Client-side template injection vulnerabilities arise when applications using a client-side template framework dynamically embed user input in web pages. When a web page is rendered, the framework will scan the page for template expressions, and execute any that it encounters. An attacker can exploit this by supplying a malicious template expression that launches a cross-site scripting (XSS) attack. As with normal cross-site scripting, 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 that 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 client-side template injection vulnerabilities is dependent upon the nature of the vulnerable application, the kinds of data and functionality that it contains, and the other applications that belong to the same domain and organization. If the application is used only to display non-sensitive public content, with no authentication or access control functionality, then a client-side template injection flaw may be considered low risk. However, if the same application resides on a domain that 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 organization that 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 organization in order to capture credentials for other applications that it owns. In many kinds of application, such as those providing online banking functionality, client-side template injection should always be considered high risk.
Browser cross-site scripting filters are typically unable to detect or prevent client-side template injection attacks.
If possible, avoid using server-side code to dynamically embed user input into client-side templates. If this is not practical, consider filtering out template expression syntax from user input prior to embedding it within client-side templates.
Note that HTML-encoding is not sufficient to prevent client-side template injection attacks, because frameworks perform an HTML-decode of relevant content prior to locating and executing template expressions.