Didier Stevens

Wednesday 28 September 2016

decoder-search.py Beta

Filed under: Beta,My Software — Didier Stevens @ 0:00

I’ve been developing a new Python program similar to XORSearch. decoder-search.py does brute-forcing and searching of a file like XORSearch, but it stead of simple operations like XOR, ROL, …, it can handle more complex translations. Templates for these translations have to be provided in a configuration file, for example like this:

expression ((byte + %i1:1-10%) ^ %i2:1-32%) % 0x100

This template specifies a translation expression that adds a number to each byte in the file, and then XORs the sum. The first integer added to each byte is brute-forced from 1 to 10 (%i1:1-10%), and the second integer used for the XOR operation is brute-forced from 1 to 32 (%i2:1-32%). Such an encoding has been used in the last hancitor maldoc samples.

Here is the result on a sample that contains an encoded EXE:
20160927-201559

And here is the result on a sample that contains encoded URLs:

20160927-201914

For me this tool is still in beta phase, because I might change the format of the configuration file in later versions, without providing backwards compatibility. You can find it in my GitHub Beta repository.

Monday 19 September 2016

Update: translate.py Version 2.3.1

Filed under: My Software,Update — Didier Stevens @ 0:00

I needed to decompress the content of a Flash file (.swf). I thought of using my translate.py program with a command to inflate (zlib) the content (minus the header of 8 bytes): lambda b: zlib.decompress(b[8:])

Quite simple, but the problem is that translate.py doesn’t import zlib. I have to do that, but that can’t be done in a lambda function. So I added option -e (execute) to execute extra statements:

20160918-222119

translate_v2_3_1.zip (https)
MD5: A3C30A3534DC96B28C1C18B425E2A82D
SHA256: BBD24406BC3038620807E8C4116B325BE6124BE92D041173A8E4BAB56D06C7E2

Sunday 18 September 2016

Overview of Content Published In August

Filed under: Announcement — Didier Stevens @ 18:35

Here is an overview of content I published in August:

Blog posts:

YouTube videos:

SANS ISC Diary entries:

Monday 29 August 2016

Update: rtfdump Version 0.0.4

Filed under: maldoc,My Software — Didier Stevens @ 0:00

This version has a user-friendlier handling of files that are not rtf:

20160812-131850

Last months, I’ve seen many maldocs that disguise .doc files as .rtf.

rtfdump_V0_0_4.zip (https)
MD5: C384FD5356DA4E2129E44903BA20966A
SHA256: 0B73AB16577BDB1DC0B1431013E28893004DD563DD4C4D00BA1D20B1DBAED917

Monday 22 August 2016

Update: xor-kpa.py Version 0.0.3 With Man Page

Filed under: Encryption,My Software — Didier Stevens @ 0:00

This new version has a man page now (option -m):

Usage: xor-kpa.py [options] filename-plaintext [filename-ciphertext]
XOR known-plaintext attack

Predefined plaintext:
 dos: This program cannot be run in DOS mode

Source code put in the public domain by Didier Stevens, no Copyright
Use at your own risk
https://DidierStevens.com

Options:
  --version             show program's version number and exit
  -h, --help            show this help message and exit
  -m, --man             Print manual
  -n, --name            Use predefined plaintext
  -e EXTRA, --extra=EXTRA
                        Minimum number of extras
  -d, --decode          Decode the ciphertext

Manual:

xor-kpa performs a known-plaintext attack (KPA) on an XOR-encoded file. Take a
file with content "This is a secret message, do not share!". This file is XOR-
encoded like this: the key is ABC, the first byte of the file is XORed with A,
the second byte of the file is XORed with B, the third byte of the file is
XORed with C, the fourth byte of the file is XORed with A, the fifth byte of
the file is XORed with B, ...
If you know part of the plaintext of this file, and that plaintext is longer
than the key, then xor-kpa can recover the key.

xor-kpa tries to recover the key as follows. xor-kpa encodes the encoded file
with the provided plaintext: if you XOR-encode an XOR-encoded file
(ciphertext) again with its plaintext, then the result is the keystream (the
key repeated): ABCABCABC... xor-kpa detects such keystreams and extracts the
key.

