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. PHPGIFpngJPEG; PHPForA-Z26AA,AB,AC; WebPHPCodeigniter; Ubuntu PHP; EosPHP; ctfphp message.png message.png ADS Squashfs is one popular implementation of an embedded device filesystem. Example 1:You are provided an image named computer.jpg.Run the following command to dump the file in hex format. And of course, like most CTF play, the ideal environment is a Linux system with occasionally Windows in a VM. You may need to download binwalk on your system. Please * Use an hexadecimal editor like `bless`,`hexeditor`,`nano` with a specific option or many more. 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. Use Git or checkout with SVN using the web URL. ``` Sox is another useful command-line tool for converting and manipulating audio files. checksums, and decompressing the image data); it can optionally dump almost all of the chunk-level information in the image in human-readable form. The images will be stored at this GIT repository if youd like to download them and try the commands and tools for yourself. Other times, a message might be encoded into the audio as DTMF tones or morse code. pngcheck -v mystery_solved_v1.png The participant or team with the highest score wins the event. This SVG image compressor shrinks your SVG logos, illustrations or icons to the smallest file size and best quality possible. 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. ezgif. ``` Below are a few more that you can research to help expand your knowledge. The width of the PNG must be 958. ## Statement of the challenge So let's change the name of the chunck picoCTF 2019 - [Forensic] c0rrupted (250 points) Also, if a file contains another file embedded somewhere inside it, the file command is only going to identify the containing filetype. For more information, please see our Also, a snapshot of memory often contains context and clues that are impossible to find on disk because they only exist at runtime (operational configurations, remote-exploit shellcode, passwords and encryption keys, etc). An open-source alternative has emerged called Kaitai. ERRORS DETECTED in mystery_solved_v1.png Gimp provides the ability to alter various aspects of the visual data of an image file. The term for identifying a file embedded in another file and extracting it is "file carving." === 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). Written by Maltemo, member of team SinHack. Thanks for reading. A PNG image always starts with those 4 bytes: Without a strategy, the only option is looking at everything, which is time-prohibitive (not to mention exhausting). Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. 9-CTF. `89 50 4E 47 0D 0A 1A 0A` The best tool to repair and fix your corrupted or broken PNG image PNG image repair tool Made in Germany EU GDPR compliant Select file Drag & Drop Drag your image file onto this website. Why we see the red compression artifacts so well and what we can do about them. 1642 x 1095 image, 24-bit RGB, non-interlaced Having the PNG magic number doesn't mean it is a well formed PNG file. The width of Underscore_in_C is also 958 so we can try to use the bytes of Underscore_in_C as the keys. # L | IDAT | DATA | CHECKSUM ---> {L} {DATA, CHECKSUM, L} {DATA, CHECKSUM, L} {DATA, CHECKSUM} Example 1:You are provided an image named ocean.jpg.Running the exiftool command reveals the following information. 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. There is a hint with the `D` and `T` letters, which help us to deduce that it is a `IDAT` chunk. For debugging and detect CRC problem, you can use : pngcheck -v [filename] An analysis of the image compression pipeline of the social network Twitter. Also, I saw the length anounced for this chunk was enormous : `AA AA FF A5`. 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. Information# Version# By Version Comment noraj 1.0 Creation CTF# Name : IceCTF 2016 Website : https://icec.tf/ Type : Online Format : Jeopardy CTF Time : link Description# We intercepted t. Linux; Security; . The newer scheme for password-protecting zip files (with AES-256, rather than "ZipCrypto") does not have this weakness. Click inside the file drop area to upload a file or drag & drop a file. Dig deeper to find what was hidden! ctf Viewing the image, we get the flag: picoCTF{c0rrupt10n_1847995}. Cannot retrieve contributors at this time, 00000000: 89 65 4e 34 0d 0a b0 aa 00 00 00 0d 43 22 44 52 .eN4..C"DR. 00000010: 00 00 06 6a 00 00 04 47 08 02 00 00 00 7c 8b ab jG..|.. 