How Server-Side Template Injection Works Behind the Scenes
A server-side template injection occurs when user input is embedded directly into a templating engine and evaluated without proper sanitization or isolation. This creates an SSTI vulnerability that allows an attacker to inject specially crafted SSTI payloads (for example, {{7*7}} in Jinja2), which the engine will evaluate, enabling everything from data disclosure to arbitrary code execution in the server context. Because different template engines expose different objects and APIs, SSTI payloads vary by platform but share the same danger: they let untrusted input escape the expected rendering flow and execute in the application’s runtime, often leading to full remote code execution or lateral movement if left unchecked.
Minimal vulnerable example in Jinja2
from flask import request, render_template_string
@app.route("/hello")
def hello():
name = request.args.get("name", "world")
# ❌ Vulnerable: directly rendering user input
return render_template_string("Hello " + name)
If a user sends ?name={{7*7}}, the app will evaluate it, returning Hello 49. That’s a textbook SSTI vulnerability.
Minimal vulnerable example in Twig
// ❌ Vulnerable Twig usage
$template = $twig->createTemplate("Welcome " . $_GET['user']);
echo $template->render([]);
An attacker can inject SSTI payloads like {{7*7}} to prove code execution. The danger: simple injection can escalate to reading files, executing OS commands, or pivoting deeper into infrastructure.
Real-World Exploits: SSTI Payloads That Trigger Remote Code Execution
Once an SSTI vulnerability exists, attackers try to move from proof-of-concept math to full RCE. Different template engines handle payloads differently.
Jinja2 payloads
- {{7*7}} → arithmetic execution
- {{config.items()}} → leaks server configs.
- {{ ”.__class__.__mro__[2].__subclasses__() }} → path to RCE
Velocity payloads
- #set($x=”7″)${x} → injection bypass
- #set($a=$class.inspect(“java.lang.Runtime”)) → direct runtime access
Twig payloads
- {{7*7}} → arithmetic
- {{app.request.server.all}} → environment variables
- {{_self.env.registerUndefinedFilterCallback(‘system’)}} → code execution
These SSTI payloads demonstrate how the same vulnerability in different engines leads to different exploit paths, but they are always dangerous.
Where Server Side Template Injection Hides in CI/CD-Driven Applications
Server side template injection isn’t just a web app risk; it shows up in modern CI/CD pipelines too. Typical hiding places include:
- Helm charts in Kubernetes, where template values are rendered dynamically
- Email templates that concatenate user-controlled inputs
- Dashboards where query strings or config data are injected into templates
- DevOps scripts that generate HTML/Markdown using templating engines.
Example:
# ❌ Insecure Helm values with user input
configMap:
appMessage: "{{ .Values.message }}"
If.Values.message comes from untrusted input, it introduces a server-side template injection in your deployment pipeline itself.
Preventing SSTI with Safer Template Patterns and Static Analysis
Mitigating SSTI vulnerabilities requires better coding patterns and early detection.
Secure patterns
- ❌ Don’t use render_template_string or equivalent
- ✅ Use predefined template files and pass sanitized variables
- ✅ Sandbox template engines when available
- ✅ Validate and escape user input before rendering
Insecure vs secure cookie handling (related input risk)
# ❌ Insecure: session cookie without flags
response.set_cookie("session", token)
# ✅ Secure: session cookie hardened
response.set_cookie("session", token, httponly=True, secure=True, samesite="Strict")
Mini Checklist for Developers
- Never render raw user input directly
- Use sandboxed templates when supported
- Sanitize and validate all template variables
- Avoid custom template evaluators
- Scan code for render_template_string or string concatenation patterns
Static analysis and linters can flag risky constructs before they reach production.
Embedding SSTI Checks in DevSecOps Pipelines
Catching server-side template injection early is cheaper and safer than fixing it later. DevSecOps teams should embed checks into pipelines:
- Commit hooks: reject commits with dangerous functions (render_template_string)
- Static analyzers: scan for server side template injection risk in templating code
- Dependency validation: flag outdated template engines with known SSTI vulnerabilities
- Pipeline gates: block merges until SSTI checks pass
By making SSTI payload detection part of CI/CD, you prevent exploitable code from ever shipping.
Don’t Let Server-Side Template Injection Slip Into Your Stack
A single server-side template injection can escalate from math tricks ({{7*7}}) to full remote code execution. SSTI vulnerabilities appear not only in web apps but also in CI/CD pipelines, Helm charts, and email templates.
Key takeaways
- Never render raw user input into templates
- Validate and sanitize all dynamic variables
- Different engines (Jinja2, Velocity, Twig) have different SSTI payloads, but all can be weaponized
- Use static analysis and fail-fast gates in pipelines
- Audit templates in your stack regularly
Tools like Xygeni help teams detect unsafe template usage, block SSI vulnerabilities, and enforce secure practices across pipelines and dependencies. In DevSecOps, treating SSTI payloads as a top-tier risk is essential because a single injection in your pipeline or app can compromise your entire environment.
