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

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