SSTI (Server-Side Template Injection)

What Is SSTI (Server-Side Template Injection) ?

SSTI (Server-Side Template Injection) ek attack hai jisme ek attacker template ki native syntax ka fayda uthakar usme malicious payload inject karta hai. Yeh compromised template phir server-side execute hota hai.

Template engine ek web page generate karta hai jisme ek fixed template ko dynamic data ke saath combine kiya jata hai.

Attackers SSTI ka use karke user input ko directly template me insert karte hain, jisse wo arbitrary directives add karke template engine ka behavior change kar sakte hain. Isse attackers ko target server par full control milne ka chance hota hai.

What Is the Impact of Server-Side Template Injection?

Server-side template injection vulnerabilities ek website ko alag-alag attacks ke liye expose kar sakti hain, jo template engine ke type aur application ke sath uske kaam karne ke tareeke par depend karta hai. Kabhi-kabhi yeh vulnerabilities zyada bada security risk nahi hoti hain, lekin aksar inka impact severe hota hai.

Sabse severe cases me, attackers remotely code execute kar sakte hain aur backend servers ko poori tarah se control kar sakte hain. Fir yeh attackers in servers ka use karke internal infrastructure par aur bhi attacks launch kar sakte hain. Agar attackers remotely code execute nahi kar pate, tab bhi wo sensitive data ya server par stored files read kar sakte hain. Read access hone par bhi attackers SSTI ka use karke kai aur attacks perform kar sakte hain.

SSTI (Server-Side Template Injection) ka impact kaafi serious ho sakta hai, kyunki yeh directly server-side code execution ka risk create karta hai. Kuch major impacts yeh hain:

Remote Code Execution (RCE) – Agar attacker template engine ke through arbitrary code execute karne me successful ho jaye, toh wo server par full control le sakta hai.
Data Exposure – Attackers sensitive information jaise database credentials, API keys, aur user data ko access kar sakte hain.
Privilege Escalation – Agar attacker low-privilege access le kar server ke internal components aur configurations ko manipulate kare, toh wo higher privileges gain kar sakta hai.
Lateral Movement – Ek compromised server ka use karke attacker dusre internal servers ya applications par attack kar sakta hai.
Denial of Service (DoS) – Attackers malicious templates inject karke server crash kar sakte hain ya uska performance degrade kar sakte hain.

Overall, SSTI ek critical vulnerability hai jo proper validation aur security measures ke bina kisi bhi web application ke liye ek bada security risk ban sakti hai.

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How to Detect SSTI Vulnerabilities

Plain Text vs. Code-Based SSTI Detection

SSTI vulnerability do alag-alag context me ho sakti hai, jisme har ek ke liye alag detection techniques chahiye hoti hain.

Plain Text SSTI Detection

Plain text context me SSTI vulnerabilities detect karne ke liye, testers template expressions ka use payload ke roop me karte hain, jo alag-alag template engines me kaam karte hain. Agar server HTTP response me error message show kare, toh isse SSTI vulnerability hone ka indication mil sakta hai.

Common Template Expressions:

Yeh kuch commonly used template expressions hain jo alag-alag template engines ke liye use hoti hain:

- Jinja2 (Python – Flask, Django, etc.)
  {{7*7}}
- Twig (PHP – Symfony, Drupal, etc.)
  {{7*7}}
- Freemarker (Java – Spring, etc.)
  ${7*7}
- Velocity (Java – Apache Velocity, etc.)
  #set($x = 7*7) $x
- Smarty (PHP – Smarty Template Engine)
  {$7*7}

Agar in expressions ko inject karne par server ka response unexpected behavior dikhata hai, jaise ki actual calculation ka result return hota hai (e.g., 49), toh iska matlab hai ki application SSTI vulnerable ho sakti hai.

Code-Based SSTI Detection

Code context me SSTI vulnerabilities detect karne ke liye, testers specially crafted payloads ka use karte hain jo ya toh server se error message retrieve kar sakein ya phir blank response generate karein.

Basic SSTI Testing Example

Agar ek application HTTP request me personal_greeting variable use kar rahi hai, toh tester is variable me SSTI payload inject karke response analyze kar sakta hai.

1. Normal Request:
  GET /?personal_greeting=username HTTP/1.1 Host: victim-website.com
  Response:
  Hello user01
2. HTML Tag Injection Test:
  GET /?personal_greeting=username}}<tag> HTTP/1.1 Host: victim-website.com
  Response:
  Hello user01 <tag>
  Agar response me HTML tag inject ho raha hai, toh iska matlab hai ki input properly sanitized nahi hai, aur further testing se SSTI vulnerability confirm ki ja sakti hai.

