CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

Report generated by XSS.CX at Fri Apr 01 10:26:12 CDT 2011.

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1. Cross-site scripting (reflected) next

Summary

Severity:	Low
Confidence:	Certain
Host:	http://www.synovate.net
Path:	/favicon.ico

Issue detail

The value of the Referer HTTP header is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 348c0"><script>alert(1)</script>95eff1b825a was submitted in the Referer HTTP header. 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.

Because the user data that is copied into the response is submitted within a request header, the application's behaviour is not trivial to exploit in an attack against another user. In the past, methods have existed of using client-side technologies such as Flash to cause another user to make a request containing an arbitrary HTTP header. If you can use such a technique, you can probably leverage it to exploit the XSS flaw. This limitation partially mitigates the impact of the vulnerability.

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 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.

Request

GET /favicon.ico HTTP/1.1
User-Agent: curl/7.21.0 (amd64-pc-win32) libcurl/7.21.0 OpenSSL/0.9.8o zlib/1.2.3
Host: www.synovate.net
Accept: */*
Proxy-Connection: Keep-Alive
Expect: <script>alert(1)</script>
Referer: http://www.google.com/search?hl=en&q=348c0"><script>alert(1)</script>95eff1b825a

Response

HTTP/1.1 404 Not found
Server: Netscape-Enterprise/3.5.1C
Date: Fri, 01 Apr 2011 03:19:55 GMT
Content-type: text/html
Content-length: 363

<TITLE>Not Found</TITLE><H1>Not Found</H1> The requested object does not exist on this server. The link you followed is either outdated, inaccurate, or the server has been instructed not to let you ha
...[SNIP]...
<A HREF="http://www.google.com/search?hl=en&q=348c0"><script>alert(1)</script>95eff1b825a">
...[SNIP]...

2. HTML does not specify charset previous

Summary

Severity:	Information
Confidence:	Certain
Host:	http://www.synovate.net
Path:	/favicon.ico

Issue description

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.

Request

Response

HTTP/1.1 404 Not found
Server: Netscape-Enterprise/3.5.1C
Date: Fri, 01 Apr 2011 03:19:33 GMT
Content-type: text/html
Content-length: 207

<TITLE>Not Found</TITLE><H1>Not Found</H1> The requested object does not exist on this server. The link you followed is either outdated, inaccurate, or the server has been instructed not to let you ha
...[SNIP]...

Report generated by XSS.CX at Fri Apr 01 10:26:12 CDT 2011.