Mitra Save

A generator of weird files (binary polyglots, near polyglots...)

Project README

Mitra

A tool to generate binary polyglots (files that are valid with several file formats).

Loosely named after Μιθραδάτης, a famous polyglot. Pronounced mɪtrə.

What's new.

How to use

mitra.py file1.png file2.dcm gives you a working PNG/DICOM polyglot.

Check Corkami mini or tiny PoCs for input files. and the formats repository for some extra technical info.

Features

It tries different layouts: Stacks (appended data), Cavities (blank space), Parasites (comments), Zippers (mutual comments).

It stores the offsets where the payloads 'switch sizes' between parenthesis for ambiguous ciphertexts.

Ex: Z(80-162-286)-DICOM^TIFF.be3b767b.dcm.tif is a DICOM/TIFF zipper where the payloads switch side at offsets 0x80, 0x162 and 0x286.

The -s option extracts the 2 payloads separately, mixed with pseudo-random bytes (it doesn't fix checksums afterwards).

Mitra can generate near-polyglots (with overlapping bytes) with the --overlap parameter. It stores the overlap data between curly braces of the output file - for example, O(d-40a){4D5A}.wasm.exe.

Goals

Exploration

The goal of this project was to explore polyglots layouts, formats abuses, and understand which format features enables which kind of abuse.

For example, in a chunk-based format, just find where to cut the file, then wrap foreign data in a new chunk and insert the chunk. So you just need to teach Mitra how to identify the type, where to cut, and how to wrap.

Demonstration

You can prevent a software launch if you find a security risk, but a bad format design is not considered a risk in itself: it's just the parser's fault, not the designer's !

So if you review a bad file format, then maybe with Mitra you can quickly generate polyglots with many other formats and demonstrate the risks.

Status

                          Delayed                 Magic at offset zero,                  No appended
      Any offset  Cavities start                 tolerated appended data                     data    Footer

        Z 7 A R   P I D T   P M   A B B C C E E F F G G I I I I J J N O P L P P R R T W   B J P P W   I X
        i Z r A   D S C A   S P   R M Z A P B L L l I Z C C D L P P E G S N E N I T I A   P a C C A   D Z
        p   j R   F O M R     4     P 2 B I M F V a F   C O 3 D 2 G S G D K   G F F F D   G v A A S   3
                                          O L     c         v A                 F   F       a P P M   v
                                                            2                                   N     1

Zip     . X X X   X X X X   X X   X X X X X X X X X X X X X X X X X X X X X X X X X X X   X X X X X       41
7Z      X . X X   X X X X   X X   X X X X X X X X X X X X X X X X X X X X X X X X X X X   X X X X X       41
Arj     X X . X   X X X X   X X   X X X X X X X X X X X X X X X X X X X X X X X X X X X   X X X X X       41
RAR     X X X .   X X X X   X X   X X X X X X X X X X X X X X X X X X X X X X X X X X X   X X X X X       41

PDF     X X X X   . X X X   X X   X X X X X X X X X X X X X X X X X X X X X X X X X X X   X X X X X       41
ISO     X X X X   X . X X   X X   X X X X X X X X X X X X X X X X X X X X X X X X X X X   X X X X X       41
DCM     X X X X   X X .     X X   X X X X X   X X X X X   X X X X X X X X   X X X X X X   X X X X X       37
TAR     X X X X   X X   .   X X   X     X X     X X   X   X X X X X   X X     X X X X X   X   X X X       30

PS      X X X X   X X X X   .                                                                              8
MP4     X X X X   X X X X     .                                                                            8

