Tcl 1.16 Cross Site Scripting

`  
SektionEins GmbH  
www.sektioneins.de  
  
-= Security Advisory =-  
  
Advisory: Cross-Site-Scripting (XSS) in tcllib's html::textarea  
Release Date: 26 February 2015  
Last Modified: 26 February 2015  
Author: Ben Fuhrmannek [ben.fuhrmannek[at]sektioneins.de]  
  
Application: tcllib - Tcl standard library - versions 1.0.0 to 1.16;  
html package versions lower than 1.4.4  
Severity: The use of html::textarea always results in XSS.  
Risk: High  
Vendor Status: resolved with html package version 1.4.4  
Reference: https://www.sektioneins.de/en/advisories/advisory-012015-xss-tcllib-html-textarea.html  
http://core.tcl.tk/tcllib/tktview/09110adc430de8c91d26015f9697cdd099755e63  
  
Overview:  
  
"The Tcl Library is a kitchen sink of packages across a broad spectrum of  
things." - Tcl Library Home (http://core.tcl.tk/tcllib/home)  
  
Applications using tcllib's ::html::textarea functions are vulnerable to  
Cross-Site-Scripting. This function is usually used to programmatically add  
an HTML <textarea> to the output stream of a CGI script.  
  
No publicly available software has been found to be vulnerable. However it is  
suspected that many non-public Tcl web applications using the  
::html::textarea function are in operation.  
  
Details:  
  
User supplied input is directly inserted into the <textarea> as default  
value, e.g. a textarea named 'ta' with a parameter of ta=XXX results in  
`<textarea>XXX</textarea>`  
  
This can be used to break out of the <textarea>-context and insert arbitrary  
HTML content such as <script>-Tags.  
  
The attack is possible using HTTP GET requests as well as POST and multipart  
form encoded POST requests.  
  
Code:  
  
modules/html/html.tcl  
(http://core.tcl.tk/tcllib/artifact/9a43f5efda2b74a5e61b60f261afdaf9ce1f1221)  
lines 914-919  
  
proc ::html::textarea {name {param {}} {current {}}} {  
::set value [ncgi::value $name $current]  
return "<[string trimright \  
"textarea name=\"$name\"\  
[tagParam textarea $param]"]>$value</textarea>\n"  
}  
  
Proof of Concept:  
  
test.cgi  
  
#!/usr/bin/env tclsh  
package require ncgi  
package require html  
  
::ncgi::parse  
::ncgi::header  
puts [::html::textarea ta]  
  
http://example.com/test.cgi?ta=%3C/textarea%3E%3Cscript%3Ealert%281%29%3C%2fs  
cript%3E  
  
Recommendation:  
  
It is recommended to upgrade tcllib's html package to the current trunk  
version.  
  
In the meantime, a quick application level bugfix would be to encode the  
input variable in question manually. Example with 'ta' as name:  
  
set ::ncgi::value(ta) [::html::quoteFormValue [::ncgi::value ta {}]]  
  
Disclosure Timeline:  
  
28 January 2015 - submitted bug to Tcl Library bugtracker  
http://core.tcl.tk/tcllib/tktview/09110adc430de8c91d26015f9697cdd099755e63  
  
28 January 2015 - ticket was changed to priority '8', severity 'Severe'  
  
24 February 2015 - ticket was changed to priority '9 Immediate', severity 'Critical'  
  
26 February 2015 - ticket resolved and closed with revision 212d1feefe  
http://core.tcl.tk/tcllib/info/212d1feefe  
  
26 February 2015 - released advisory  
  
CVE Information:  
  
The Common Vulnerabilities and Exposures project (cve.mitre.org) has not  
assigned a name to this vulnerability yet.  
  
GPG-Key:  
  
pub 4096R/B379CF81 2014-08-01 [expires: 2016-09-06]  
Key fingerprint = FD91 EEC1 F7F1 48E3 DBBE AB10 9168 88DF B379 CF81  
  
Copyright 2015 SektionEins GmbH. All rights reserved.  
  
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