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 defences:
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.
The value of the error_email request parameter is copied into the HTML document as plain text between tags. The payload 27082<script>alert(1)</script>021743a385e was submitted in the error_email parameter. This input was echoed unmodified 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 value of the error_password request parameter is copied into the HTML document as plain text between tags. The payload 35bcd<script>alert(1)</script>e7128760b76 was submitted in the error_password parameter. This input was echoed unmodified in the application's response.
This proof-of-concept attack demonstrates that it is possible to inject arbitrary JavaScript into the application's response.
...[SNIP]... <div class='field-help error'> Sorry, your password must be at least 6 characters long.35bcd<script>alert(1)</script>e7128760b76 </div> ...[SNIP]...
The value of the error_username request parameter is copied into the HTML document as plain text between tags. The payload 6c186<script>alert(1)</script>c129cdbc708 was submitted in the error_username parameter. This input was echoed unmodified in the application's response.
This proof-of-concept attack demonstrates that it is possible to inject arbitrary JavaScript into the application's response.
Most browsers have a facility to remember user credentials that are entered into HTML forms. This function can be configured by the user and also by applications which employ user credentials. If the function is enabled, then credentials entered by the user are stored on their local computer and retrieved by the browser on future visits to the same application.
The stored credentials can be captured by an attacker who gains access to the computer, either locally or through some remote compromise. Further, methods have existed whereby a malicious web site can retrieve the stored credentials for other applications, by exploiting browser vulnerabilities or through application-level cross-domain attacks.
Issue remediation
To prevent browsers from storing credentials entered into HTML forms, you should include the attribute autocomplete="off" within the FORM tag (to protect all form fields) or within the relevant INPUT tags (to protect specific individual fields).
Unless directed otherwise, browsers may store a local cached copy of content received from web servers. Some browsers, including Internet Explorer, cache content accessed via HTTPS. If sensitive information in application responses is stored in the local cache, then this may be retrieved by other users who have access to the same computer at a future time.
Issue remediation
The application should return caching directives instructing browsers not to store local copies of any sensitive data. Often, this can be achieved by configuring the web server to prevent caching for relevant paths within the web root. Alternatively, most web development platforms allow you to control the server's caching directives from within individual scripts. Ideally, the web server should return the following HTTP headers in all responses containing sensitive content:
<!DOCTYPE HTML> <html id='signup-namechecker' role='document'> <head> <!--[if IE]><![endif]--> <!-- Google Website Optimizer Control Script --> <script> function utmx_section(){}function u ...[SNIP]...
If a web response states that it contains HTML content but does not specify a character set, then the browser may analyse the HTML and attempt to determine which character set it appears to be using. Even if the majority of the HTML actually employs a standard character set such as UTF-8, the presence of non-standard characters anywhere in the response may cause the browser to interpret the content using a different character set. This can have unexpected results, and can lead to cross-site scripting vulnerabilities in which non-standard encodings like UTF-7 can be used to bypass the application's defensive filters.
In most cases, the absence of a charset directive does not constitute a security flaw, particularly if the response contains static content. You should review the contents of the response and the context in which it appears to determine whether any vulnerability exists.
Issue remediation
For every response containing HTML content, the application should include within the Content-type header a directive specifying a standard recognised character set, for example charset=ISO-8859-1.
<!DOCTYPE HTML> <html id='signup-namechecker' role='document'> <head> <!--[if IE]><![endif]--> <!-- Google Website Optimizer Control Script --> <script> function utmx_section(){}function u ...[SNIP]...
The server presented a valid, trusted SSL certificate. This issue is purely informational.
The server presented the following certificates:
Server certificate
Issued to:
mailchimp.com
Issued by:
Thawte DV SSL CA
Valid from:
Tue Apr 26 18:00:00 GMT-06:00 2011
Valid to:
Fri Apr 26 17:59:59 GMT-06:00 2013
Certificate chain #1
Issued to:
Thawte DV SSL CA
Issued by:
thawte Primary Root CA
Valid from:
Wed Feb 17 18:00:00 GMT-06:00 2010
Valid to:
Mon Feb 17 17:59:59 GMT-06:00 2020
Certificate chain #2
Issued to:
thawte Primary Root CA
Issued by:
thawte Primary Root CA
Valid from:
Thu Nov 16 18:00:00 GMT-06:00 2006
Valid to:
Wed Jul 16 17:59:59 GMT-06:00 2036
Issue background
SSL helps to protect the confidentiality and integrity of information in transit between the browser and server, and to provide authentication of the server's identity. To serve this purpose, the server must present an SSL certificate which is valid for the server's hostname, is issued by a trusted authority and is valid for the current date. If any one of these requirements is not met, SSL connections to the server will not provide the full protection for which SSL is designed.
It should be noted that various attacks exist against SSL in general, and in the context of HTTPS web connections. It may be possible for a determined and suitably-positioned attacker to compromise SSL connections without user detection even when a valid SSL certificate is used.Report generated by XSS.CX at Sat Aug 20 09:19:19 GMT-06:00 2011.
