A Guide to Understanding Server-Side Template Injection

A server-side template injection (SSTI) attack occurs when cyber attackers execute remote code by exploiting how template engines process user input as executable code, rather than data. This is a critical vulnerability that can lead to complete system compromise.

This comprehensive guide provides the framework for detecting, preventing, and responding to SSTI attacks in your organization.

SSTI turns the convenience of template engines into a direct attack vector. Think of it as a technique where cybercriminals exploit a vulnerability in how web applications handle templates.

For instance, template engines such as Jinja2 for Python, Twig for PHP, and FreeMarker for Java simplify development by replacing placeholders like {{ user.name }} with real data at render time. When applications pass untrusted input directly into those placeholders, the engine interprets the cyber attacker's payload as executable code.

The result escalates from data leakage to complete remote code execution, granting adversaries the same privileges your server runs with.

This vulnerability typically appears in seemingly harmless features, such as contact forms, custom email templates, or any field where users control the message content. If that text renders without validation, an cyber attacker can inject {{ 7*7 }} as a test and, on success, chain it into {{self._TemplateReference__context.cycler.__init__.__globals__.os.popen('id').read() }} for system-level access.

What starts as a simple form submission can expose personal information, compromise email addresses, or provide cyber attackers with a phishing link into your infrastructure. A successful exploit can bypass even the most robust email security gateway and deliver malicious templates directly to your support inbox.

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. SSTI may sound niche, but it's often the same starting point as the classic Nigerian prince phishing attack, just wrapped in server-side code instead of an email message.

That said, three defenses neutralize the threat: validate every input, switch to logic-less or sandboxed templates, and keep template engines fully patched. The sections ahead detail how template engines process data, how to detect SSTI during development, and the specific steps to eliminate this attack vector before it reaches production.

Template engines accelerate development by separating presentation from business logic, but user input that shapes template logic creates vulnerabilities. Engines like Jinja2, Twig, and FreeMarker accept static templates, merge them with dynamic data, and return rendered HTML.

When user input remains data rather than code, the attack surface stays minimal.# SAFE: user data inserted into a predefined template

from flask import Flask, render_template_string, request
app = Flask(__name__)

TEMPLATE = "Hello, {{ name | e }}"
@app.route("/safe")
def safe():
 name = request.args.get("name", "world")
 return render_template_string(TEMPLATE, name=name)

The template remains fixed, and Jinja2 escapes the name variable. Even {{7*7}} renders literally because the engine never reevaluates the string.

Vulnerabilities emerge when user input controls the template itself. Jinja2 parses everything you pass, turning cyber attacker payloads into executable Python during rendering.

<p dir="ltr"># UNSAFE: user controls the template</p><p dir="ltr">@app.route("/unsafe")</p><p dir="ltr">def unsafe():</p><p dir="ltr"> user_input = request.args.get("template") # ?template=Hello+{{7*7}}</p> return render_template_string(user_input) # returns "Hello 49"

The function runs inside the full Python runtime, allowing cybercriminals to climb the object hierarchy (''.__class__.mro()[1].__subclasses__()) and reach os.popen for remote code execution. "Sandboxed" engines still evaluate expressions—misconfiguration or debugging flags can reopen dangerous filters and functions.

This pattern can only occur in insecurely configured environments: Twig could process {{ system('id') }} if the function is explicitly exposed, and FreeMarker would require dangerous Java object exposure for similar code execution.

The principle remains constant: template engines that interpret user-controlled logic become interpreters. Also, you must never render untrusted templates, always escape variables, and remember that mixing code with data transforms your template engine into a server gateway.

SSTI vulnerabilities emerge from six standard development practices that blur the line between user data and executable template logic.

These include the following:

• When developers inject untrusted user input into templates and render them with Jinja2 (e.g., using render_template_string), Jinja2 may interpret expressions like {{7*7}} from user input and return 49. This can potentially escalate to remote code execution in vulnerable Flask applications if the input is not properly sanitized. This seemingly harmless mistake escalates instantly to remote code execution.

