Didier Stevens

Wednesday 9 April 2014

PDF Rainbow Tables

Filed under: Encryption,PDF — Didier Stevens @ 0:57

Looks I hadn’t blogged this video:

Monday 3 March 2014

Forensic Use of CAT Files

Filed under: Encryption,Forensics,Malware — Didier Stevens @ 0:16

I found this executable A0000623.sys with 6 detections on VirusTotal. Are these false positives or true positives?

The file was found in the _restore system folder. It looks like it is a Windows system file (tcp.sys), but maybe it is infected. It has no digital signature.

With the help of Google, I was able to trace it back to MS05-019: WindowsXP-KB893066-x86-ENU.exe. But unfortunately, WindowsXP-KB893066-x86-ENU.exe can no longer be downloaded from Microsoft’s site, as they published a new release for this patch: WindowsXP-KB893066-v2-x86-ENU.exe.

Fortunately, I found another file in this _restore folder: A0000615.cat. This is a catalog file that Microsoft uses to sign Windows executables. With Sysinternals’ sigcheck tool and this catalog file, I was able to confirm that this is a signed Windows executable and conclude that the detections are false positives.

I will release a new version of my AnalyzePESig tool that accepts an optional catalog file.

Monday 6 January 2014

Video: Checking the Digital Signature of Windows Executables

Filed under: Encryption,My Software — Didier Stevens @ 4:09

I produced a new video: a simple howto for users who don’t know how to use Windows explorer’s properties dialog to check a digital signature.

Later in the video, it gets a bit more technical by using tools (AnalyzePESig and sigcheck) to check signatures.

Wednesday 11 December 2013

MS13-098: Fixing Authenticode

Filed under: Encryption,Hacking — Didier Stevens @ 23:17

In 2009 I added a command to my Disitool to inject data “into” an Authenticode signature without invalidating it.

This year I reported on some installer programs using this padding trick.

With MS13-098, Microsoft releases a patch to prevent this signature padding trick. This change in behavior will become active June 10th 2014.

But you can already activate it now by setting reg_sz key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Cryptography\Wintrust\Config\EnableCertPaddingCheck to “1″.

Here is the effect illustrated with my AnalyzePESig tool:

20131211-230933

But beware of a potential issue with this regkey. Setting it to “0″ will not revert to the old behavior (tested in VM with Windows XP SP3).

I had to deleted the key (actually, I renamed it) and reboot to revert to the old behavior. I informed Microsoft.

Tuesday 13 August 2013

A Bit More Than A Signature

Filed under: Encryption,Forensics,Hacking,My Software — Didier Stevens @ 19:07

Soon I’ll release new versions of my Authenticode Tools.

Detecting extra data in the signature field is one of the new features. For example, it will analyze the size specified in the optional header data directory for security, the size specified in the WIN_CERTIFICATE structure and the size specified in the PKCS7 signature itself. These should be the same, taking into account some zero-byte padding.

In case you didn’t know: extra data can be added in the data directory that contains the signature, without invalidating the signature. My Disitool can do this.

With this new version of AnalyzePESig, I found some setup programs that contain extra data after the signature; data that seems to contain installation options for the installer. For example, the Google Chrome installer has this:

20130813-205011

As you can see, the size specified in the optional header data directory for security and the size specified in the WIN_CERTIFICATE structure are both 6272 bytes, but the size of the PKCS7 signature is 6079. So that leaves 181 extra bytes. You can see them here:

20130813-205744

And I found some other installers with extra data (config data or license information) in the signature directory: GotoMyPc, PowerGrep, RegexBuddy.

Wednesday 15 May 2013

Quickpost: Signed PDF Stego

Filed under: Encryption,Hacking,PDF,Quickpost — Didier Stevens @ 14:08

A signed PDF file is just like all signed files with embedded signatures: the signature itself is excluded from the hash calculation.

Open a signed PDF document in a hex editor and search for string /ByteRange. You’ll find something like this:

36 0 obj
<</ByteRange[0 227012 248956 23362 ]            /Contents<308226e106092a864886f7

This indicates which byte sequences  are used for the hash calculation (position and length of each sequence). So in this example, byte sequence 227013-248955 is excluded, because it contains the signature in hex format padded with 0×00 bytes. This padding is not part of the DER signature, you can change it without changing or invalidating the signature.


