Arbitrary Code Execution Affecting ejs package, versions <2.5.3

ejs is a popular JavaScript templating engine. Affected versions of the package are vulnerable to Remote Code Execution by letting the attacker under certain conditions control the source folder from which the engine renders include files. You can read more about this vulnerability on the Snyk blog.

There's also a Cross-site Scripting & Denial of Service vulnerabilities caused by the same behaviour.

ejs provides a few different options for you to render a template, two being very similar: ejs.render() and ejs.renderFile(). The only difference being that render expects a string to be used for the template and renderFile expects a path to a template file.

Both functions can be invoked in two ways. The first is calling them with template, data, and options:

ejs.render(str, data, options);

ejs.renderFile(filename, data, options, callback)

The second way would be by calling only the template and data, while ejs lets the options be passed as part of the data:

ejs.render(str, dataAndOptions);

ejs.renderFile(filename, dataAndOptions, callback)

If used with a variable list supplied by the user (e.g. by reading it from the URI with qs or equivalent), an attacker can control ejs options. This includes the root option, which allows changing the project root for includes with an absolute path.

ejs.renderFile('my-template', {root:'/bad/root/'}, callback);

By passing along the root directive in the line above, any includes would now be pulled from /bad/root instead of the path intended. This allows the attacker to take control of the root directory for included scripts and divert it to a library under his control, thus leading to remote code execution.

The fix introduced in version 2.5.3 blacklisted root options from options passed via the data object.

• November 27th, 2016 - Reported the issue to package owner.
• November 27th, 2016 - Issue acknowledged by package owner.
• November 28th, 2016 - Issue fixed and version 2.5.3 released.

• Snyk Blog
• Fix commit