Server-Side Template Injection (SSTI): When Your Template Engine Executes Attacker Code 🎨

Understanding the Hidden Danger in Modern Web Applications

In the landscape of web application security, Server-Side Template Injection (SSTI) represents one of the most critical yet often overlooked vulnerabilities. When threat actors exploit a template’s native syntax and inject malicious payloads into templates, the compromised template is then executed server-side, potentially allowing attackers to gain full control of a targeted server. This vulnerability class has become increasingly prevalent, with one out of every 16 organizations impacted weekly by SSTI attacks over the past three months.

What is Server-Side Template Injection?

Template engines are the backbone of dynamic web content generation. They work by combining fixed templates with volatile data to generate web pages. Popular template engines include Jinja2 (Python), Handlebars (JavaScript), Thymeleaf (Java), Twig (PHP), and many others. These powerful tools separate presentation logic from business logic, making development faster and code more maintainable.

However, this power comes with a price. SSTI vulnerabilities occur when unsanitized user input is directly concatenated into template engines, allowing attackers to inject malicious template syntax that gets evaluated on the server side. Instead of treating user input as plain data, the application processes it as executable template code—a dangerous mishandling that can lead to catastrophic consequences.

How Template Engines Work

Template engines generate web pages by merging static templates with dynamic data. For example, a simple template might look like:

<h1>Welcome, {{ username }}!</h1>

In a secure implementation, the username variable is passed as data and properly escaped. However, when developers concatenate user input directly into the template string itself, disaster can strike:

# Vulnerable code
template = "Welcome, " + user_input + "!"
render_template_string(template)

This seemingly innocent code opens the door for attackers to inject template expressions that the engine will execute.

The Threat Landscape: SSTI in 2024-2025

High-profile platforms such as Atlassian Confluence, CrushFTP, and Rejetto HTTP File Server were specifically targeted and successfully exploited through SSTI vulnerabilities, with CISA highlighting these incidents to emphasize their critical nature. Recent vulnerabilities continue to emerge, including CVE-2024-56085 in Logpoint versions prior to 7.5.0, where authenticated users could inject payloads while creating Search Template Dashboards.

The statistics paint a concerning picture. During the recent three-month period, the Retail/Wholesale sector saw the highest impact, with SSTI attacks affecting one out of every 11 organizations weekly. This sector’s vulnerability stems from high transaction volumes, valuable customer data, and complex third-party integrations—all factors that expand the attack surface.

Furthermore, cloud-based organizations face approximately 30% more frequent SSTI attacks on a weekly basis compared to their on-premises counterparts, likely due to misconfigurations, third-party integrations, and security coverage gaps between cloud providers and customers.

Popular Template Engines and Their Vulnerabilities

Jinja2 (Python/Flask)

Jinja2 is widely used in Python web frameworks, particularly Flask. A classic vulnerable Flask application might look like this:

@app.route("/page")
def page():
    name = request.values.get('name')
    output = Jinja2.from_string('Hello ' + name + '!').render()
    return output

An attacker could exploit this by sending {{7*7}} as the name parameter, which would evaluate to 49 instead of displaying the literal text. This pattern demonstrates both XSS and SSTI vulnerabilities, as the user input is directly embedded in the template without proper sanitization.

For remote code execution, attackers can access Python’s object introspection capabilities to reach dangerous modules:

{{ self._TemplateReference__context.cycler.__init__.__globals__.os }}

This payload navigates through Python’s object hierarchy to access the os module, enabling command execution on the server.

Handlebars (JavaScript/Node.js)

Handlebars is a popular logic-less template engine for JavaScript. While designed to be safer by restricting logic in templates, misconfigurations can still lead to vulnerabilities. Attackers can exploit helper functions or prototype pollution to achieve code execution.

