[Cryptography-dev] Parsing DER from PE File

[Paul Kehrer]

One day I will learn to run the code I write before I ask people to use it. The missing signers variable should go after the pkcs7 assignment. It looks like this:

signers = backend._lib.PKCS7_get0_signers(pkcs7, backend._ffi.NULL, 0)

With that in place and using the extracted.der you previously provided I can parse a cert, which has the following subject/issuer data:

        Issuer: C=GB, ST=Greater Manchester, L=Salford, O=COMODO CA Limited, CN=COMODO RSA Code Signing CA
        Validity
            Not Before: Oct 19 00:00:00 2018 GMT
            Not After : Sep 25 23:59:59 2019 GMT
        Subject: C=GB/postalCode=WA1 1RG, ST=UK, L=WARRINGTON/street=Brunel House, 340 Firecrest Court, O=TATIANA PUK, LIMITED, CN=TATIANA PUK, LIMITED

I've also attached the cert. If this is what you're looking for then your use case is covered by the existing issue, although I still need to decide on an API for this.

-Paul



On December 23, 2018 at 2:17:54 AM, Robert Simmons (rsimmons0 at gmail.com) wrote:

import os
import pathlib
import pefile

target = pathlib.Path().home().joinpath('Desktop').joinpath('HWID_4_0_6YMBWX.exe')
fname = str(target)
totsize = os.path.getsize(target)
pe = pefile.PE(fname)
pe.parse_data_directories(directories=[pefile.DIRECTORY_ENTRY['IMAGE_DIRECTORY_ENTRY_SECURITY']])
sigoff = 0
siglen = 0
for s in pe.__structures__:
    if s.name == 'IMAGE_DIRECTORY_ENTRY_SECURITY':
        sigoff = s.VirtualAddress
        siglen = s.Size
pe.close()
with open(fname, 'rb') as fh:
    fh.seek(sigoff)
    thesig = fh.read(siglen)

from cryptography.hazmat.backends.openssl.backend import backend
from cryptography.hazmat.backends.openssl import x509

bio = backend._bytes_to_bio(thesig[8:])
pkcs7 = backend._lib.d2i_PKCS7_bio(bio.bio, backend._ffi.NULL)
certs = []
for i in range(backend._lib.sk_X509_num(signers)):
    x509_ptr = backend._lib.sk_X509_value(signers, i)
    certs.append(x509._Certificate(backend, x509_ptr))

That's the exact code I'm trying to run with the provided code snippet at the end. If you want to follow along with the exact file I'm working with:
hxxps://dangerous[.]link/d9b72c43-1bdd-415b-b15f-3a436b26bca8

The password to that file is "infected" and btw: it is live malware, so please treat it accordingly. Run code on it in a safe environment for handling malware.

On Sun, Dec 23, 2018 at 4:10 AM Robert Simmons <rsimmons0 at gmail.com> wrote:
I've added the use case to the issue as requested. I tried the code snippet, but the contents of signers is missing. What should that be?

NameError: name 'signers' is not defined

On Fri, Dec 21, 2018 at 11:21 AM Paul Kehrer <paul.l.kehrer at gmail.com> wrote:
Out of curiosity, does the following code load the cert you expect? der should be the bytes of extracted.der:

from cryptography.hazmat.backends.openssl.backend import backend
from cryptography.hazmat.backends.openssl import x509

bio = backend._bytes_to_bio(der)
pkcs7 = backend._lib.d2i_PKCS7_bio(bio.bio, backend._ffi.NULL)
certs = []
for i in range(backend._lib.sk_X509_num(signers)):
    x509_ptr = backend._lib.sk_X509_value(signers, i)
    certs.append(x509._Certificate(backend, x509_ptr))

Certs will be a list of signer certificates -- in this case, just one cert in the list. Please note that this code does not manage memory correctly so it should strictly be used to test if the cert you need is being properly extracted :)

-Paul (reaperhulk)


On December 21, 2018 at 8:02:13 AM, Paul Kehrer (paul.l.kehrer at gmail.com) wrote:

Thanks, that's perfect. Looking at this data it's actually a PKCS7 envelope holding multiple certificates and at the moment cryptography unfortunately has no interface for parsing PKCS7. If you wouldn't mind sharing your use case directly on https://github.com/pyca/cryptography/issues/3983 then it will help me when I'm prioritizing features for upcoming releases.

-Paul


On December 20, 2018 at 2:23:11 PM, Robert Simmons (rsimmons0 at gmail.com) wrote:

Definitely. I've attached the DER data as extracted from the PE file using the following code:

pe = pefile.PE(fname)
pe.parse_data_directories(directories=[pefile.DIRECTORY_ENTRY['IMAGE_DIRECTORY_ENTRY_SECURITY']])
sigoff = 0
siglen = 0
for s in pe.__structures__:
    if s.name == 'IMAGE_DIRECTORY_ENTRY_SECURITY':
        sigoff = s.VirtualAddress
        siglen = s.Size
pe.close()
with open(fname, 'rb') as fh:
    fh.seek(sigoff)
    thesig = fh.read(siglen)
with open('extracted.der', 'wb') as fh:
    fh.write(thesig[8:])

I've attached extracted.der as a zip file to maintain integrity as an attachment.

Thanks!

On Thu, Dec 20, 2018 at 11:12 AM Paul Kehrer <paul.l.kehrer at gmail.com> wrote:
Could you give us an example (in hex or b64 or something) so we can easily reproduce? Make sure any certs you're giving us don't contain sensitive data of course.

-Paul


On December 19, 2018 at 11:55:04 PM, Robert Simmons (rsimmons0 at gmail.com) wrote:

I've asked this question on Stack Overflow here:
https://stackoverflow.com/q/53862702/1033217

I have compared my code to Dider Stevens's disitool here (examine the function ExtractDigitalSignature):
https://github.com/DidierStevens/DidierStevensSuite/blob/master/disitool.py

When I load that extracted file into a variable and try to parse it with cryptography, it fails. If I pipe the same file to openssl on the command line, it works.

I am thinking this has to do with the number of certificates in the directory in the PE file. There can be three (cert, intermediate CA, and CA, etc).

Any idea what's going on?
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