Example:
 xor-kpa.py "#secret message" encoded.txt
Output:
 Key:       ABC
 Extra:     11
 Keystream: BCABCABCABCABC

In this example, we assume that the plaintext contains "secret message". xor-
kpa finds one keystream: BCABCABCABCABC. From this keystream, xor-kpa extracts
the key: ABC.
Extra is the number of extra charecters in the keystream: the keystream is 14
characters longh, the key is 3 characters long, so extra is 14 - 3 = 11. It is
a measure for the probability that the recovered key is the actual key. The
longer it is, the better.
In this case, because the ciphertext is a small file, xor-kpa found only one
keystream. But for larger files or small plaintext, it will identify more than
one potential keystream.

Example:
 xor-kpa.py #secret encoded.txt
Output:
 Key:       ABC
 Extra:     3
 Keystream: BCABCA

 Key:       'KUW^'
 Extra:     1
 Keystream: '^KUW^'

 Key:       'S@E'
 Extra:     1
 Keystream: 'S@ES'

In this example, xor-kpa has identified 3 potential keys. The potential keys
are sorted by descending extra-value. So the most promising keys are listed
first.
Keystreams with an extra value of 1 (1 extra character) rarely contain the
correct key.
Option -e (--extra) allows us to reduce the amount of displayed potential keys
by specifying the minimum value for extras.

Example:
 xor-kpa.py -e 2 #secret encoded.txt
Output:
 Key:       ABC
 Extra:     3
 Keystream: BCABCA

With option -e 2 we specify that the keystream must at least have 2 extras.
That's why the keystreams with 1 extra are not listed.

xor-kpa can also decode the ciphertext file with the recovered key (the key
with the highest extra value). Use option -d (--decode) to do this:

Example:
 xor-kpa.py -d #secret encoded.txt
Output:
 This is a secret message, do not share!

xor-kpa takes one or two arguments. The first argument is a file containing
the plaintext, the second argument is a file containing the ciphertext.
xor-kpa can also read the ciphertext from stdin (for example via a pipe), in
that case the second argument is omitted.
The files can also be ZIP files containing one file (optionally password-
protected with 'infected'), in that case xor-kpa will decompress the content
of the ZIP file and use it.

In stead of putting the plaintext or the ciphertext in a file, it can also be
passed in the argument. To achieve this, precede the text with character #
(this is what we have done in all the examples up till now).
If the text to pass via the argument contains control characters or non-
printable characters, hexadecimal (#h#) or base64 (#b#) can be used.

Example:
 xor-kpa.py -d #h#736563726574 encoded.txt
Output:
 This is a secret message, do not share!

Example:
 xor-kpa.py -d #b#c2VjcmV0 encoded.txt
Output:
 This is a secret message, do not share!

Finally, the plaintext can be selected from a predefined list. For the moment,
the only text in the predefined list is 'This program cannot be run in DOS
mode', identified by the keyword dos. Use option -n (--name) to use predefined
plaintext.

Example:
 xor-kpa.py -n dos malware.vir



xor-kpa_V0_0_3.zip (https)
MD5: 228B9DE1D3005F75190113369A91E1D4
SHA256: A30C20668BA0939DD936BB2706AEC636E5260EFB0B0F16F4770F9B1B59E780A9

Monday 15 August 2016

Video: mimikatz: Golden Ticket + DCSync

Filed under: Encryption — Didier Stevens @ 0:00

I also have a video for my mimikatz: Golden Ticket + DCSync blog post.

Friday 12 August 2016

mimikatz: Golden Ticket + DCSync

Filed under: Encryption — Didier Stevens @ 8:04

This blog post aims to provide a bit more information about what Benjamin Delpy wrote in this tweet:

20160805-092138

For this demo I run mimikatz as a least privilege, local user on a Windows workstation that is a member of my demo domain. The first step is to generate and use a golden ticket to obtain domain admin rights. The second step is to use dcsync to retrieve hashes from the domain controller.

As a freshly logged-on local user, I have no tickets:

20160805-090019

Then I create a golden ticket for the domain admin:

20160805-090730

20160805-090713

And I use it:

20160805-090827

Now my least privilege, local user is impersonating the domain administrator:

20160805-090904

Then I retrieve the hashes for user user01 from the domain control via the DRSR protocol:

20160805-091005

Compare the LM and NTLM hashes with the hashes in this blogpost: they are the same.