00000020: 78 00 00 00 01 73 52 47 42 00 ae ce 1c e9 00 00 x.sRGB. 00000030: 00 04 67 41 4d 41 00 00 b1 8f 0b fc 61 05 00 00 ..gAMAa 00000040: 00 09 70 48 59 73 aa 00 16 25 00 00 16 25 01 49 ..pHYs%%.I. We solved many challenges and overall placed second (CTFtime). Microsoft has created dozens of office document file formats, many of which are popular for the distribution of phishing attacks and malware because of their ability to include macros (VBA scripts). If partial trials are present, this function will % remove them from the last meg4 file. Open your mystery data as "raw image data" in Gimp and experiment with different settings. The ImageMagick toolset can be incorporated into scripts and enable you to quickly identify, resize, crop, modify, convert, and otherwise manipulate image files. ### Correcting the IHDR chunk Le flag est sous la forme APRK{SHA1(NOMPRENOM)}. Lets start with the PNG header. Example of using hexdump format strings to output the first 50 bytes of a file as a series of 64-bit integers in hex: Binary is 1's and 0's, but often is transmitted as text. E N 4`| But most of the time, as the file is corrupted, you will obtain this answer : data. The next chunks after the IHDR were alright until it ends with an unknown header name : Before going further with the challenge details, Id like to quickly summarize how a PNG file actually is. If you need to dig into PNG a little deeper, the pngtools package might be useful. The premiere open-source framework for memory dump analysis is Volatility. It will give you 4 bytes more than the right result. Confused yet? and our You can do this also on the image processing page. If you like this post, consider a small donation. |Hexa Values|Ascii Translation| byte 2: X movement. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. 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. The flag is **picoCTF{c0rrupt10n_1847995}** AperiCTF 2019 - [OSINT] Hey DJ (175 points) It may also lack the "black hat attacker" appeal that draws many players to participate in CTFs. 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. 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 example, it can be used to print the basic statistics about an image (dimensions, bit depth, etc. Made for fixed-function low-resource environments, they can be compressed, single-file, or read-only. Select the issues we can fix for you, and click the repair button Download link of repaired file will be available instantly after repaired. As far as that is possible, all formats supported by the PNG standard are represented. We can read `0xffa5 bytes`. 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. . But malicious VBA macros are rarely complicated, since VBA is typically just used as a jumping-off platform to bootstrap code execution. We can use binwalk to search images for embedded files such as flags or files that may contain clues to the flag. We intercepted this image, but it must have gotten corrupted during the transmission. On October 14th and 15th 2022 we participated in the Reply Cyber Security Challenge 2022. scalpel, now a part of SleuthKit (discussed further under Filesystems) is another tool for file-carving, formerly known as Foremost. chunk sRGB at offset 0x00025, length 1 `89 50 4E 47 0D 0A B0 AA` chunk IDAT at offset 0x20008, length 65524 `00 00 FF A5` pngcheck says that the expected checksum as stated in the file (0x495224f0) doesn't match the computed checksum. We can simply try replacing the expected hex values with the computed CRC. |Hexa Values|Ascii Translation| 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. Fix each invalid chunk with a combinatoric, brute-force approach. You can decode an image of a QR code with less than 5 lines of Python. 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. We got another image inside 3.png. In this article, we will focus on finding hidden data in images and introduce commands and tools that you can use to help you find the flag. It also uses an identification heuristic, but with certainty percentages. Cookie Notice author: Maltemo The closest chunk type is IDAT, let's try to fix that first: Now let's take a look at the size. When analyzing file formats, a file-format-aware (a.k.a. Commands and Tools to help you find hidden data in images while participating in Capture The Flag events. |`89 65 4E 34`|`. Example 1:You are provided an image named dog.jpg.Run the following command to see if Binwalk finds any embedded files. Let's see if that fixes the checksum: That fixed the problem, we remain with a "invalid chunk length (too large)" message. SharkyCTF 2020 - [Web] Containment Forever (300pts) |Hexa Values|Ascii Translation| CTF PNG Critical Chunk Size Fixer This is a tool I created intended to be used in forensics challenges for CTFs where you are given a corrupted PNG file. Prouvez-lui le contraire en investiguant. A typical VBA macro in an Office document, on Windows, will download a PowerShell script to %TEMP% and attempt to execute it, in which case you now have a PowerShell script analysis task too. Can you try and fix it? ``` D E T`| For some reason, I thought the 1 was an l at first! 3. By default, it only checks headers of the file for better performance. This is a collection of graphics images created to test PNG applications like viewers, converters and editors. So I correct it with `bless`. TrID is a more sophisticated version of file. 