Thymeleaf (Java/Spring)

Thymeleaf requires expressions to be placed within specific attributes, but it supports expression inlining using syntax like [[…]] or [(…)], making [[${7*7}]] a simple SSTI test payload. However, Thymeleaf’s default configuration doesn’t support dynamic template generation, as templates must be predefined, requiring developers to implement their own TemplateResolver to create templates from strings on-the-fly.

When exploitable, Thymeleaf SSTI can be devastating:

${T(java.lang.Runtime).getRuntime().exec('malicious_command')}

Twig (PHP)

Twig follows similar syntax to Jinja2 and is commonly used in PHP applications. Vulnerable implementations allow attackers to execute arbitrary PHP code:

{{_self.env.registerUndefinedFilterCallback("exec")}}{{_self.env.getFilter("id")}}

Why SSTI is More Dangerous Than SQL Injection

While SQL injection remains a critical vulnerability, SSTI often presents an even greater risk to organizations. Here’s why:

1. Direct Server Access

SQL injection typically limits attackers to database operations—reading, modifying, or deleting data. In the most severe SSTI cases, attackers can remotely execute code and fully take over backend servers, then use these servers to launch additional attacks on internal infrastructure. This direct access to server execution environments is fundamentally more powerful than database access alone.

2. Bypass Security Controls

SQL injection attacks must contend with database permissions, stored procedures, and database-level security controls. SSTI, however, executes in the context of the web application itself, often running with full application privileges. This means attackers inherit all the permissions of the application server process.

3. Multi-Stage Attack Platform

Even when attackers cannot execute code remotely, they might be able to read sensitive data or files stored on the server, using SSTI as the basis for many other attacks. SQL injection typically requires data exfiltration through database channels, while SSTI enables direct file system access, credential theft from configuration files, and pivoting to internal networks.

4. Harder to Detect

Traditional security tools excel at detecting SQL injection patterns. However, conventional security layers like EDRs and CWPPs watch operating-system events, but the layer where SSTI starts lies outside their line of sight. Additionally, existing WAF signature-based detections are typically handcrafted for well-known payloads and patterns, with those rules rarely mapping to different template engines’ syntax.

5. Complex Exploitation Paths

Sandboxing complicates but rarely prevents exploitation, as documented escapes exploit reflection APIs or serialization flaws to break containment. Modern exploitation tools can automatically generate hundreds of unique payloads to overwhelm defenses, making SSTI attacks increasingly sophisticated.

Real-World Attack Scenarios

Scenario 1: E-Greeting Card Application

Consider a web application allowing users to create customized greeting cards. Users can input their message, which is rendered into an HTML template. If the application directly embeds user input into the template string, an attacker could inject:

{{config.items()}}

This payload would expose all application configuration variables, potentially revealing database credentials, API keys, and other sensitive information.

Scenario 2: PDF Invoice Generation

Generated PDF files, invoices, and emails usually use templates, making them prime targets for SSTI. An attacker could inject malicious code into invoice fields that gets executed when the PDF is generated server-side, leading to remote code execution with the privileges of the PDF generation service.

Scenario 3: Marketing Email Campaigns

Because template engines often power transactional email, misconfigurations such as open SMTP relays or incorrect MX records compound the risk by exposing internal message flows. Attackers exploiting SSTI in email templates can read internal messages, exfiltrate customer data, or send phishing emails from legitimate infrastructure.

Detecting SSTI Vulnerabilities

Initial Detection

Detection of SSTI requires layering static application security testing, dynamic testing, and runtime monitoring directly into your development pipeline, catching vulnerabilities before they reach production.

The most common detection approach involves fuzzing with polyglot payloads:

${{<%[%'"}}%\

In most cases, this polyglot payload will trigger an error in the presence of an SSTI vulnerability. The Hackmanit Template Injection Table provides an interactive reference containing efficient template injection polyglots for 44 major template engines.

Mathematical Expression Testing

A simple initial test involves injecting mathematical expressions:

{{7*7}}
${7*7}
<%= 7*7 %>
${{7*7}}
#{7*7}

If the server returns 49 instead of the literal expression, template injection is confirmed.

