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CAPEC Details
Name Stored XSS
Likelyhood of attack Typical severity
High Very High
Summary This type of attack is a form of Cross-site Scripting (XSS) where a malicious script is persistenly "stored" within the data storage of a vulnerable web application. Initially presented by an adversary to the vulnerable web application, the malicious script is incorrectly considered valid input and is not properly encoded by the web application. A victim is then convinced to use the web application in a way that creates a response that includes the malicious script. This response is subsequently sent to the victim and the malicious script is executed by the victim's browser. To launch a successful Stored XSS attack, an adversary looks for places where stored input data is used in the generation of a response. This often involves elements that are not expected to host scripts such as image tags (<img>), or the addition of event attibutes such as onload and onmouseover. These elements are often not subject to the same input validation, output encoding, and other content filtering and checking routines.
Prerequisites An application that leverages a client-side web browser with scripting enabled. An application that fails to adequately sanitize or encode untrusted input. An application that stores information provided by the user in data storage of some kind.
Execution Flow
Step Phase Description Techniques
1 Explore [Survey the application for stored user-controllable inputs] Using a browser or an automated tool, an adversary follows all public links and actions on a web site. They record all the links, the forms, the resources accessed and all other potential entry-points for the web application. The adversary is looking for areas where user input is stored, such as user profiles, shopping carts, file managers, forums, blogs, and logs.
  • Use a spidering tool to follow and record all links and analyze the web pages to find entry points.
  • Use a proxy tool to record all links visited during a manual traversal of the web application.
  • Use a browser to manually explore the website and analyze how it is constructed. Many browsers' plugins are available to facilitate the analysis or automate the discovery.
2 Experiment [Probe identified potential entry points for stored XSS vulnerability] The adversary uses the entry points gathered in the "Explore" phase as a target list and injects various common script payloads and special characters to determine if an entry point actually represents a vulnerability and to characterize the extent to which the vulnerability can be exploited.
  • Use a list of XSS probe strings to submit script in input fields that could be stored by the web application. If possible, the probe strings contain a unique identifier so they can be queried for after submitting to see if they are stored.
  • Use a list of HTML special characters to submit in input fields that could be stored by the web application and check if they were properly encoded, replaced, or filtered out.
3 Experiment [Store malicious XSS content] Once the adversary has determined which stored locations are vulnerable to XSS, they will interact with the web application to store the malicious content. The adversary can have many goals, from stealing session IDs, cookies, credentials, and page content from a victim.
  • Store a malicious script on a page that will execute when viewed by the victim.
  • Use a tool such as BeEF to store a hook into the web application. This will alert the adversary when the victim has accessed the content and will give the adversary control over the victim's browser, allowing them access to cookies, user screenshot, user clipboard, and more complex XSS attacks.
4 Exploit [Get victim to view stored content] In order for the attack to be successful, the victim needs to view the stored malicious content on the webpage.
  • Send a phishing email to the victim containing a URL that will direct them to the malicious stored content.
  • Simply wait for a victim to view the content. This is viable in situations where content is posted to a popular public forum.
Solutions Use browser technologies that do not allow client-side scripting. Utilize strict type, character, and encoding enforcement. Ensure that all user-supplied input is validated before being stored.
Related Weaknesses
CWE ID Description
CWE-79 Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Related CAPECS
CAPEC ID Description
CAPEC-63 An adversary embeds malicious scripts in content that will be served to web browsers. The goal of the attack is for the target software, the client-side browser, to execute the script with the users' privilege level. An attack of this type exploits a programs' vulnerabilities that are brought on by allowing remote hosts to execute code and scripts. Web browsers, for example, have some simple security controls in place, but if a remote attacker is allowed to execute scripts (through injecting them in to user-generated content like bulletin boards) then these controls may be bypassed. Further, these attacks are very difficult for an end user to detect.