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. You may need to install exiftool on your system. The file within the zip file is named hidden_text.txt. Look at man strings for more details. |`AB 44 45 54`|`. 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. Once that is done, type sfc/scannow' in the command prompt window and press the 'Enter' button again. ```sh So I decided to change the PNG header **again** to correct this problem : The traditional heuristic for identifying filetypes on UNIX is libmagic, which is a library for identifying so-called "magic numbers" or "magic bytes," the unique identifying marker bytes in filetype headers. Reddit and its partners use cookies and similar technologies to provide you with a better experience. Paste a Base64 Data URI from your clipboard into this website. For EXT3 and EXT4 filesystems, you can attempt to find deleted files with extundelete. Most audio and video media formats use discrete (fixed-size) "chunks" so that they can be streamed; the LSBs of those chunks are a common place to smuggle some data without visibly affecting the file. P N G`| Paste an image from your clipboard into this website. chunk IDAT at offset 0x00057, length 65445, zlib: deflated, 32K window, fast compression, chunk IDAT at offset 0x10008, length 65524, chunk IDAT at offset 0x20008, length 65524, chunk IDAT at offset 0x30008, length 6304. When our hope was gone and our PCs were slowly turning in frying pans, esseks another awesome teammate, came to the rescue. The next IDAT chunk is at offset 0x10004. Additionally, a lesser-known feature of the Wireshark network protocol analyzer is its ability to analyze certain media file formats like GIF, JPG, and PNG. There might be a gold mine of metadata, or there might be almost nothing. https://mega.nz/#!aKwGFARR!rS60DdUh8-jHMac572TSsdsANClqEsl9PD2sGl-SyDk, you can also use bless command to edit the header or hexeditor, check the header format has the hint says and edit the header format After that try to open the file and see what goes on, After that you can use the gif speed control online and slow the speed of the encoded message and finally your get the message but being encoded, https://upload.wikimedia.org/wikipedia/commons/5/59/Gifs_in_txt_and_hex.gif File: mystery_solved_v1.png (202940 bytes) [TOC] Its advantage is its larger set of known filetypes that include a lot of proprietary and obscure formats seen in the real world. Another note about zip cracking is that if you have an unencrypted/uncompressed copy of any one of the files that is compressed in the encrypted zip, you can perform a "plaintext attack" and crack the zip, as detailed here, and explained in this paper. Some of the PNG chunks must have been corrupted as well then. To verify correcteness or attempt to repair corrupted PNGs you can use pngcheck. Flag. Hi, I'm Christoph, the developer of compress-or-die.com. Learn more. Embedded device filesystems are a unique category of their own. Beyond that, you can try tcpxtract, Network Miner, Foremost, or Snort. It can also find the visual and data difference between two seemingly identical images with its compare tool. The first chunk is IHDR and has the length of 0xD, so let's fix that as well. * 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 : Rating: 5.0 # crcket > Category: Forensics > Description: ``` DarkArmy's openers bagging as many runs as possible for our team. Zip is the most common in the real world, and the most common in CTFs. .. 00000000: 89 50 4e 47 0d 0a 1a 0a 00 00 00 0d 43 22 44 52 .PNG..C"DR, 00000000: 89 50 4e 47 0d 0a 1a 0a 00 00 00 0d 49 48 44 52 .PNG..IHDR, fixed.png: PNG image data, 1642 x 1095, 8-bit/color RGB, non-interlaced, 1642 x 1095 image, 24-bit RGB, non-interlaced, chunk gAMA at offset 0x00032, length 4: 0.45455, chunk pHYs at offset 0x00042, length 9: 2852132389x5669 pixels/meter, CRC error in chunk pHYs (computed 38d82c82, expected 495224f0), 00000040: 00 09 70 48 59 73 00 00 16 25 00 00 16 25 01 49 ..pHYs%%.I, chunk pHYs at offset 0x00042, length 9: 5669x5669 pixels/meter (144 dpi), 00000053: aa aa ff a5 ab 44 45 54 ..DET, DECIMAL HEXADECIMAL DESCRIPTION, --------------------------------------------------------------------------------, 87 0x57 Raw signature (IDAT), 65544 0x10008 Raw signature (IDAT), 131080 0x20008 Raw signature (IDAT), 196616 0x30008 Raw signature (IDAT). Even in IR work, computer forensics is usually the domain of law enforcement seeking evidentiary data and attribution, rather than the commercial incident responder who may just be interested in expelling an attacker and/or restoring system integrity. One would typically not bust a criminal case by carefully reassembling a corrupted PNG file, revealing a photo of a QR code that decodes to a password for a zip archive containing an NES rom that when played will output the confession. Additional meta-information within files may be useful depending on the challenge. For a more local converter, try the xxd command. Regardless, many players enjoy the variety and novelty in CTF forensics challenges. For these, try working with multimon-ng to decode them. Stegsolve (JAR download link) is often used to apply various steganography techniques to image files in an attempt to detect and extract hidden data. Example of using xxd to do text-as-ascii-to-hex encoding: We've discussed the fundamental concepts and the tools for the more generic forensics tasks. 