Identification of Template Engine

Once injection is confirmed, attackers identify the specific template engine by testing various syntax patterns:

• Jinja2/Twig: {{7*7}} returns 49
• Smarty: {7*7} returns 49
• Thymeleaf: [[${7*7}]] returns 49
• FreeMarker: ${7*7} returns 49
• Velocity: #set($x=7*7)$x returns 49

Different template engines have different syntax, features, and potential for exploitation, making this identification step critical.

Advanced Exploitation Techniques

Breaking Out of Sandboxes

Many template engines implement sandboxing to restrict dangerous operations. However, attackers have developed sophisticated techniques to escape these restrictions.

In Jinja2, for example, attackers can navigate Python’s object hierarchy to access restricted modules:

{{''.__class__.__mro__[1].__subclasses__()}}

This payload enumerates all Python classes, allowing attackers to find classes with dangerous methods like os.system() or subprocess.Popen().

Reading Sensitive Files

Attackers can read arbitrary files from the server:

{{''.__class__.__mro__[1].__subclasses__()[40]('/etc/passwd').read()}}

Achieving Remote Code Execution

Command execution enables persistence and lateral movement, as cyber attackers write SSH keys to authorized_keys, schedule cron jobs, or deploy webshells in writable directories.

Example RCE payloads:

Jinja2:

{{config.__class__.__init__.__globals__['os'].popen('whoami').read()}}

Thymeleaf:

__${T(java.lang.Runtime).getRuntime().exec("touch /tmp/pwned")}__::.x

Ruby ERB:

<%= system('whoami') %>

Automated Exploitation Tools

Tools like Tplmap automatically select appropriate payloads, handle encoding requirements, and deliver shells within seconds, while AI-driven frameworks can generate hundreds of unique SSTI payloads to overwhelm signature-based defenses.

Popular tools include: - Tplmap: Automatic SSTI detection and exploitation - SSTImap: Interactive SSTI scanner with multi-engine support - Tinja: Efficient SSTI detection using novel polyglots

Prevention and Mitigation Strategies

1. Never Concatenate User Input

The golden rule: never concatenate user input directly into template strings. Always pass user data as parameters:

Vulnerable:

template = "Hello " + user_input + "!"
render_template_string(template)

Secure:

render_template('hello.html', username=user_input)

By using render_template with predefined templates, Flask passes the name variable to the template, and Jinja2 automatically escapes it, rendering it safe from injection attacks.

2. Use Logic-Less Template Engines

Three defenses neutralize the threat: validate every input, switch to logic-less or sandboxed templates, and keep template engines fully patched. Logic-less template engines like Mustache or Handlebars (when properly configured) minimize the attack surface by restricting executable logic in templates.

3. Implement Input Validation

Static application security testing serves as the first line of defense, with configuration to flag functions that build templates with untrusted input or call dangerous render helpers. Pattern-matching rules identifying render_template_string(request.*) in Flask or string concatenation inside template rendering functions catch most issues.

4. Content Security Policy

Implement strict Content Security Policies (CSP) to prevent execution of malicious scripts, providing defense-in-depth even if SSTI vulnerabilities exist.

5. Web Application Firewalls

Deploy WAFs with SSTI-specific rules to detect template syntax patterns like {{...}}, ${...}, or <%...%> in user inputs. However, remember that attackers can sidestep WAF rules with small changes in variable names, whitespace, or encoding.

6. Runtime Application Self-Protection

Modern security solutions observe execution as templates are rendered, identifying malicious behavior at its origin and preventing it from propagating, providing zero-day coverage because detection is driven by runtime behavior.

7. Least Privilege Principles

Run template rendering processes with minimal necessary privileges. If an SSTI vulnerability is exploited, limiting the application’s system access reduces the potential impact.