Quickpost info

Monday 13 May 2013

Adobe Reader and CRLs

Filed under: Encryption,PDF — Didier Stevens @ 18:08

There’s something that I wanted to test out for quite some time, but kept postponing until recently. Adobe Reader will ask confirmation before it retrieves a URL when a PDF document contains an action to do so. But what about the Certificate Revocation List in a signed PDF document?

When you open a signed PDF document with Adobe Reader, the signature gets checked automatically. If the signature is not OK, for example because it doesn’t chain up to a trusted root CA, revocations checks are not performed. In other words, the CRL is not downloaded:

20130426-141512

But when I change the settings so that my root CA is trusted, the signature is considered valid and the CRL is retrieved. No warning is given to the user, it happens automatically and silently. Here is the log entry on my server:

192.168.1.1 – - [26/Apr/2013:11:33:35 -0400] “GET /root.crl HTTP/1.1″ 200 709 “-” “PPKHandler”

PPKHandler is the User Agent String.

20130426-173447

20130426-173632

The CRL file can’t be an empty file, and must be signed by the root CA, otherwise the signature is considered invalid.

So when you open a signed PDF document with Adobe Reader, the signature is automatically checked and the CRL is silently downloaded. This is done with a request to the webserver of the commercial CA which issued the certificate (crl.adobe.com, crl.geotrust.com, …). You can change automatic checking with Preferences / Signatures / Verification.

A quick check with Foxit Reader reveals it doesn’t check the signature automatically.

Wednesday 8 May 2013

Howto: Make Your Own Cert And Revocation List With OpenSSL

Filed under: Encryption — Didier Stevens @ 10:34

Here is a variant to my “Howto: Make Your Own Cert With OpenSSL” method. This time, I needed a signing cert with a Certificate Revocation List (CRL) extension and an (empty) CRL. I used instructions from this post.

Adding a CRL extension to a certificate is not difficult, you just need to include a configuration file with one line. But creating a CRL file requires more steps, that’s why I needed this howto. The start of this howto is the same as my previous howto.

First we generate a 4096-bit long RSA key for our root CA and store it in file ca.key:

openssl genrsa -out ca.key 4096

Generating RSA private key, 4096 bit long modulus
...................................................................................++
........................................................................++
e is 65537 (0x10001)

If you want to password-protect this key, add option -des3.

Next, we create our self-signed root CA certificate ca.crt; you’ll need to provide an identity for your root CA:

openssl req -new -x509 -days 1826 -key ca.key -out ca.crt

You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [XX]:BE
State or Province Name (full name) []:Brussels
Locality Name (eg, city) [Default City]:Brussels
Organization Name (eg, company) [Default Company Ltd]:Didier Stevens
Organizational Unit Name (eg, section) []:
Common Name (eg, your name or your server's hostname) []:Didier Stevens CA
Email Address []:

The -x509 option is used for a self-signed certificate. 1826 days gives us a cert valid for 5 years.

Next step: create our subordinate CA that will be used for the actual signing. First, generate the key:

openssl genrsa -out ia.key 4096

Generating RSA private key, 4096 bit long modulus
.....++
.............................................................................++
e is 65537 (0x10001)

Then, request a certificate for this subordinate CA:

openssl req -new -key ia.key -out ia.csr

You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [XX]:BE
State or Province Name (full name) []:Brussels
Locality Name (eg, city) [Default City]:Brussels
Organization Name (eg, company) [Default Company Ltd]:Didier Stevens
Organizational Unit Name (eg, section) []:
Common Name (eg, your name or your server's hostname) []:Didier Stevens IA
Email Address []:

Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:

Make sure the Common Name is different for both certs, otherwise you’ll get an error. Now, before we process the request for the subordinate CA certificate and get it signed by the root CA, we need to create a couple of files (this step is done with Linux; to create empty file certindex on Windows, you could use Notepad in stead of touch).

touch certindex
echo 01 > certserial
echo 01 > crlnumber

And also create this configuration file (ca.conf):

# Mainly copied from:
# http://swearingscience.com/2009/01/18/openssl-self-signed-ca/

[ ca ]
default_ca = myca

[ crl_ext ]
# issuerAltName=issuer:copy  #this would copy the issuer name to altname
authorityKeyIdentifier=keyid:always

