SSTI (Server Side Template Injection) is an incredibly hazardous as well as often overlooked method of code injection that has been used against modern websites since long before 2026. Due to the fact that developers continually use untrusted user input directly in template engines there are still a large number of successful attacks being executed by attackers (through the use of SSTI) against popular application frameworks.
SSTI works differently from traditional code injection (which usually requires direct manipulation of back-end application logic) because it manipulates the template engine itself into executing instructions provided by the attacker (i.e., the template engine sees the attacker's data as executable file/data). This ultimately leads to access to sensitive data, arbitrary file access (ie, read any file) or complete remote code execution (RCE) on the targeted server.
For website owners, startups, and development teams building with Laravel, Symfony, Django, Flask, Spring Boot, or similar frameworks, understanding this trick is essential for strong website security and data privacy protection.
What Is Server-Side Template Injection (SSTI)?
SSTI involves using a template engine (like Jinja2, Twig, Blade, or Freemarker) that renders user injected input that has not been sanitized or separated sufficiently from the code being executed, rendering the injected content to be evaluated as part of the templating engine’s syntax. Using SSTI, an attacker can break the standard data rendering methods of the templating engine and execute arbitrary expressions or commands.
Here are some of the possible entry points for accessing an application and performing SSTI on it:
1. Profile fields (i.e., name, biography, etc.)
2. Search parameters.
3. Custom email and notification templates.
4. Exporting & reporting functionally.
5. Dynamically rendered input fields within a template
How the Trick Works in Practice
Basic detection payloads are utilized to check the vulnerability before proceeding to information disclosure and, subsequently, RCE.
Step 1: Detection Detecting the SSTI vulnerability.
When you send simple arithmetic calculations to the server, if the server evaluates the calculations and produces a result versus simply returning them as text (simple arithmetic calculations versus non-executed SQL statements), the SSTI will indicate that a vulnerability exists.
Example payload: {{7*7}} or ${7*7} or {{7+'7'}}
When you see the number 49 in response (rather than a literal string), this indicates that your input is being processed by a template engine.
Step 2: Identifying template engines.
All template engines have a unique behavior. Here are the ways to recognize each template engine:
1. Jinja2 (which is for Python/Flask/Django) - If you enter the equation {{7*7}}, you will receive an answer of 49 and you can traverse objects using the __class__ property of an object.
2. Twig (PHP/Symfony/Laravel): will perform a similar evaluation with math equations as well; however, it will have filters available such as \|join.
3. Freemarker (Java/Spring): ${7*7} or ${"freemarker.template.utility.Execute"?new()("id")}
Step 3: Escalating to RCE
Once the attacker has confirmed that there is an SSTI, they will find all object traversal methods available and access dangerous functions such as system(), exec() or Runtime.getRuntime().exec().
Practical Examples on Major Frameworks
1. Jinja2 (applications created with Flask or Django) A typical instance of a vulnerability is if the developer were to do render_template_string(user_input).
Detection: {{7*7}} → returns 49
Remote Code Execution (RCE) Example for: {{ ''.__class__.__mro__[2].__subclasses__()[40]('/etc/passwd').read() }} (file read)
Or as a means for executing commands: {{ self.__init__.__globals__['os'].popen('id').read() }}
This is made possible because Jinja2 gives access to objects within the Python language, enabling an assailant to access the os module to execute operating system commands.
2. Twig (applications created with Symfony or Laravel) This would generally fall into the category of common PHP frameworks that have a template engine named twig. You can create a vulnerability if you allow user input to control the template's output or filter to the output.
Detection: {{7*7}} → 49
RCE Example (Registering a filter callback): {{_self.env.registerUndefinedFilterCallback('system')}}{{_self.env.getFilter('id')}}
Alternative using array_map and chr for obfuscation: {{passthru(implode(null,array_map(chr(99).chr(104).chr(114),[105,100])))}}
It would take time and have many variations to construct and execute the id command, character by character.
3. Freemarker (Java/Spring Boot) Common template engine for many Enterprise Java applications.
Detection: ${7*7}
RCE payload: ${"freemarker.template.utility.Execute"?new()("id")}
Or: ${''.getClass().forName('java.lang.Runtime').getRuntime().exec('id')}
These tricks continue to work against developers who have the false belief that template engines are "safe" when rendering User Data, particularly when they are creating features like custom reports or view rendering of User created content.
