Preventing server-side template injection (SSTI) requires four aspects: 1. Use security configurations, such as disabling method calls and restricting class loading; 2. Avoid user input as template content, only variable replacement and strictly verify input; 3. Adopt sandbox environments, such as Pebble, Mustache or isolating rendering context; 4. Regularly update the dependent version and review the code logic to ensure that the template engine is configured reasonably and prevent the system from being attacked due to user-controllable templates.

Server-side template injection (SSTI) is a security issue that is easily overlooked but has a very high potential risk in Java applications. The template engine is originally intended to make it easier for developers to generate dynamic content, but if used improperly, the attacker may execute arbitrary code through template injection, which directly leads to the system being controlled. Although Java does not easily trigger SSTI like Python or PHP, the consequences will be equally serious if there is a problem.

The following is a few common perspectives to talk about how to prevent SSTI in Java projects.

Use a secure template engine configuration

Many template engines allow expression execution by default, such as Thymeleaf, Freemarker, and Velocity. If these engines do not have restrictions, they will easily become the entrance to SSTI.

Take Freemarker as an example. By default, it allows calling Java methods, such as ${'abc'.getClass().getName()} to get the class name and further execute arbitrary code. To avoid this, you can:

• Disable method calls: cfg.setMethodExposureLevel(Configuration.MethodExposure.LIMITED);
• Disable class template loading: Avoid user input as template content
• Set a whitelisting mechanism to limit accessible classes and methods

Similarly, Thymeleaf has disabled method calls in expressions by default starting with 3.0, but it is still recommended to check the configuration to make sure that SpringEL 's high-risk features are not enabled.

Avoid using user input as template content

The core problem of SSTI is that the template content is controllable. If your app allows users to enter a piece of text and render it as a template, it's almost like opening the door to thieves.

For example, some systems allow users to customize email templates, page content, etc. If these contents are directly handed over to the template engine for processing, it is easy to be injected into malicious expressions.

suggestion:

• Don't let users control the template structure, only variable replacement is allowed
• If you have to let the user edit the template, consider using whitelist variables, or rendering with a sandbox environment
• Strict checksum escape of user input, especially special characters such as ${} , #{} , etc.

Render user templates using sandboxes or isolated environments

If your application does require user-defined templates, consider using some sandboxed template engines, or isolate risk by limiting context.

For example:

• Using Pebble , it does not execute arbitrary method calls by default
• Using Mustache , it is a logically independent template language and is not easy to trigger SSTI
• For Velocity, you can inherit EventCartridge to limit template behavior

In addition, you can also consider running the rendering of the user template in a separate JVM or container, restricting its permissions, such as not loading sensitive classes, prohibiting reflection, etc.

Regularly review how templates are used and dependent versions

The template engine itself may also have security vulnerabilities, such as SSTI-related CVEs that have appeared in older versions of Freemaker or Velocity.

suggestion:

• Keep template engine dependencies updated with the latest stable version
• Scan project dependencies using OWASP Dependency-Check or similar tools
• Review the way template calls in the code to avoid using eval -like logic

In general, although SSTI in Java is not as common as other languages, it cannot be taken lightly. The key point is: don't let users control the template content, configure the template engine reasonably, and use the sandbox environment if necessary. Basically all this is it, security issues are often not very complicated, but are easily overlooked.

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