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 REST URL parameter 2 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 586b3"><script>alert(1)</script>fd5d373da6f was submitted in the REST URL parameter 2. 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.
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 13:52:07 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Content-Length: 351 Content-Type: text/html
The value of REST URL parameter 3 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 4cb57"><script>alert(1)</script>c4ecd9035a5 was submitted in the REST URL parameter 3. 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.
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 13:52:10 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Content-Length: 351 Content-Type: text/html
The value of REST URL parameter 4 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 671e0"><script>alert(1)</script>b6a54dfe7d0 was submitted in the REST URL parameter 4. 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.
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 13:52:12 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Content-Length: 351 Content-Type: text/html
The value of REST URL parameter 5 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 4b9dc"><script>alert(1)</script>a898bbb80ea was submitted in the REST URL parameter 5. 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.
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 13:52:15 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Content-Length: 351 Content-Type: text/html
The value of REST URL parameter 6 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload a022f"><script>alert(1)</script>50f86d23164 was submitted in the REST URL parameter 6. 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.
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 13:52:17 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Content-Length: 351 Content-Type: text/html
The value of REST URL parameter 7 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 3a98c"><script>alert(1)</script>ded47c83d85 was submitted in the REST URL parameter 7. 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.
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 13:52:19 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Content-Length: 344 Content-Type: text/html
The value of REST URL parameter 2 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 61b08"><script>alert(1)</script>fce45bf1d87 was submitted in the REST URL parameter 2. 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.
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 13:51:48 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Content-Length: 327 Content-Type: text/html
The value of REST URL parameter 3 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 30c02"><script>alert(1)</script>ee3df7c2248 was submitted in the REST URL parameter 3. 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.
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 13:51:51 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Content-Length: 328 Content-Type: text/html
<A HREF="http://b3.mookie1.com/RealMedia/ads/click_lx.ads/B3DM/DLX30c02"><script>alert(1)</script>ee3df7c2248/1541632537/x71/default/empty.gif/4d686437616b354e4750554143347542?x" target="_top"><IMG SR ...[SNIP]...
The value of REST URL parameter 4 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload b07d5"><script>alert(1)</script>7c06b5c6b4a was submitted in the REST URL parameter 4. 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.
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 13:51:53 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Content-Length: 319 Content-Type: text/html
The value of REST URL parameter 2 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 4cfc3"><script>alert(1)</script>6bf04e3a3dc was submitted in the REST URL parameter 2. 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.
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 13:52:33 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Content-Length: 394 Content-Type: text/html
The value of REST URL parameter 3 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload a1eab"><script>alert(1)</script>d0178fb1b3b was submitted in the REST URL parameter 3. 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.
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 13:52:35 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Content-Length: 395 Content-Type: text/html
The value of REST URL parameter 4 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 6d4b9"><script>alert(1)</script>128efcae1a was submitted in the REST URL parameter 4. 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.
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 13:52:38 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Content-Length: 394 Content-Type: text/html
The value of REST URL parameter 5 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 7e390"><script>alert(1)</script>100d6cc6b90 was submitted in the REST URL parameter 5. 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.
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 13:52:40 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Content-Length: 395 Content-Type: text/html
The value of REST URL parameter 6 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload c3a6b"><script>alert(1)</script>a0c2c728d97 was submitted in the REST URL parameter 6. 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.
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 13:52:42 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Content-Length: 393 Content-Type: text/html
The value of REST URL parameter 7 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 51505"><script>alert(1)</script>cd8f0da8f39 was submitted in the REST URL parameter 7. 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.
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 13:52:45 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Content-Length: 387 Content-Type: text/html
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.
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 13:51:26 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Content-Length: 508 Content-Type: text/html
<SCRIPT TYPE="text/javascript" language="JavaScript"> var B3d=new Date(); var B3m=B3d.getTime(); B3d.setTime(B3m+30*24*60*60*1000); document.cookie="ATT=247B3;expires="+B3d.toGMTString()+";path=/;doma ...[SNIP]...
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 13:51:34 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Content-Length: 1931 Content-Type: text/html
<html><head></head><body> <script> function cookie_check(ifd,ife){ var s=ife.indexOf(ifd); if(s==-1)return ""; s+=ifd.length; var e=ife.indexOf(";",s); if(e==-1)e=ife.length; return ife.substring(s, ...[SNIP]...
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 13:51:42 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Content-Length: 238 Content-Type: text/html
<SCRIPT TYPE="text/javascript" language="JavaScript"> var B3d=new Date(); var B3m=B3d.getTime(); B3d.setTime(B3m+30*24*60*60*1000); document.cookie="ATTW=ZapTraderB3;expires="+B3d.toGMTString()+";path ...[SNIP]...
The response contains the following Content-type statement:
Content-Type: text/plain
The response states that it contains plain text. However, it actually appears to contain unrecognised content.
Issue background
If a web response specifies an incorrect content type, then browsers may process the response in unexpected ways. If the specified content type is a renderable text-based format, then the browser will usually attempt to parse and render the response in that format. If the specified type is an image format, then the browser will usually detect the anomaly and will analyse the actual content and attempt to determine its MIME type. Either case can lead to unexpected results, and if the content contains any user-controllable data may lead to cross-site scripting or other client-side vulnerabilities.
In most cases, the presence of an incorrect content type statement 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 a message body, the application should include a single Content-type header which correctly and unambiguously states the MIME type of the content in the response body.
HTTP/1.1 200 OK Date: Thu, 18 Aug 2011 14:25:45 GMT Server: Apache/2.2.3 (Red Hat) P3P: CP="NON NID PSAa PSDa OUR IND UNI COM NAV STA",policyref="/w3c/p3p.xml" Last-Modified: Wed, 13 Oct 2010 18:42:29 GMT ETag: "2000223-1cee-49283f49acf40" Accept-Ranges: bytes Content-Length: 7406 Content-Type: text/plain
