Server-Side Template Injection (SSTI) Pattern Regex for Python

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching server-side template injection (ssti) pattern, ported and verified for Python. In security-sensitive code, using an unverified regex can open the door to both false positives and denial-of-service attacks. The snippet below is ready to drop into your Python project — whether you're validating in a Django view, a FastAPI endpoint, or a standalone data processing script.

Python Implementation

# Server-Side Template Injection (SSTI) Pattern
# ReDoS-safe | RegexVault — Security > Injection Patterns

import re

serverside_template_injection_ssti_pattern_pattern = re.compile(r'(?:\{\{[^}]*\}\}|\{%[^%]*%\}|\$\{[^}]*\}|#\{[^}]*\}|<#[^>]*>|@\{[^}]*\}|\[\[.*?\]\]|\{#[^}]*#\}|\{\{7\*7\}\}|\{\{config\}\}|\{\{self\}\}|__class__|__mro__|__import__)')

def validate_serverside_template_injection_ssti_pattern(value: str) -> bool:
    return bool(serverside_template_injection_ssti_pattern_pattern.fullmatch(value))

# Example
print(validate_serverside_template_injection_ssti_pattern("{{7*7}}"))  # True

Test Cases

When to use this pattern

This pattern is drawn from the Security > Injection Patterns category and carries a ReDoS-safe certification. That matters for Python developers because particularly important in Python web servers where CPU-bound regex operations can stall concurrent request handling. RegexVault audits patterns against known backtracking attack vectors, ensuring you have the necessary context before using this regex in a high-stakes production environment.

Common Pitfalls

Technical Notes

Template engines: Jinja2/Twig/Nunjucks use {{ }}, {% %}, Freemarker uses ${}, Ruby ERB uses <%=%>. The probe {{7*7}} (expected result 49) is the canonical SSTI detection test. __class__ and __mro__ are Python object introspection used in Jinja2 sandbox escapes.