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.==. There will be images associated with each command and tool. [TOC] In the case where you do need to understand a complicated VBA macro, or if the macro is obfuscated and has an unpacker routine, you don't need to own a license to Microsoft Office to debug this. CTF Example WDCTF-finals-2017 Download the challenge here If you look at the file, you can see that the header and width of the PNG file are incorrect. And we got the final image : file mystery 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. |**Values (hex)** | **Purpose**| There are a handful of command-line tools for zip files that will be useful to know about. Therefore, we get the length of 0x10004 - 0x5B - 0x4 = 0xFFA5 which is good since the original value is 0xAAAAFFA5. I H C R it should be . I'm not going to beat around the bush here; I need your help. pngcheck verifies the integrity of PNG, JNG and MNG files (by checking the internal 32-bit CRCs, a.k.a. --- This an introduction to finding data hidden in image files. Creator: 2phi. The rest is specified inside the XML files. ``` File: mystery_solved_v1.png (202940 bytes) Drag your image file onto this website. Find all corrupted PNG files: find . Based on the output, the 19th key value must be 0x3 and the 20th key must be 0xbe. Although it's closed-source, it's free and works across platforms. MacOS is not a bad environment to substitute for Linux, if you can accept that some open-source tools may not install or compile correctly. Hints Recon In the Recon stage, we look around the repaired file systems for clues as stated in the hints and the following clues were found : #message png, #message png ADS, #broken pdf. Challenges incorporate several hacking skills such as web exploitation, reverse engineering, cryptography, and steganography. |`0A`| **A Unix-style line ending (LF) to detect Unix-DOS line ending conversion. |`49 48 44 52`|`I H D R`| You could also interface Wireshark from your Python using Wirepy. Follow my twitter for latest update. With the help of a hex editor we added the missing 0x0D byte, renamed the file and. I noticed that it was not correct ! ### Correcting the IDAT chunk Description Plus it will highlight file transfers and show you any "suspicious" activity. * https://hackmd.io/k4zl24xaSHqntmIR6SsdZA#Step-2--Correcting-the-PLTE-length-of-the-PNG-file When you are on the file, search for known elements that give hints . Understand the technical background of online image compression tools and learn which image compressor you should use from now on. Determine which chunks are invalid due to CRC and/or length errors. Xor the extracted image with the distorted image with . We use -n 7 for strings of length 7+, and -t x to view- their position in the file. Another is a framework in Ruby called Origami. Both formats are structured, compound file binary formats that enable Linked or Embedded content (Objects). If trying to repair a damaged PCAP file, there is an online service for repairing PCAP files called PCAPfix. If you already know what you're searching for, you can do grep-style searching through packets using ngrep. For initial analysis, take a high-level view of the packets with Wireshark's statistics or conversations view, or its capinfos command. The binary objects can be compressed or even encrypted data, and include content in scripting languages like JavaScript or Flash. Description Much joy. 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 . In a CTF, part of the game is to identify the file ourselves, using a heuristic approach. It is also extensible using plugins for extracting various types of artifact. I broke my solution down into 5 steps: Read the corrupted PNG into memory. 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. To verify the correctness or attempt to repair corrupted PNGs you can use pngcheck Try fixing the file header Extract all the files within the image, we find what we needed. So, we just need to override 0xAAAA with zeroes again. Audacity is the premiere open-source audio file and waveform-viewing tool, and CTF challenge authors love to encode text into audio waveforms, which you can see using the spectogram view (although a specialized tool called Sonic Visualiser is better for this task in particular).

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