Posted by admin-csnv on July 10, 2019
I got to thinking about file dumps in hexadecimal this week. This is something I do at least a few times a week – usually to look at file headers or non-printable characters for one reason or another.
File headers will usually let you know what type of file you’re looking at (no matter what the file extension is). More here on that: https://linux.die.net/man/1/file
When looking at or for non-printable characters, this can be for any number of reasons, but almost always it’s to figure out what some crazy application is doing with CRLF (Carriage Return / Line Feed) so that I can fix the output to properly feed the next script or tool, or so that Word will read it correctly (which I guess is the same thing).
Anyway, the go-to tool for this is XXD:
# xxd /usr/bin/vi | more
00000000: 7f45 4c46 0201 0100 0000 0000 0000 0000 .ELF…………
00000010: 0300 3e00 0100 0000 b0b4 0600 0000 0000 ..>………….
00000020: 4000 0000 0000 0000 a066 3000 0000 0000 @……..f0…..
00000030: 0000 0000 4000 3800 0900 4000 1d00 1c00 …[email protected]…@…..
00000040: 0600 0000 0500 0000 4000 0000 0000 0000 ……[email protected]…….
00000050: 4000 0000 0000 0000 4000 0000 0000 0000 @……[email protected]…….
00000060: f801 0000 0000 0000 f801 0000 0000 0000 …………….
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More on XXD here (or type”man xxd”): https://linux.die.net/man/1/xxd
If you’re on a stripped-down Linux version, something like busybox, XXD won’t be there (it comes with VIM, not VI), but often those distro’s will still have the “hexdump” command:
# hexdump -C /bin/vi | more
00000000 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 |.ELF…………|
00000010 03 00 03 00 01 00 00 00 88 63 00 00 34 00 00 00 |………c..4…|
00000020 6c bf 05 00 00 00 00 00 34 00 20 00 08 00 28 00 |l…….4. …(.|
00000030 1f 00 1c 00 06 00 00 00 34 00 00 00 34 00 00 00 |……..4…4…|
00000040 34 00 00 00 00 01 00 00 00 01 00 00 05 00 00 00 |4……………|
00000050 04 00 00 00 03 00 00 00 34 01 00 00 34 01 00 00 |……..4…4…|
00000060 34 01 00 00 13 00 00 00 13 00 00 00 04 00 00 00 |4……………|
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But what if you’re on a customer Windows host? And what if they haven’t installed any of the Linux tools? Well, as you might guess, “PowerShell to the rescue!” Powershell’s “format-hex” command gives you much the same output:
PS C:> Get-Content windowssystemcmd.exe |format-hex |more
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
00000000 4D 5A 3F 00 03 00 00 00 04 00 00 00 3F 3F 00 00 MZ?………??..
00000010 3F 00 00 00 00 00 00 00 40 00 00 00 00 00 00 00 ?……[email protected]…….
00000020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 …………….
00000030 00 00 00 00 00 00 00 00 00 00 00 00 3F 00 00 00 …………?…
00000040 0E 1F 3F 0E 00 3F 09 3F 21 3F 01 4C 3F 21 54 68 ..?..?.?!?.L?!Th
00000050 69 73 20 70 72 6F 67 72 61 6D 20 63 61 6E 6E 6F is program canno
00000060 74 20 62 65 20 72 75 6E 20 69 6E 20 44 4F 53 20 t be run in DOS
00000070 6D 6F 64 65 2E mode.
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Better yet, format-hex handles multiple encodings, so if you have a specific character encoding to work with, “-encoding” is your friend! The default is UTF8BOM (for “byte order marker”), “unicode” encoding will give you UTF-16
The full “format-hex” docs are here (along with dozens of other places that google will find for you): https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.utility/format-hex?view=powershell-6
(or “get-help format-hex”)
More on the various encoding options here: https://docs.microsoft.com/en-us/dotnet/api/system.text.encoding.codepage?view=netcore-2.2
If you’ve seen a situation where you needed a different method to accomplish this task, please use our comment form to share!!
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Rob VandenBrink
Coherent Security
(c) SANS Internet Storm Center. https://isc.sans.edu Creative Commons Attribution-Noncommercial 3.0 United States License.
Reposted from SANS. View original.