The name of an arbitrarily supplied request parameter is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload d331b"style%3d"x%3aexpression(alert(1))"259b7b94635 was submitted in the name of an arbitrarily supplied request parameter. This input was echoed as d331b"style="x:expression(alert(1))"259b7b94635 in the application's response.
This proof-of-concept attack demonstrates that it is possible to inject arbitrary JavaScript into the application's response. The PoC attack demonstrated uses a dynamically evaluated expression with a style attribute to introduce arbirary JavaScript into the document. Note that this technique is specific to Internet Explorer, and may not work on other browsers.
Issue background
Reflected cross-site scripting vulnerabilities arise when data is copied from a request and echoed into the application's immediate response in an unsafe way. An attacker can use the vulnerability to construct a request which, if issued by another application user, will cause JavaScript code supplied by the attacker to execute within the user's browser in the context of that user's session with the application.
The attacker-supplied code can perform a wide variety of actions, such as stealing the victim's session token or login credentials, performing arbitrary actions on the victim's behalf, and logging their keystrokes.
Users can be induced to issue the attacker's crafted request in various ways. For example, the attacker can send a victim a link containing a malicious URL in an email or instant message. They can submit the link to popular web sites that allow content authoring, for example in blog comments. And they can create an innocuous looking web site which causes anyone viewing it to make arbitrary cross-domain requests to the vulnerable application (using either the GET or the POST method).
The security impact of cross-site scripting vulnerabilities is dependent upon the nature of the vulnerable application, the kinds of data and functionality which it contains, and the other applications which belong to the same domain and organisation. If the application is used only to display non-sensitive public content, with no authentication or access control functionality, then a cross-site scripting flaw may be considered low risk. However, if the same application resides on a domain which can access cookies for other more security-critical applications, then the vulnerability could be used to attack those other applications, and so may be considered high risk. Similarly, if the organisation which owns the application is a likely target for phishing attacks, then the vulnerability could be leveraged to lend credibility to such attacks, by injecting Trojan functionality into the vulnerable application, and exploiting users' trust in the organisation in order to capture credentials for other applications which it owns. In many kinds of application, such as those providing online banking functionality, cross-site scripting should always be considered high risk.
Issue remediation
In most situations where user-controllable data is copied into application responses, cross-site scripting attacks can be prevented using two layers of defenses:
Input should be validated as strictly as possible on arrival, given the kind of content which it is expected to contain. For example, personal names should consist of alphabetical and a small range of typographical characters, and be relatively short; a year of birth should consist of exactly four numerals; email addresses should match a well-defined regular expression. Input which fails the validation should be rejected, not sanitised.
User input should be HTML-encoded at any point where it is copied into application responses. All HTML metacharacters, including < > " ' and =, should be replaced with the corresponding HTML entities (< > etc).
In cases where the application's functionality allows users to author content using a restricted subset of HTML tags and attributes (for example, blog comments which allow limited formatting and linking), it is necessary to parse the supplied HTML to validate that it does not use any dangerous syntax; this is a non-trivial task.
Request
GET /bcStripesDemo.aspx?d331b"style%3d"x%3aexpression(alert(1))"259b7b94635=1 HTTP/1.1 Host: www.myphotoid.net Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
Response
HTTP/1.1 200 OK Connection: close Date: Sat, 20 Nov 2010 15:13:50 GMT Server: Microsoft-IIS/6.0 MicrosoftOfficeWebServer: 5.0_Pub X-Powered-By: ASP.NET X-AspNet-Version: 2.0.50727 Set-Cookie: ASP.NET_SessionId=carvnznjeh5uz0frv2dc0qnk; path=/; HttpOnly Cache-Control: private Content-Type: text/html; charset=utf-8 Content-Length: 20128
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
When a web browser makes a request for a resource, it typically adds an HTTP header, called the "Referer" header, indicating the URL of the resource from which the request originated. This occurs in numerous situations, for example when a web page loads an image or script, or when a user clicks on a link or submits a form.
If the resource being requested resides on a different domain, then the Referer header is still generally included in the cross-domain request. If the originating URL contains any sensitive information within its query string, such as a session token, then this information will be transmitted to the other domain. If the other domain is not fully trusted by the application, then this may lead to a security compromise.
You should review the contents of the information being transmitted to other domains, and also determine whether those domains are fully trusted by the originating application.
Today's browsers may withhold the Referer header in some situations (for example, when loading a non-HTTPS resource from a page that was loaded over HTTPS, or when a Refresh directive is issued), but this behaviour should not be relied upon to protect the originating URL from disclosure.
Note also that if users can author content within the application then an attacker may be able to inject links referring to a domain they control in order to capture data from URLs used within the application.