All the arguments (krbtgt, domain, domain admin username, domain SID) needed for the kerberos::golden command can be extracted from the ntds.dit file we obtained. More info on alternative methods to obtain the arguments can be found here.

@gentilkiwi told me that the domain admin username and RID can also be faked, as long that it is part of the domain admins group. It will work for about 20 minutes without checks.

If we don’t have the necessary rights (for example domain admin) to query a DC with DRSR, we get an error 5 (access denied):

20160805-090342

You also get this error when the krbtgt NTLM hash has changed. Command ptt will seem to succeed however:

20160805-121604

Remember that unless the password for user krbtgt is changed (which is not a standard practice), the krbtgt NTLM hash never changes. So even very old copies of ntds.dit can be used to recover hashes as described in this method.

The ticket is stored on file using asn1:

20160805-100151

Benjamin has a YARA rule (mimikatz_kirbi_ticket) to detect such tickets:

20160805-101142

Unfortunately, the mimikatz I use (version 2.1) uses another asn1 encoder and the rule no longer works.

Until Benjamin makes a more generic rule, you can use this updated rule:

rule mimikatz_kirbi_ticket
{
	meta:
		description		= "KiRBi ticket for mimikatz"
		author			= "Benjamin DELPY (gentilkiwi); Didier Stevens"

	strings:
		$asn1			= { 76 82 ?? ?? 30 82 ?? ?? a0 03 02 01 05 a1 03 02 01 16 }
		$asn1_84		= { 76 84 ?? ?? ?? ?? 30 84 ?? ?? ?? ?? a0 84 00 00 00 03 02 01 05 a1 84 00 00 00 03 02 01 16 }

	condition:
		$asn1 at 0 or $asn1_84 at 0
}

This ticket file is created on disk because I use kerberos::golden’s option /ticket:, but if I use option /ptt, the ticket is immediately passed, and not written to disk.

@gentilkiwi also told me that if you impersonate a domain controller account for kerberos::dcsync, then no events are logged.

Monday 8 August 2016

Howto CreateCertGUI: Create Your Own Certificate On Windows (OpenSSL Library)

Filed under: Encryption,My Software — Didier Stevens @ 0:00

I created a program with a graphical user interface to create a simple certificate. This program uses the OpenSSL library. Extract the program from the zip file (below) and run it:

20160807-232138

You don’t have to install any dependencies, everything is linked into the program.

If you need more help, here is a video:

Download:

CreateCertGUI_V1_0_0_1.zip (https)
MD5: F5400736E7E38F30D35A02FEB6D99651
SHA256: 82D59AC494FEF1A8B219C591717359712C19E8845D02A457017045A9A4C3D989

And if you are interested, here is the source code:

CreateCertGUI_source_V1_0_0_1.zip (https)
MD5: 790CA083407032434A8DA1FF8AC1E512
SHA256: B15BB8A3504EF56D1C6C84CA181FFB6E5A73956EC79757C62B87B520C136AA2D

Tuesday 2 August 2016

rtfdump: Update And Videos

Filed under: maldoc,My Software,Update — Didier Stevens @ 0:00

I made a small update to rtfdump and added new rules to rtf.yara.

This video is an intro to rtfdump:

This is a video on an RTF maldoc (MD5 07884483f95ae891845caf0d50ce507f) that contains an exploit for MS12-027 CVE-2012-0158:

This is a video on an RTF maldoc (MD5 4483ad299158eb54f6ff58b5346a36ee) that contains an exploit for MS10-087 CVE-2010-3333:

rtfdump_V0_0_3.zip (https)
MD5: 59DC23EE55F76C065A2A718DDFDB0E4E
SHA256: 46F9D768C6976AD5D4018EFDFD35DAE4212FEAE57871434A33CAEF028CB4CBA2

Monday 1 August 2016

Overview of Content Published In July

Filed under: Announcement — Didier Stevens @ 0:00

Here is an overview of content I published in July:

Blog posts:

YouTube videos:

Videoblog posts:

SANS ISC Diary entries:

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