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.
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.
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.
GET /data/feed/api/user/kennycarlile/albumid/5245720683775232113?kind=photo54546%3balert(1)//7aa94a199c9&alt=rss HTTP/1.1 Host: picasaweb.google.com Proxy-Connection: keep-alive Referer: http://picasaweb.google.com/s/c/bin/slideshow.swf Accept: */* User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.13 (KHTML, like Gecko) Chrome/9.0.597.107 Safari/534.13 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: S=sorry=6N0zPerLQtZIDQDH-_qUMQ; GDSESS=ID=e1a16a9b7ce43f1c:EX=1299345465:S=ADSvE-cUtO3XgnQSw6g77AjY8znezaDTsA; NID=44=u2XW7LojnxksmroQ7tLOJH9iTdx0ZE4aJZEmpm79dDPuyRSgPEj_FPvzR_qlOMRWHUOw8qy4vEbL4IaHouYJFpHUibauoR4eQwQw207Tb3UvOzU5XoNP7tDP3nznjsVn; PREF=ID=b1572e52fc3cd4d5:U=846c862c779dbef7:FF=4:LD=en:CR=2:TM=1297804539:LM=1299334711:GM=1:SG=1:S=yYP0BOQvGyQXDqgs
The application publishes a Flash cross-domain policy which uses a wildcard to specify allowed domains, and allows access from specific other domains.
Using a wildcard to specify allowed domains means that any domain matching the wildcard expression can perform two-way interaction with this application. You should only use this policy if you fully trust every possible web site that may reside on a domain which matches the wildcard expression.
Allowing access from specific domains means that web sites on those domains can perform two-way interaction with this application. You should only use this policy if you fully trust the specific domains allowed by the policy.
The Flash cross-domain policy controls whether Flash client components running on other domains can perform two-way interaction with the domain which publishes the policy. If another domain is allowed by the policy, then that domain can potentially attack users of the application. If a user is logged in to the application, and visits a domain allowed by the policy, then any malicious content running on that domain can potentially gain full access to the application within the security context of the logged in user.
Even if an allowed domain is not overtly malicious in itself, security vulnerabilities within that domain could potentially be leveraged by a third-party attacker to exploit the trust relationship and attack the application which allows access.
You should review the domains which are allowed by the Flash cross-domain policy and determine whether it is appropriate for the application to fully trust both the intentions and security posture of those domains.
GET /crossdomain.xml HTTP/1.0 Host: picasaweb.google.com
The cookie does not appear to contain a session token, which may reduce the risk associated with this issue. You should review the contents of the cookie to determine its function.
By default, cookies are scoped to the issuing domain and all subdomains. If you remove the explicit domain attribute from your Set-cookie directive, then the cookie will have this default scope, which is safe and appropriate in most situations. If you particularly need a cookie to be accessible by a parent domain, then you should thoroughly review the security of the applications residing on that domain and its subdomains, and confirm that you are willing to trust the people and systems which support those applications.
GET /data/feed/api/user/kennycarlile/albumid/5245720683775232113?kind=photo&alt=rss HTTP/1.1 Host: picasaweb.google.com Proxy-Connection: keep-alive Referer: http://picasaweb.google.com/s/c/bin/slideshow.swf Accept: */* User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.13 (KHTML, like Gecko) Chrome/9.0.597.107 Safari/534.13 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: S=sorry=6N0zPerLQtZIDQDH-_qUMQ; GDSESS=ID=e1a16a9b7ce43f1c:EX=1299345465:S=ADSvE-cUtO3XgnQSw6g77AjY8znezaDTsA; NID=44=u2XW7LojnxksmroQ7tLOJH9iTdx0ZE4aJZEmpm79dDPuyRSgPEj_FPvzR_qlOMRWHUOw8qy4vEbL4IaHouYJFpHUibauoR4eQwQw207Tb3UvOzU5XoNP7tDP3nznjsVn; PREF=ID=b1572e52fc3cd4d5:U=846c862c779dbef7:FF=4:LD=en:CR=2:TM=1297804539:LM=1299334711:GM=1:SG=1:S=yYP0BOQvGyQXDqgs
The TRACE method is designed for diagnostic purposes. If enabled, the web server will respond to requests which use the TRACE method by echoing in its response the exact request which was received.
Although this behaviour is apparently harmless in itself, it can sometimes be leveraged to support attacks against other application users. If an attacker can find a way of causing a user to make a TRACE request, and can retrieve the response to that request, then the attacker will be able to capture any sensitive data which is included in the request by the user's browser, for example session cookies or credentials for platform-level authentication. This may exacerbate the impact of other vulnerabilities, such as cross-site scripting.
The TRACE method should be disabled on the web server.
The file robots.txt is used to give instructions to web robots, such as search engine crawlers, about locations within the web site which robots are allowed, or not allowed, to crawl and index.
The presence of the robots.txt does not in itself present any kind of security vulnerability. However, it is often used to identify restricted or private areas of a site's contents. The information in the file may therefore help an attacker to map out the site's contents, especially if some of the locations identified are not linked from elsewhere in the site. If the application relies on robots.txt to protect access to these areas, and does not enforce proper access control over them, then this presents a serious vulnerability.
The robots.txt file is not itself a security threat, and its correct use can represent good practice for non-security reasons. You should not assume that all web robots will honour the file's instructions. Rather, assume that attackers will pay close attention to any locations identified in the file. Do not rely on robots.txt to provide any kind of protection over unauthorised access.
GET /robots.txt HTTP/1.0 Host: picasaweb.google.com