Issue remediation
The application should never transmit any sensitive information within the URL query string. In addition to being leaked in the Referer header, such information may be logged in various locations and may be visible on-screen to untrusted parties.
When an application includes a script from an external domain, this script is executed by the browser within the security context of the invoking application. The script can therefore do anything that the application's own scripts can do, such as accessing application data and performing actions within the context of the current user.
If you include a script from an external domain, then you are trusting that domain with the data and functionality of your application, and you are trusting the domain's own security to prevent an attacker from modifying the script to perform malicious actions within your application.
Issue remediation
Scripts should not be included from untrusted domains. If you have a requirement which a third-party script appears to fulfil, then you should ideally copy the contents of that script onto your own domain and include it from there. If that is not possible (e.g. for licensing reasons) then you should consider reimplementing the script's functionality within your own code.
The response dynamically includes the following script from another domain:
http://www.google-analytics.com/urchin.js
Request
GET / HTTP/1.1 Host: www.myphotoid.net Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
Response
HTTP/1.1 200 OK Connection: close Date: Sat, 20 Nov 2010 15:13:46 GMT Server: Microsoft-IIS/6.0 MicrosoftOfficeWebServer: 5.0_Pub X-Powered-By: ASP.NET X-AspNet-Version: 2.0.50727 Cache-Control: private Content-Type: text/html; charset=utf-8 Content-Length: 16959
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
The response dynamically includes the following script from another domain:
http://www.google-analytics.com/urchin.js
Request
GET /bcStripesDemo.aspx HTTP/1.1 Host: www.myphotoid.net Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
Response
HTTP/1.1 200 OK Connection: close Date: Sat, 20 Nov 2010 15:13:46 GMT Server: Microsoft-IIS/6.0 MicrosoftOfficeWebServer: 5.0_Pub X-Powered-By: ASP.NET X-AspNet-Version: 2.0.50727 Set-Cookie: ASP.NET_SessionId=t1wusm55jgzspy2jdk5p5bv5; path=/; HttpOnly Cache-Control: private Content-Type: text/html; charset=utf-8 Content-Length: 20018
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
The page contains a form which is used to submit a user-supplied file to the following URL:
http://www.myphotoid.net/default.aspx
Note that Burp has not identified any specific security vulnerabilities with this functionality, and you should manually review it to determine whether any problems exist.
Issue background
File upload functionality is commonly associated with a number of vulnerabilities, including:
File path traversal
Persistent cross-site scripting
Placing of other client-executable code into the domain
Transmission of viruses and other malware
Denial of service
You should review the file upload functionality to understand its purpose, and establish whether uploaded content is ever returned to other application users, either through their normal usage of the application or by being fed a specific link by an attacker.
Some factors to consider when evaluating the security impact of this functionality include:
Whether uploaded content can subsequently be downloaded via a URL within the application.
What Content-type and Content-disposition headers the application returns when the file's content is downloaded.
Whether it is possible to place executable HTML/JavaScript into the file, which executes when the file's contents are viewed.
Whether the application performs any filtering on the file extension or MIME type of the uploaded file.
Whether it is possible to construct a hybrid file containing both executable and non-executable content, to bypass any content filters - for example, a file containing both a GIF image and a Java archive (known as a GIFAR file).
What location is used to store uploaded content, and whether it is possible to supply a crafted filename to escape from this location.
Whether archive formats such as ZIP are unpacked by the application.
How the application handles attempts to upload very large files, or decompression bomb files.
Issue remediation
File upload functionality is not straightforward to implement securely. Some recommendations to consider in the design of this functionality include:
Use a server-generated filename if storing uploaded files on disk.
Inspect the content of uploaded files, and enforce a whitelist of accepted, non-executable content types. Additionally, enforce a blacklist of common executable formats, to hinder hybrid file attacks.
Enforce a whitelist of accepted, non-executable file extensions.
If uploaded files are downloaded by users, supply an accurate non-generic Content-type header, and also a Content-disposition header which specifies that browsers should handle the file as an attachment.
Enforce a size limit on uploaded files (for defense-in-depth, this can be implemented both within application code and in the web server's configuration.
Reject attempts to upload archive formats such as ZIP.
Request
GET / HTTP/1.1 Host: www.myphotoid.net Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
Response
HTTP/1.1 200 OK Connection: close Date: Sat, 20 Nov 2010 15:13:46 GMT Server: Microsoft-IIS/6.0 MicrosoftOfficeWebServer: 5.0_Pub X-Powered-By: ASP.NET X-AspNet-Version: 2.0.50727 Cache-Control: private Content-Type: text/html; charset=utf-8 Content-Length: 16959
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">