The original request is generally innocent, such as a link to another page or a Common Gateway Interface CGI script providing a common service such as a guestbook. The injected script generally attempts to access privileged information or services that the second website does not intend to allow.
The response or the request generally reflects results back to the malicious website. If JavaScript is detected, the XSS Filter searches evidence of reflection, information that would be returned to the attacking website if the attacking request were submitted unchanged. If reflection is detected, the XSS Filter sanitizes the original request so that the additional JavaScript cannot be executed.
The following image shows an example of a site that is modified to prevent a cross-site scripting attack. Web developers may wish to disable the filter for their content. They can do so by setting an HTTP header:. Guidelines for Setting Security Headers. If you want to have extra security that better user agents can provide, use a strict Content-Security-Policy header.
The idea is to apply some kind of heuristics to try to detect reflection XSS attack and automatically neuter the attack. The problematic parts are " heuristics " and " neutering ". The heuristics causes false positives and neutering cannot be safely done because it causes side-effects that can be used to implement XSS attacks and side-channel data attacks on perfectly safe web sites.
The worst part is that this value is the least-safe value of all possible values for this header! For a given secure web site that is, the site does not have reflected XSS vulnerabilities this "XSS protection" feature allows following attacks:. This is possible because the heuristics of this feature are simply "if value of any GET parameter is found in the scripting part of the page source, the script will be automatically modified in user agent dependant way".
In practice, the attacker can e. Note that the rest of the page continues execute JavaScript and the attacker just selectively removed this part of page security. In practice, any JS in the page source can be modified.
For some cases, a page without XSS vulnerability having reflected content can be used to run selected JavaScript on page due the neutering incorrectly turning plain text data into executable JavaScript code.
This allows the attacker to know that the first digit of the secret is 5. The attacker then continues to guess the next digit. In the end, if your site is full of XSS reflection attacks, using the default value of 1 will reduce the attack surface a little bit. However, if your site is secure and you don't emit X-XSS-Protection: 0 , your site will be vulnerable with any browser that supports this feature. If you want defense in depth support from browsers against yet-unknown XSS vulnerabilities on your site, use a strict Content-Security-Policy header and keep sending 0 for this mis-feature.
That doesn't open your site to any known vulnerabilities. This used to be enabled in Edge but Microsoft already removed this mis-feature from Edge. Mozilla Firefox never implemented this. It's usually enabled by default anyway, so the role of this header is to re-enable the filter for this particular website if it was disabled by the user. The anti-XSS filter was added in Chrome 4. Its unknown if that version honored this header.
This header is getting somehow deprecated. This means that if you do not need to support legacy browsers, it is recommended that you use Content-Security-Policy without allowing unsafe-inline scripts instead. Imperva crowdsourcing technology automatically collects and aggregates attack data from across its network, for the benefit of all customers.
The crowdsourcing approach enables extremely rapid response to zero-day threats, protecting the entire user community against any new threat, as soon as a single attack attempt is identified. Crowdsourcing also enables the use of IP reputation system that blocks repeated offenders, including botnet resources which tend to be re-used by multiple perpetrators.
Cross site scripting XSS attacks What is cross site scripting XSS Cross site scripting XSS is a common attack vector that injects malicious code into a vulnerable web application. Cross site scripting attacks can be broken down into two types: stored and reflected. What is stored cross site scripting To successfully execute a stored XSS attack, a perpetrator has to locate a vulnerability in a web application and then inject malicious script into its server e.
Request demo Learn more. Article's content. Latest Blogs. DDoS Mitigation Application Security. Grainne McKeever. Yohann Sillam , Ron Masas. Matthew Hathaway. Research Labs When the same-origin policy is not properly enforced, attackers can inject a script that modifies the web page. For example, the script can allow an attacker to impersonate a pre-authenticated user. It also allows attackers to input malicious code, which is then executed by the browser, or execute JavaScript that modifies content on the page.
XSS can cause serious issues. Attackers often leverage XSS to steal session cookies and impersonate the user. Attackers can also use XSS to deface websites, spread malware, phish for user credentials, support social engineering techniques, and more.
Learn more in our detailed guide to XSS vulnerabilities. Unsanitized user input can put any web application at risk of an XSS attack. Cross site scripting attacks can have devastating consequences. Attackers can masquerade as authorized users via session cookies, allowing them to perform any action allowed by the user account.
A hacker can also change the instructions given to users who visit the target website, misdirecting their behavior. This scenario is particularly dangerous if the target is a government website or provides vital resources in times of crisis.
As a result of an XSS vulnerability, the application accepts malicious code from the user and includes it in its response. For example, suppose a website encodes a message in a URL parameter. If the application does not sanitize the input provided by the URL parameter, an attacker can inject a malicious script into it, like this:.
This is also known as second-order or persistent XSS, because it persists in the system. The data can come from any untrusted source that sends an HTTP request to the application, such as comments posted on a blog or an application that displays email messages using SMTP. An example of a stored XSS attack is an Ecommerce website that allows customers to post reviews of products. Now consider that the mechanism used to publish reviews does not properly sanitize user inputs, allowing attackers to embed HTML tags in the text they submit.
Really enjoyed this product, highly recommend it. The review is published on the page, and loads for every user who views the page hence this is a stored XSS attack. Rather, a malicious change in the DOM environment causes client code to run unexpectedly. Reflected and stored cross-site scripting can be sanitized on the server-side and there are multiple ways of doing it.
What you need to do is whitelist what is allowed.
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