• Disabling security features compounds the risk. Developers often disable auto-escaping or sandbox features for performance gains, thereby stripping away the engine's protective guardrails. This exposes configuration objects, environment variables, and ultimately shell access, creating direct pathways for attacks to take place.

• Next, you need to debug and preview endpoints provide another common attack vector. These convenient development tools compile arbitrary templates on demand, allowing cybercriminals to jump from template preview to full system compromise. CVE-2024-32651 in ChangeDetection.io exemplifies how these endpoints become critical vulnerabilities in production systems.

• Third-party integrations multiply the attack surface when plugins and microservices accept raw template fragments. The security gap widens when one library assumes inputs are safe while your code assumes they're sanitized, creating injection channels that bypass both systems' protections.

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

• Legacy code written before the widespread awareness of template injection became prevalent continues to operate in production environments. These older modules rarely receive modern security hardening but still process live traffic, providing persistent entry points that often go unnoticed during security reviews.

Before addressing these risks, it's essential to understand how SSTI attacks occur. Let’s explore this in detail in the next section.

A successful SSTI attack follows a repeatable four-stage routine: discover inputs, identify the engine, craft a weaponized payload, then escalate to a full compromise. Understanding each stage lets you break the chain before remote code execution becomes inevitable.

These include:

Attackers scan your application for any input that echoes back to users. Before sending probes, they frequently enumerate MX records to profile mail infrastructure that might relay responses containing injectable templates. Search forms, profile fields, URL parameters, and HTTP headers become prime targets because they often feed directly into server-side templates.

The reconnaissance starts with simple arithmetic expressions: {{7*7}}, ${7*7}, or <%= 7*7 %>. When the response shows 49 instead of the literal text, the input field is executing code rather than displaying data.

Automated tools help accelerate this discovery phase. A single wfuzz or Burp Intruder scan sprays dozens of test expressions across every parameter, flagging fields that compile and execute code. Once mathematical evaluation is confirmed, attackers know the template engine is interpreting user input as executable instructions.

Each engine uses distinct syntax, so attackers probe with engine-specific expressions until one responds:

{{7*7}}

${7*7}

<%= 7*7 %>

[[${7*7}]]

Error messages often reveal the engine's identity and version through stack traces. When errors are suppressed, cybercriminals can fingerprint a platform by testing whitespace handling, filters, or control structures unique to it. Precise identification is critical because escape sequences, sandbox designs, and available built-ins vary dramatically between engines.

With the engine identified, cyber attackers shift from proof-of-concept to system access. Jinja2 exploitation uses object traversal like {{''.__class__.mro()[1].__subclasses__()}} to enumerate Python subclasses, eventually reaching os.popen. Twig misconfigurations enable {% if 1==1 %}{{system('id')}}{% endif %}. FreeMarker does not permit ${"id".exec()}, even when security managers are disabled.

Sandboxing complicates but rarely prevents exploitation. Documented escapes exploit reflection APIs or serialization flaws to break containment. Tools like Tplmap automatically select appropriate payloads, handle encoding requirements, and deliver shells within seconds. At the same time, AI-driven frameworks such as wormgpt can generate hundreds of unique SSTI payloads to overwhelm signature-based defenses.

Command execution enables persistence and lateral movement. The cyber attackers write SSH keys to authorized_keys, schedule cron jobs, or deploy webshells in writable directories. Reading ~/.ssh/id_rsa, dumping cloud metadata endpoints, or pivoting through trust relationships expands the breach from one container to the entire environment.

In addition, social engineering often runs in parallel; even the classic Nigerian prince email lure now embeds SSTI payloads in unsuspecting links, and financial-themed impersonations such as the recent BBT Bank campaign demonstrate how phishing tactics can funnel victims directly into vulnerable template endpoints.

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

Your defense against SSTI attacks should start with implementing a series of protective measures, which are covered in the following sections.

