CWE-1336 Base Incomplete

Improper Neutralization of Special Elements Used in a Template Engine

This vulnerability occurs when an application uses a template engine to process user-controlled input but fails to properly sanitize special syntax characters. Attackers can inject template…

Definition

What is CWE-1336?

This vulnerability occurs when an application uses a template engine to process user-controlled input but fails to properly sanitize special syntax characters. Attackers can inject template expressions or directives that the engine executes, leading to unintended code execution.
Modern web applications frequently use template engines like Twig, Jinja2, FreeMarker, or Pug to dynamically generate content. These engines have their own syntax for expressions and commands. If user input containing this syntax isn't neutralized before processing, attackers can inject malicious template code—potentially accessing sensitive data, modifying application logic, or taking control of the rendering process. This vulnerability affects both server-side and client-side template engines, though attack methods differ. While sometimes manifesting as Cross-Site Scripting (XSS), the root cause is template injection rather than HTML injection. Developers should validate and sanitize all external inputs before passing them to template engines, treating template syntax as potentially dangerous code rather than plain text.
Real-world impact

Real-world CVEs caused by CWE-1336

  • CVE-2024-34359

    Chain: Python bindings for LLM library do not use a sandboxed environment when parsing a template and constructing a prompt, allowing jinja2 Server Side Template Injection and code execution - one variant of a "prompt injection" attack.

  • CVE-2017-16783

    server-side template injection in content management server

  • CVE-2020-9437

    authentication / identity management product has client-side template injection

  • CVE-2020-12790

    Server-Side Template Injection using a Twig template

  • CVE-2021-21244

    devops platform allows SSTI

  • CVE-2020-4027

    bypass of Server-Side Template Injection protection mechanism with macros in Velocity templates

  • CVE-2020-26282

    web browser proxy server allows Java EL expressions from Server-Side Template Injection

  • CVE-2020-1961

    SSTI involving mail templates and JEXL expressions

How attackers exploit it

Step-by-step attacker path

  1. 1

    Identify a code path that handles untrusted input without validation.

  2. 2

    Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.

  3. 3

    Deliver the payload through a normal request and observe the application's reaction.

  4. 4

    Iterate until the response leaks data, executes attacker code, or escalates privileges.

Vulnerable code example

Vulnerable pseudo

MITRE has not published a code example for this CWE. The pattern below is illustrative — see Resources for canonical references.

Vulnerable pseudo 
// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
  // Untrusted input flows directly into the sensitive sink.
  return executeUnsafe(input);
}
Secure code example

Secure pseudo

Secure pseudo 
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
  const safe = validateAndEscape(input);
  return executeWithGuards(safe);
}
What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist

How to prevent CWE-1336

  • Architecture and Design Choose a template engine that offers a sandbox or restricted mode, or at least limits the power of any available expressions, function calls, or commands.
  • Implementation Use the template engine's sandbox or restricted mode, if available.
Detection signals

How to detect CWE-1336

SAST High

Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.

DAST Moderate

Run dynamic application security testing against the live endpoint.

Runtime Moderate

Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.

Code review Moderate

Code review: flag any new code that handles input from this surface without using the validated framework helpers.

Plexicus auto-fix

Plexicus auto-detects CWE-1336 and opens a fix PR in under 60 seconds.

Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.

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Frequently asked questions

Frequently asked questions

What is CWE-1336?

This vulnerability occurs when an application uses a template engine to process user-controlled input but fails to properly sanitize special syntax characters. Attackers can inject template expressions or directives that the engine executes, leading to unintended code execution.

How serious is CWE-1336?

MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.

What languages or platforms are affected by CWE-1336?

MITRE lists the following affected platforms: Java, PHP, Python, JavaScript, Interpreted, Not OS-Specific, AI/ML, Client Server.

How can I prevent CWE-1336?

Choose a template engine that offers a sandbox or restricted mode, or at least limits the power of any available expressions, function calls, or commands. Use the template engine's sandbox or restricted mode, if available.

How does Plexicus detect and fix CWE-1336?

Plexicus's SAST engine matches the data-flow signature for CWE-1336 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.

Where can I learn more about CWE-1336?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/1336.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-1336

CWE-94 Parent

Improper Control of Generation of Code ('Code Injection')

This vulnerability occurs when an application builds executable code using unvalidated external input, such as user data. Because the…

CWE-95 Sibling

Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection')

This vulnerability occurs when an application takes user input and passes it directly into a dynamic code execution function, like eval(),…

CWE-96 Sibling

Improper Neutralization of Directives in Statically Saved Code ('Static Code Injection')

Static Code Injection occurs when an application incorporates unvalidated or improperly sanitized user input directly into a static,…

Resources

Further reading

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