A1 - 1 Server Side JS Injection


When eval(), setTimeout(), setInterval(), Function()are used to process user provided inputs, it can be exploited by an attacker to inject and execute malicious JavaScript code on server.

Attack Mechanics

Web applications using the JavaScript eval()function to parse the incoming data without any type of input validation are vulnerable to this attack. An attacker can inject arbitrary JavaScript code to be executed on the server. Similarly setTimeout(), and setInterval()functions can take code in string format as a first argument causing same issues as eval().

This vulnerability can be very critical and damaging by allowing attacker to send various types of commands.

Denial of Service Attack:

An effective denial-of-service attack can be executed simply by sending the commands below to eval()function:


This input will cause the target server's event loop to use 100% of its processor time and unable to process any other incoming requests until process is restarted.

An alternative DoS attack would be to simply exit or kill the running process:




File System Access

Another potential goal of an attacker might be to read the contents of files from the server. For example, following two commands list the contents of the current directory and parent directory respectively:



Once file names are obtained, an attacker can issue the command below to view the actual contents of a file:


An attacker can further exploit this vulnerability by writing and executing harmful binary files using fsand child_processmodules.

How Do I Prevent It?

To prevent server-side js injection attacks:

  • Validate user inputs on server side before processing
  • Do not use eval()function to parse user inputs. Avoid using other commands with similar effect, such as setTimeOut(), setInterval(), and Function().
  • For parsing JSON input, instead of using eval(), use a safer alternative such as JSON.parse(). For type conversions use type related parseXXX()methods.
  • Include "use strict"at the beginning of a function, which enables strict mode within the enclosing function scope.

Source Code Example

In routes/contributions.js, the handleContributionsUpdate()function insecurely uses eval()to covert user supplied contribution amounts to integer.

        // Insecure use of eval() to parse inputs
        var preTax = eval(req.body.preTax);
        var afterTax = eval(req.body.afterTax);
        var roth = eval(req.body.roth);

This makes application vulnerable to SSJS attack. It can fixed simply by using parseInt()instead.

        //Fix for A1 -1 SSJS Injection attacks - uses alternate method to eval
        var preTax = parseInt(req.body.preTax);
        var afterTax = parseInt(req.body.afterTax);
        var roth = parseInt(req.body.roth);

In addition, all functions begin with use strictpragma.

Further Reading