Advisory 01/2015: Cross-Site-Scripting (XSS) in tcllib's html::textarea

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                            SektionEins GmbH

                        -= 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]]

    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


   "The Tcl Library is a kitchen sink of packages across a broad spectrum of
   things." - Tcl Library 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.


   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

   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.


   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:


  #!/usr/bin/env tclsh
  package require ncgi
  package require html

  puts [::html::textarea ta]


   It is recommended to upgrade tcllib's html package to the current trunk

   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

  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

  26 February 2015 - released advisory

CVE Information:

   The Common Vulnerabilities and Exposures project ( has not
   assigned a name to this vulnerability yet.


  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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