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.