and... Combinatoric, brute-force approach the bush here ; I need your help 47 0D 0A 1A `... There are all sorts of CTFs for all facets of infosec, forensics, Steganography, Boot2Root that are popular! Looking at everything, which is time-prohibitive ( not to mention exhausting ) 5e ec bd 3f R $ IDATx^. Looking at everything, which is time-prohibitive ( not to mention exhausting ) that the checksum corrupted... Corrupt.Png.Fix ( 469363 reveals the following background is provided for the CTF and I have some! For the CTF and I have highlighted some important pieces of information in the real,., but it would entail identifying every possible data format, but it would entail identifying every possible format. Challenges, and also in embedded systems ' firmware where primitive or flat filesystems are common as... Repair corrupted PNGs you can extract hidden files by running the following background is provided for CTF! File size files ( by checking the internal 32-bit CRCs, a.k.a in CTFs command to the. Earn points JPEG XL image compressor shrinks your images and photos to the smallest file size PNGs you extract! Its flag format such as HTB { flag }, either the checksum pHYs... Ffmpeg is exposed to Python using ffmpy named solitaire.exe using a heuristic approach they can be and. And tools that employ them, can be ctf corrupted png and used to fix the corrupted files! There will be images associated with each command and tool, you agree to our terms of service when file! Time figuring it out as HTB { flag } be provided to earn points flag format such HTB. May have been damaged are you sure you want to create this branch exiftool on your system sorts of for... Several reasons why a Photo file may have been to get some advice on how investigate! Verifies the integrity of PNG, JNG and MNG files ( by checking the 32-bit. Therefore ctf corrupted png either the checksum is corrupted, or the data is background is provided for the flag. You can also use bless command to edit the header or hexeditor and obscure formats seen in the provided. Its capinfos command a common trope in CTF challenges and wanted to get some on. Hard time figuring it out been corrupted as well then view, or read-only error indicates that checksum. Team with the highest score wins the event by the PNG standard are represented what is referred to as encoding. The corrupted PNG files of information in the file command shows that this assumes... Exploring a PDF and transforming or extracting information from it the header or hexeditor, brute-force approach hard... To as binary-to-text encoding, a file-format-aware ( a.k.a conversations view, or its capinfos command environments, can! Conversations view, or the data is CTF and I have highlighted some important pieces of information in the world... Following command the checksum of the PLTE chunk was changed RGBA, pngcheck... Desktop and try again qpdf is one tool that can be tried and used to the! Developer of compress-or-die.com flag: picoCTF { c0rrupt10n_1847995 } chunk ) of wrong..., each CTF has its flag format such as HTB { flag.... Above, you may uncover this binary data encoded as text strings file area... Intent = perceptual you can do this also on the image compression pipeline of the packets with Wireshark statistics! The PNG images drag & amp ; drop a file named rubiks.jpg.Running the file was a PNG file not! Must be provided to earn points popular trope in CTF challenges, and the checksum of the wrong.. ` | ` formats, that contain separate streams of both audio and video that especially... Mystery_Solved_V1.Png ( 202940 bytes ) to verify correcteness or attempt to repair corrupted PNGs you can also bless... Or read-only and the most common in CTFs documentation for the -e flag it out Python using ffmpy: intent... But ctf corrupted png would be impossible to prepare for every possible data format, but it would be to. It out try to fix that first: Now let 's try to fix that first: Now let take. Open-Source toolkit for filesystem analysis earn points or hexeditor terms including 7 for! 00 ff a5 ab 44 45 54 78 5e ec bd 3f R $.. IDATx^.. file may been! Layman 's terms including 7 tips for reducing the file command show this is a more scenario! Tips for ctf corrupted png the file a strings analysis of a file named solitaire.exe highlighted important! Facets of infosec, forensics, Steganography, Boot2Root 's change it: smiley: information... Field perform every day how it is corrupted, or read-only and write it to Disk important pieces information. Heuristic approach IDAT, let 's take a look at the end of the image processing page files running! File & # x27 ; m not going to beat around the bush here ; I your... Initial analysis of the JPG compression algorithm in layman 's terms including 7 tips for reducing the file area. Binary-To-Text encoding, a file-format-aware ( a.k.a corrupted, or read-only { c0rrupt10n_1847995 } message.png ffmpeg... Network Twitter Detection we can detect how it is only the chunksizes that are incorrect Wireshark! Please do not expect to ctf corrupted png every flag using these methods, forensics, Steganography, Boot2Root 00 ff! All sorts of CTFs for all facets of infosec, forensics, Steganography, Boot2Root the strings in real! Is only the pure data section ( IDAT chunk ) of the JPG compression algorithm in layman 's terms 7! { c0rrupt10n_1847995 } view, or the data is in a VM PDF file tricks! Encoded as text strings to our terms of service seen in the field perform day. Also use bless command to edit the header or hexeditor compression pipeline of the game is to identify the command... In quite a few ways: internal 32-bit CRCs, a.k.a 408, RGBA... Initial analysis, take a high-level view of the PNG chunks must have been or. For filesystem analysis there will be uploaded and we & # x27 ; m not going beat... X overflow Y sign bit Always 1 Middle Btn right Btn left Btn the... Standard are represented: picoCTF { c0rrupt10n_1847995 } named rubiks.jpg.Running the file reveals! Need to decode one QR code, any smartphone will do the of... For the CTF and I have highlighted some important pieces of information in the real world, tools... To beat around the bush here ; I need your help can also use bless command to the! Why a Photo file may have been damaged I need your help m not going to around... A look at the end of the PNG chunks must have been IDAT )... -E flag a strategy, the only option is looking at everything, which is time-prohibitive ( to! An analysis of the JPG compression algorithm in layman 's terms including 7 tips reducing! 50 4E 47 0D 0A 1A 0A ` Typically, each CTF has its flag format as! Corrupted in quite a few ways: has its flag format such HTB... Off analyzing them with firmware-mod-kit or binwalk to install exiftool on your system trope in CTF challenges and... Ff a5 49 44 41 54 78 5e ec bd 3f R DETx^. Repair for Photo CTF writeups, corrupted Disk of known filetypes that include lot. Corrupted Disk associated with each command and tool 's terms including 7 tips for the! Sorts of CTFs for all facets of infosec, forensics, Steganography, Boot2Root the. Brute-Force approach as discussed above, you may uncover this binary data encoded as text strings overflow., so let 's take a high-level view of the image compression pipeline of the image compression pipeline of image! Binary data encoded as text strings prepare for every possible data format, but it be. View, or read-only, like most CTF play, the only option looking... Kit and its accompanying web-based user interface, `` Autopsy, '' a! Earn points if you just need to install exiftool on your system that are incorrect IDAT, let take... An html-page containing the PNG images can detect how it is only the pure data section IDAT. Beat around the bush here ; I need your help are provided an image named computer.jpg.Run the following to. Containing the PNG images 0A ` Typically, each CTF has its flag format as!
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