AR      X X X X   X X X X         .                                                                        8
BMP     X X X X   X X X             .                                                                      7
BZ2     X X X X   X X X               .                                                                    7
CAB     X X X X   X X X X               .                                                                  8
CPIO    X X X X   X X X X                 .                                                                8
EBML    X X X X   X X                       .                                                              6
ELF     X X X X   X X X                       .                                                            7
FLV     X X X X   X X X X                       .                                                          8
Flac    X X X X   X X X X                         .                                                        8
GIF     X X X X   X X X                             .                                                      7
GZ      X X X X   X X X X                             .                                                    8
ICC     X X X X   X X                                   .                                                  6
ICO     X X X X   X X X X                                 .                                                8
ID3v2   X X X X   X X X X                                   .                                              8
ILDA    X X X X   X X X X                                     .                                            8
JP2     X X X X   X X X X                                       .                                          8
JPG     X X X X   X X X X                                         .                                        8
NES     X X X X   X X X                                             .                                      7
OGG     X X X X   X X X X                                             .                                    8
PSD     X X X X   X X X X                                               .                                  8
LNK     X X X X   X X                                                     .                                6
PE      X X X X   X X X                                                     .                              7
PNG     X X X X   X X X X                                                     .                            8
RIFF    X X X X   X X X X                                                       .                          8
RTF     X X X X   X X X X                                                         .                        8
TIFF    X X X X   X X X X                                                           .                      8
WAD     X X X X   X X X X                                                             .                    8

BPG     X X X X   X X X X                                                                 .                8
Java    X X X X   X X X                                                                     .              7
PCAP    X X X X   X X X X                                                                     .            8
PCAPN   X X X X   X X X X                                                                       .          8
WASM    X X X X   X X X X                                                                         .        8

ID3v1                                                                                                 .    0
XZ                                                                                                      .  0

Formats combinations: 286

Notes that some formats are containers and apply to several file types.

Near polyglots

With the --overlap option (disabled by default), filetypes starting at the same offsets can be combined.

The overwritten content might be restored via any operation, such as decryption via specific bruteforced parameters (CBC, GCM...), and is stored in the filename between curly brakets. Ex: O(5-204){424D4E0100}.bmp.jpg

Since it's pure bruteforcing, it's not practical if the filetype requires too many bytes to be restored.

                                                               Variable  Unsupported
                                                                offset     parasite
Minimal start offset
         1 2 4 8     9  16  20  23  28  34  40  64  94  132    12  28   
                      12          26  32  36      68  112 226    16     

         P P J F M T F W G P R I R B C I P C J P E A P I I J    W B O      B E G L N
         S E P l P I L A Z N I D T M P L S A P C L R C C C a    A P G      Z B I N E
             G a 4 F V D   G F 3 F P I D D B 2 A F   A O C v    S G G      2 M F K S
               c   F         F v     O A       P     P     a    M            L
                               2               N
                                               G

1* PS    . M A ? ? ? ? ? ? A ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?    ? ? ?      ? ? ? ? ?
2^ PE    M . A A A A A A A A A A A A A A A A A A ! ! ! ! ! !    M M M      ! ! ! ! !
4+ JPG   A A . A A A A A A A A A A A A A A A A A A A A A A A    A A A      A A A A A
.  .
.  .     [the table could go on but would take too long to bruteforce]

           X: automated     ?: likely possible
           M: manual        !: unknown
  • *: hack that relies on line comments with GhostScript - requires the parasite not to contain any new line, after encryption.
  • ^: hack relying on overwritting the Dos Header, therefore restricting the parasite space to offsets 2-60.
  • +: Signature, comment declaration and length takes all 2 bytes each, in total:
    • to set an exact length, 6 bytes are required.
    • the length is big endian, so you can round up the length and leave the lowest byte uncontrolled, requiring only 5 bytes.
    • If the entire length is left undefined, and if your payload fits in the resulted encoded length after encryption, then only 4 bytes are required.

Script polyglots

Some script language (JavaScript, HTML, PHP, Ruby...) tolerate binary contents and can be combined with binary formats.

Typical exploits target browser-supported formats such as images, GZip, Flash, Mp3...

Mitra can be used to make room in a binary file.