8. Regular Security Audits

Developers likely skipped the “Security Considerations” section of template engine documentation, which could provide useful security insights. Regular code reviews focusing on template usage patterns help identify vulnerabilities before they reach production.

Common Vulnerable Patterns to Avoid

Pattern 1: User-Controlled Template Selection

# Dangerous
template_name = request.args.get('template')
return render_template(template_name)

Pattern 2: Dynamic Template Generation

// Dangerous
String template = "Hello " + userInput;
templateEngine.process(template, context);

Pattern 3: Debug Endpoints in Production

Debug and preview endpoints provide another common attack vector, as these convenient development tools compile arbitrary templates on demand, allowing cybercriminals to jump from template preview to full system compromise.

Pattern 4: Embedded Business Logic

Embedding business logic directly in templates accelerates development, but it also expands risk, as each capability added—such as data loops, calculations, and API calls—provides cybercriminals with additional methods to manipulate template execution and access system resources.

Business Impact and Risk Assessment

The business impact is immediate: credential theft, data exfiltration, and potential shutdown of revenue-critical services. Organizations face:

• Financial losses from data breaches and downtime
• Regulatory penalties under GDPR, CCPA, and other privacy regulations
• Reputational damage from security incidents
• Legal liability from compromised customer data
• Operational disruption during incident response and remediation

The Retail/Wholesale sector is particularly vulnerable due to high transaction volumes with valuable customer data including personal information and payment details attractive to hackers.

Testing SSTI in Development

Code Review Checklist

• ✅ Are all template rendering functions using parameterized inputs?
• ✅ Is user input ever concatenated into template strings?
• ✅ Are debug template endpoints disabled in production?
• ✅ Do templates have access to sensitive functions or modules?
• ✅ Is input validation applied before template rendering?

Automated Testing

Integrate SSTI detection into CI/CD pipelines:

# Example using SSTImap
python3 sstimap.py -u 'https://example.com/page?name=test' --level 5

Manual Testing Techniques

The approach to detecting SSTI is similar to other injection attacks: identify points of input, fuzz, and review results:

1. Identify input fields reflected in server responses
2. Inject polyglot payloads or mathematical expressions
3. Observe server behavior (errors, evaluation, delays)
4. Identify the template engine
5. Craft exploitation payloads
6. Document and report findings

Industry Standards and Compliance

SSTI vulnerabilities fall under several compliance framework requirements:

• OWASP Top 10 2021: Category A03 (Injection)
• CWE-1336: Improper Neutralization of Special Elements Used in a Template Engine
• NIST: Secure coding practices and input validation requirements
• PCI DSS: Requirement 6.5.1 (Injection flaws)

Conclusion: The Escalating Threat of SSTI

Server-Side Template Injection represents a critical vulnerability class that combines the power of code execution with the stealth of legitimate template processing. Testing for server-side template injections in 2025 remains crucial, especially as developers continue to struggle with adequately validating user input.

The key takeaway: template engines are powerful tools that must be handled with extreme care. User input should never be treated as template code. By following secure coding practices, implementing defense-in-depth strategies, and maintaining vigilance through regular security testing, organizations can protect themselves from this devastating attack vector.

Addressing SSTI vulnerabilities is a critical priority for organizations involved in web application development and maintenance, especially given the widespread use of template engines and the common need for dynamic content generation based on user input. The stakes are high, but with proper awareness and implementation of security best practices, SSTI vulnerabilities can be effectively prevented and mitigated.

Additional Resources

• OWASP Web Security Testing Guide: Server-Side Template Injection
• PortSwigger Web Security Academy: SSTI Tutorials
• PayloadsAllTheThings: Comprehensive SSTI payload repository
• Hackmanit Template Injection Table: Interactive reference for 44 template engines
• James Kettle’s whitepaper: “Server-Side Template Injection: RCE for the Modern Web App”

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Stay secure, validate inputs, and remember: your template engine is a powerful tool—don’t give attackers the keys to the kingdom through unvalidated user input.