 [ myca ]
 dir = ./
 new_certs_dir = $dir
 unique_subject = no
 certificate = $dir/ca.crt
 database = $dir/certindex
 private_key = $dir/ca.key
 serial = $dir/certserial
 default_days = 730
 default_md = sha1
 policy = myca_policy
 x509_extensions = myca_extensions
 crlnumber = $dir/crlnumber
 default_crl_days = 730

 [ myca_policy ]
 commonName = supplied
 stateOrProvinceName = supplied
 countryName = optional
 emailAddress = optional
 organizationName = supplied
 organizationalUnitName = optional

 [ myca_extensions ]
 basicConstraints = CA:false
 subjectKeyIdentifier = hash
 authorityKeyIdentifier = keyid:always
 keyUsage = digitalSignature,keyEncipherment
 extendedKeyUsage = serverAuth
 crlDistributionPoints = URI:http://example.com/root.crl
 subjectAltName  = @alt_names

 [alt_names]
 DNS.1 = example.com
 DNS.2 = *.example.com

Notice the crlDistributionPoints and DNS. entries pointing to domain example.com. You should change them to your domain.

Now you can sign the request:

openssl ca -batch -config ca.conf -notext -in ia.csr -out ia.crt

Using configuration from ca.conf
Check that the request matches the signature
Signature ok
The Subject's Distinguished Name is as follows
countryName          : PRINTABLE:'BE'
stateOrProvinceName   :ASN.1 12:'Brussels'
localityName          :ASN.1 12:'Brussels'
organizationName      :ASN.1 12:'Didier Stevens'
commonName            :ASN.1 12:'Didier Stevens IA'
Certificate is to be certified until May  3 21:13:02 2015 GMT (730 days)

Write out database with 1 new entries
Data Base Updated

To use this subordinate CA key for Authenticode signatures with Microsoft’s signtool, you’ll have to package the keys and certs in a PKCS12 file:

openssl pkcs12 -export -out ia.p12 -inkey ia.key -in ia.crt -chain -CAfile ca.crt

Enter Export Password:
Verifying - Enter Export Password:

Finally, you can generate the empty CRL file:
openssl ca -config ca.conf -gencrl -keyfile ca.key -cert ca.crt -out root.crl.pem
openssl crl -inform PEM -in root.crl.pem -outform DER -out root.crl
rm root.crl.pem

rm is a Linux command, use del on a Windows machine.

The last step is to host this root.crl file on the webserver pointed to in the CRL extension (http://example.com/root.crl in this example).

If you need to revoke the intermediate certificate, use this command:

openssl ca -config ca.conf -revoke ia.crt -keyfile ca.key -cert ca.crt

And then regenerate the CRL file like explained above.

Friday 26 April 2013

Howto: Add a Digital Signature to a PDF File – Free Software

Filed under: Encryption,PDF — Didier Stevens @ 12:58

This is an update to my post Howto: Add a Digital Signature to a PDF File, but this time I found free software.

Again we use our certificate which we install (open the .p12 file). Install the free JSignPdf software.

Select the PDF file to sign and select an output file (if you don’t want to overwrite your original):

20130426-140107

Push Sign It:

20130426-141230

And then you can check the signature with Adobe Reader:

20130426-141337

If you get the following error, make sure you change your PDF version from %PDF-1.1 to %PDF-1.4:

Choosen configuration requires PDF version update, but it’s not possible in the “append” signature mode.

Tuesday 20 November 2012

Update: AnalyzePESig Version 0.0.0.2

Filed under: Encryption,Forensics,My Software,Update — Didier Stevens @ 20:59

I added several new fields to the output produce by my new tool AnalyzePESig:

  • countCatalogs
  • catalogFilename
  • signatureTimestamp
  • creationtime
  • lastwritetime
  • lastaccesstime
  • dwFileAttributes
  • uiCharacteristics
  • extensions
  • issuer unique id
  • sections
  • subject unique id
  • notBeforeChain
  • notAfterChain

AnalyzePESig_V0_0_0_2.zip (https)
MD5: 738F97F76921FA2220368B3F4190F534
SHA256: E0D43E04AFD242307E3E6B675A650952D2605F45FE55F0B883ACF5B22BA32A01

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