Sunday, January 24, 2016

Unusual Reflections

This post is about unusual reflections i.e., when user-supplied input reflects back or ends-up at an unusual place. I will show unusual reflections with the help of four live examples.

-- Reflection in src Attribute of <script> Tag:

This is really unusual when you're getting a reflection as a part of src attribute of <script> tag. The site FAKT.PL has a global Alexa rank of 2296 at the time of writing. Please open the following URL (our harmless probe string ("xxxxxxxx'yyyyy</img) is part of q GET parameter's value):


The screen-shot shows the reflection of our interest.


It can be seen in the screen-shot that developers're using " for holding the value of src attribute and " from the XSS probe string are not filtered. At the same time, </ is not controlled and in my experience, there is a great chance that > is also not controlled. The URL at the time of XSSing is given below followed by a screen-shot.



-- Reflection in SQL Statement:

The title of this heading sounds interesting i.e., when user-supplied input reflects back on the web page as a part of SQL statement. Isn't it? This is happening in the wild. Please open the following URL: http://search.gmw.cn/epaper.jsp. The site GMW.CN has a global Alexa rank of 54 at the time of writing. On the URL, you will find a search bar and lets input our harmless XSS probe string ("xxxxxxxx'yyyyy</img) in the search bar. The screen-shot shows the reflection of our interest. 


The screen-shot above shows our XSS probe string reflects back on the page as a part of where clause of SQL statement. Further <textarea> tag is around the input reflection (HTML context) and its style attribute has been set to "display:none". Furthermore, </ is not filtered or encoded in reflection. Though this is an HTML context and one can assume that the XSS attack payload <img src=x onerror=confirm(1)> will simply work but in-fact it does not because of <textarea> tag around. The </textarea> tag engulfs the contents except its closing counterpart. In order to make the attack payload works, first close the </textarea> tag prematurely and then you can execute the XSS attack payload of your choice. In short, the final attack payload looks like: </textarea><img src=x onerror=confirm(1)> . The screen-shot given below shows the XSS. I was also thinking: WTF SQL statement is doing on the page? Why developers're exposing their database table(s) structure on the web page?


 -- Reflection in alert() Function:

As a part of XSS proof of concept demonstration, we normally use alert() function. I found an exceptional case where user-supplied input ends-up as a part of alert(). Sometime ago, I did a write-up on JavaScript's built-in functions as reflections but alert() is somehow unique because reflections in other functions are common. I can not name the web site in this case but the following screen-shot shows the reflection of XSS probe string ("xxxxxxxx'yyyyy</img) in alert(). The screen-shot also shows " and < from the probe string are filtered but ' is not filtered while at the same time, developers're using single quotes for holding the value.  


For better understanding and XSS this case, first look at the source code of our interest without any injection: <script>alert('For input string: "83845138c47691"');</script>. The numeric value in the alert() function was part of a GET parameter i.e., it might be controlled by the attacker. In order to XSS this case, we have an apparent two choices (given " and < are filtered as can be seen in the screen-shot above).

  1. '-confirm(1)-' 
  2. ');confirm(1);//

In the first case, we break the context with ' and than with the help of JavaScript string operation (minus operator used in this case i.e., In JavaScript, you can subtract anything from anything as long as the syntax is valid), we execute arbitrary JavaScript code (i.e., during string expression evaluation inside alert(), we execute confirm(1).). The second choice lies under the category of "respect the syntax". It means first break the context with ' and than prematurely closes the alert function call with small parenthesis i.e., ) followed by a statement terminator ;. After that proof of concept confirm(1); followed by a single line comments (//) in order to neutralize the affect of closing "');. Here is the screen-shot shows the XSS. I used the second choice from above as a payload.


 -- Reflection in GET parameter name:

This is the case where you will see reflection as a part of GET parameter name along with the value. Normally, the value of a GET parameter is or may be controlled by the attacker but in this special case, an attacker may controlled the name of GET parameter. The BAIDU.COM has a global Alexa rank of 4. Please open the following URL (our harmless probe string ("xxxxxxxx'yyyyy</img) is part of q GET parameter's value):


The screen-shot shows the reflection. It can also be seen that ", ' and / are escaped so it seems no chance to XSS this particular injection point.


So far there is nothing unusual. The interesting thing is if I replace the GET parameter q with probe string ("xxxxxxxx'yyyyy</img) and see the reflection after submission again. I mean now the above URL looks like:


The screen-shot shows the reflection.


The site has recently added protection i.e., escaping of ", ' and / on the Left-Hand-Side (LHS) of =. Sometime ago, they're only doing escaping on the Right-Hand-Side (RHS). I was able to XSS it because of this exceptional reflection on the LHS of =. I conclude on a question: How often during a pen-test you guys try to manipulate a GET parameter name? 

9 comments:

  1. well explained, never thought of manipulating GET parameter !

    Thx for share :)

    ReplyDelete
  2. I only manipulate GET parameter names in situations such as what you describe in the latter part of your post: where I can see that the query params are being interpreted as some kind of map.

    As an exception I often try the []x case it fucks up most server logic :D

    Otherwise, I very rarely try to do much with them

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