4. Additional Frameworks
a) Blade (Laravel): Much like Twig, Blade is frequently open to attack through user-defined directives or views produced by the Blade view engine.
b) React Server Components / Next.js (when configured incorrectly): Certain potential forms of Remote Code Execution (RCE) exist in server function-based applications, though patches have been released to address these core issues.
c) AI-based systems: A number of AI-enabled tools that utilize workflows (like n8n) can dynamically evaluate expressions specified by users, allowing for code injection through the use of functions such as dynamic/eval.
Although frameworks that have had their core issues repaired allow for code injection into the framework again through custom code or through the use of extensions developed by someone else.
Concerns & Risks of SSTI
When SSTI works as intended, it can create several issues such as:
1. Complete server compromise and remote code execution (RCE);
2. Disclosing sensitive information such as configuration files, environment variables, database credentials, etc.;
3. Exfiltrating data or modifying it;
4. Achieving persistence on the server using web shells; and
5. Violating GDPR and similar data protection regulations around the world (e.g., US, UK, Germany, Netherlands).
SSTI acts similar to "valid" application behavior, and therefore it usually avoids many basic website security controls and WAF rules unless they have been specifically configured to protect against this attack type.
Tips on How to Protect Your Site
1. Always avoid rendering untrusted data as templates; always render template data using the appropriate functions to do so (e.g., render_template() with context variables) instead of using render_template_string().
2. When possible, sandbox template engines; configure strict sandboxes where possible (e.g., Twig has support for sandbox mode; Jinja2 also has limited support for sandboxing). Limit access to dangerous classes/methods.
3. Validate and sanitize all data/input; define an allow list of acceptable characters; and block use of any template syntax (e.g., "{{", "}}", "${", "{%", etc.) unless absolutely needed.
4. Separate Data (user content) from Code (template string). Only send user content to a template as a variable, not as template strings.
5. Keep frameworks and libraries updated. Apply security patches ASAP since many SSTI vectors are fixed in more updated versions.
6. Use Security Tools. Incorporate static application security testing (SAST), Dynamic Scanning and a Web Application Firewall (WAF) using template injection rules into your application security. Tools such as OWASP ZAP and commercial Cyber Security Services can be very helpful in detecting these issues.
7. Review custom features Pay special attention to export functions, email templates, and user-customizable views.
For small businesses/start-ups, routinely reviewing how often they are using templates can help to mitigate most risk exposures.
Why This Continues to be An Effective Method in 2026
Due to the rapid pace of modern-day development, many developers' short cuts often create unintended consequences. Dynamic previews and AI-assisted code generation, can, at times, cause the templates/environments to be treated inappropriately. Therefore, while new developments may be difficult to misuse, there are often custom and/or legacy implementations that will continue to provide opportunities to be misused.
Any organization that maintains and manages customer data should take software supply chain security (SSTI) seriously to meet customers' privacy expectations and build to sustain their trust.
Taking the Next Step
All organizations that use template engines/compilers to render user controlled content should review today how they are rendering user inputs. A quick code audit or professional penetration test can identify hidden risks before attackers do.
Strong website security practices, including proper input handling and regular updates, remain the most effective defense.
Key Takeaways
1. Through Server-Side Template Injection (SSTI), an attacker can convert user-supplied data to code that runs on the server.
2. A simple mathematical expression such as {{7 * 7}} can be executed on the server to demonstrate that there is an SSTI vulnerability present. More complex object traversals can cause Remote Code Execution(RCE) to occur.
3. It is possible to exploit this vulnerability with a wide range of framework implementations (such as Jinja2, Twig, and FreeMarker) that are improperly configured.
4. Prevent SSTI by not rendering untrusted templates; using code sandboxes; and through tight validation of user inputs.
5. By routinely conducting security reviews and performing software updates your exposure to risk can be vastly decreased.
By mitigating code injection-related risks, such as SSTI, you can better secure your company, customer data and brand from current and evolving threats.