There are different strategies for these polyglots:

  • Multiline comments such as */ <JavaScript> /* in the middle
  • Here document: In this case, you might want to use the format magic to define a dummy variable with the format magics. For example, MZ = << HereMarker in the DOS Header of a Portable Executable.
  • Terminators can also be used to make sure that the rest of the file is ignored.
Language ML Comments Here doc. Terminator
HTML <!-- -->  
JavaScript /* */  
Perl =pod =cut << __END__
PHP /* */ <<
PostScript /{( )}   stop
PowerShell <# #> @"
Python   """
Ruby =begin =end << __END__
Shell   <<
XML <![CDATA[ ]]>  

You may want to make room for a specific buffer size to encode a multiline comment via the length declaration of the comment. For example in MP4, where the file starts with the length of the first atom.

Mocky

Mocky is a script to generates polymocks.

It turn valid files into polymocks by inserting mock signature of other file types at the right offset, if possible. These polymocks might then be detected as something else while retaining their initial validity, depending on the detection engine.

The --combine parameters tries to fit as many signatures as accepted by the target file format.

Example

  1. Take a standard PDF file.
$ file pdf.pdf
pdf.pdf: PDF document, version 1.3
  1. Generate a polymock file
$ mocky.py --combine pdf.pdf
Filetype: Portable Document Format
Parasite-combined sig(s): SymbOs / netbsd_ktraceS / SoundFX / VirtualBox / ScreamTracker / Plot84 / ezd / dicom / ds / CCP4 / DRDOS / pif / mbr
> Combined Mock: mA-pdf.pdf
  1. Check the results

The file is still totally valid.

$ pdftotext mA-pdf.pdf -
PDF
$ pdfinfo mA-pdf.pdf
Tagged:         no
UserProperties: no
Suspects:       no
Form:           none
JavaScript:     no
Pages:          1
Encrypted:      no
Page size:      612 x 792 pts (letter)
Page rot:       0
File size:      1272 bytes
Optimized:      no
PDF version:    1.3

The detected file type has changed:

$ file mA-pdf.pdf
mA-pdf.pdf: DR-DOS executable (COM)

It actually contains more signatures, and still detected as PDF too:

$file --keep-going --raw mA-pdf.pdf
mA-pdf.pdf: DR-DOS executable (COM)
- Windows Program Information File for  R>>
- DOS/MBR boot sector
- Nintendo DS ROM image: "%PDF-1.3" (, Rev.116)
- Plot84 plotting file DOS/MBR boot sector
- SymbOS executable v., name: 1 0 obj
- PDF document, version 1.3
- Old EZD Electron Density Map
- Scream Tracker Sample adlib drum mono 8bit
- SoundFX Module sound file
- DICOM medical imaging data
- CCP4 Electron Density Map
- VirtualBox Disk Image (%PDF-1.3), 5715999566798081280 bytes
- data

Notes

File formats

Remarks and recommendations to design file formats

  • Enforcing a magic at offset zero should be standard.
  • Starting at offset zero and not enforcing a magic at zero is still exploitable (PS, MP4).
  • Starting at any offset makes polyglots trivial.
  • Enforcing a footer (like XZ, ID3v1) is a great way to check if a file isn't truncated, and prevents 'naturally' appended data.
  • Most formats have a way to store parasite data, except very simple ones.
  • Don't use a standard container in a non standard way. Use a different magic if you're only going to use similar structures to a limited extend.

Polyglots detection

Since these polyglots contain the magic signatures of both formats, file may be able to detect them with the --keep-going arguments. (it does not validate the formats, but at least gives you some information).

$ file --keep-going --raw "C(66)-PNG-DICOM.7e22f58e.dcm.png"
C(66)-PNG-DICOM.7e22f58e.dcm.png: PNG image data, 13 x 7, 1-bit colormap, non-interlaced
- DICOM medical imaging data
- data

Related documentation

Paper:

Talks:

  • TimeCryption - clean now, malicious later (DEFCON CH 2021) w/ Stefan Kölbl, slides / video
  • Generating weird files - an introduction to Mitra (Pass the Salt 2021) slides / video
Open Source Agenda is not affiliated with "Mitra" Project. README Source: corkami/mitra
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