Tcl Library Source Code

Overview:

   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:

   The input value should be properly HTML-escaped.

   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 {}]]