Detection of SSTI requires layering static analysis, dynamic testing, and runtime monitoring directly into your development pipeline, catching vulnerabilities before they reach production. Here’s how you can conduct it:

• Static application security testing (SAST) serves as your first line of defense. Configure scans to flag functions that build templates with untrusted input or call dangerous render helpers. Pattern-matching rules that identify render_template_string(request.*) in Flask or string concatenation inside Twig_Environment::render() catch most issues. The testing guide provides dozens of language-specific sinks you can import into tools like Semgrep or CodeQL.

• Dynamic analysis (DAST) complements static scans. Fire benign template expressions into every parameter during integration tests and monitor response changes. If {{7*7}} returns as 49, you have a live vulnerability. Security testing resources provide engine-specific probes you can script into automated testing frameworks.

• Manual code review will always remain a critical component. When inspecting pull requests that introduce email templates or PDF renderers, ask: "Can a user influence this template's logic?" If the answer isn't definitively "no," require input sanitization or sandboxing before approval.

• Runtime monitoring completes your detection strategy. Instrument template engines to log compilation counts, execution time, and unexpected errors. Metric spikes often precede exploitation attempts, with cybercriminals testing payloads minutes before launching full RCE.

You can also use this smoke-test matrix in unit or integration tests:

{{7*7}}

49

{{7*7}}

49

${7*7}

49

<%= 7*7 %>

49

The moment you confirm template injection, you need to move fast by taking immediate steps such as: containing the affected service, preserving evidence, patching the vulnerability, and then restoring production traffic.

Here are the detailed steps:

• Start by cutting the cyber attacker's access. Pull the vulnerable container or server out of rotation, block inbound traffic that matches standard payloads, and force-expire active sessions. A web application firewall rule that rejects patterns like {{.*}} or ${.*} buys breathing room, and credential rotation prevents reuse of stolen secrets. These containment moves reduce business impact while you investigate.

• Capture evidence before it disappears. Grab relevant forensic artifacts such as application logs, shell history, and a memory snapshot. NIST's incident-handling guide emphasizes that prompt forensic collection facilitates the reconstruction of the kill chain and establishes the scope of compromise. Look for evaluated expressions (49 from {{7*7}}), unexpected template compilation spikes, or outbound connections that mirror documented payloads.

• Patch or upgrade the template engine, re-enable auto-escaping, and remove debug routes that render user-supplied templates. Rotate API keys, database passwords, and cloud tokens that were accessed by the compromised service. ChangeDetection.io's team eliminated a vulnerable endpoint and rebuilt containers from scratch to ensure comprehensive remediation, following industry best practices for addressing potential security risks.

• Deploy fresh, hardened containers after fixes are in place. Run automated tests, including innocuous probes like {{7*7}}, to confirm the flaw is closed, then gradually reintroduce traffic behind feature flags. Monitor CPU spikes or new template compilations for at least one business cycle; security testing guides outline specific indicators worth flagging.

• Capture the lessons learned. Perform a root-cause analysis, update CI tests to bar raw template rendering, and brief leadership on the cost of the outage versus the speed of containment. For example, the timeline below can help keep every stakeholder aligned:

  • T+0–15 min: Isolate service, apply WAF rule, revoke sessions

  • T+15–60 min: Collect logs, memory, and template cache; start credential rotation

  • T+1–4 h: Patch engine, remove malicious templates, rebuild containers

  • T+4–24 h: Restore service in staging, validate with smoke tests

  • T+24–72 h: Resume production traffic, monitor, and publish post-mortem

These steps ensure you eradicate the threat and return to normal operations without leaving hidden footholds behind.

To reiterate, SSTI vulnerabilities create direct paths to remote code execution and complete system compromise when cyber attackers inject malicious templates into unprotected engines. The defense requires three critical layers: rigorous input sanitization before any user data reaches template processors, sandboxing or logic-less engines to eliminate execution contexts, and continuous runtime monitoring to catch exploitation attempts that bypass static defenses.

Prevent these vulnerabilities by patching template engines immediately, implementing allow-lists for template inputs, and monitoring for anomalous compilation patterns or unexpected CPU spikes. Remember, these behavioral indicators often reveal active exploitation attempts before damage occurs.

For real-world threat detections like this, book a customized demo with Abnormal.