August 2011 Archives

Wed Aug 17 22:55:42 CEST 2011

Skype - HTML/Javascript Code Injection

Author: Levent "noptrix" Kayan
Affected Software: Skype in version <=, tested on Windows XP,Vista,7
Type: Remote
Type: HTML/Javascript code injection

Skype suffers from a persistent code injection vulnerability due to a lack
of input validation and output sanitization of following profile entries:

[+] home
[+] office
[+] mobile


The following HTML codes can be used to trigger the described vulnerability:

[+] Home Phone Number: INJECTION HERE
[+] Office Phone Number: INJECTION HERE
[+] Mobile Phone Number: <a href="#">INJECTION HERE</a>

For a PoC demonstration see:



An attacker could for example inject HTML/Javascript code. It has not been
verified though, if it's possible to hijack cookies or to attack the underlying
operating system. Attacker could give a try using extern .js files...


Low - ?


Not Fixed


Skype disputes my finding and claims:

"We have had this reported to us by various media outlets and have confirmed
that the person is mistaken, this is not a web window and while it does cause a
phone number to be underlined, does nothing other than this" Skype's spokewoman quote.

Here is my answer:
First of all, they use HTML to embed all entries in Skype user's profile. The
"parser" is not validating the input, so I was able to inject HTML code (any
html tags are possible). My first Skype bug was depending on these entries.
Their fix was: Sanitize the output on their webservers. What about the
input in the client app? Does it make sense to allow users to "embed" HTML code
in their Skype profile and especially in those "phone number" fields?
Also, there is no option to define any HTML code in Skype client. I was able to
find those bugs with Linux Skype client. I guess, they don't focus so much on
that client. Watch my Skype video about first finding at and see how I used the Linux Skype
client. I will stop here, but you can test it.

Posted by Levent Kayan | Permanent link