Identifying the Template Engine

Jab injection point detect ho jaye, tab alag-alag template engines ke specific expressions ka use karke yeh confirm kiya jata hai ki backend kis template engine ka use kar raha hai.

Malicious Payload Injection

Testers malicious ya malformed payload inject karke yeh check karte hain ki server ka behavior kya hota hai. Agar server error message return kare jo template engine ka indication de, toh vulnerability confirm ho sakti hai.

Example Payload:

POST /some-endpoint HTTP/1.1 Host: victim-website.com Content-Type: application/x-www-form-urlencoded parameter=${{<%[%'”}}%\.

Possible Response (If Vulnerable):

Agar server ka response me error message template engine ka naam ya syntax error show karta hai, toh application SSTI vulnerable ho sakti hai.

Template Engine Identify Karna

Jab testers template injection detect kar lete hain, toh unhe yeh identify karna hota hai ki kaunsa template engine use ho raha hai. Yeh step simple ho sakta hai, jisme tester ek invalid syntax submit karta hai jo error message me template engine ka naam reveal kar sakta hai. Lekin kuch cases me, yeh technique kaam nahi karti kyunki error messages suppress ho sakte hain. Yeh automation ke liye bhi suitable nahi hoti.

Dusre tareeke se, testers Burp Suite me decision tree ka use karke identification automate kar sakte hain. Yeh tree language-specific payloads ko map karta hai, jisme red arrows failure aur green arrows success indicate karte hain. Kabhi-kabhi ek payload multiple successful responses generate kar sakta hai, jaise {{7*’7’}} ka result Jinja2 me 7777777 aur Twig me 49 hota hai.

Vulnerability Exploit Karna

Jab testers template injection aur underlying template engine identify kar lete hain, toh next step hota hai documentation study karna. Yeh step zaroori hai taaki zero-day exploits dhundhe ja sakein aur verify ho ki SSTI vulnerability exploit ho sakti hai ya nahi.

Key Points Jo Document Me Dhundhne Chahiye:

1. “For Template Authors” Section – Basic syntax aur injection points samajhne ke liye.
2. “Security Considerations” Section – Developer ne agar yeh section skip kiya hoga, toh isme security loopholes mil sakte hain.
3. Built-in Features List – Yeh functions, methods, variables, aur filters cover karta hai jo exploit kiye ja sakte hain.
4. Extension & Plugin List – Kuch external features default enabled hote hain jo attack vectors provide kar sakte hain.

Template Environment Ki Accessibility Check Karna

Agar koi direct exploit nahi milta, toh testers check karte hain ki template environment kitna accessible hai.

✅ Namespace ya “self” Object – Agar yeh expose ho raha hai, toh object ke saare attributes aur methods list kiye ja sakte hain.
✅ Developer-Provided Objects – In objects me sensitive data hone ke chances zyada hote hain. Yeh application-specific hote hain aur alag-alag templates me vary kar sakte hain.

Testers ko yeh process har template ke liye separately apply karna chahiye taaki maximum data expose ho sake.

Final Step: Security Auditing

Ab tak tester ko attack surface ka clear idea mil chuka hoga. Ab conventional security auditing methods apply karni hoti hain taaki har function ko review karke exploitable vulnerabilities identify ki ja sakein.

✅ Kuch functions application-specific hote hain jo attackers ko critical exploitation ka mauka de sakte hain.
✅ Is step ko overall application security assessment ka part maana chahiye.

Agar sab sahi se analyze kiya jaye, toh SSTI ka Remote Code Execution (RCE) me escalate hone ka chance hota hai, jo backend server takeover tak lead kar sakta hai. 🚨

Server-Side Template Injection (SSTI) Attack Se Bachav

Server se template engine ko completely remove karna possible nahi hota, kyunki yeh application ke changes ko bina disruption ke support karta hai. Isliye, templates ko securely use karna aur SSTI prevent karna zaroori hai.

1️⃣ ‘Edit’ Access Ko Restrict Karna

✅ Public templates hackers ke liye easy target hote hain.
✅ Access control rules apply karke sirf authorized users ko access dena chahiye.
✅ Templates ko modify karne ka access sirf developers aur administrators tak limited hona chahiye.
✅ Production templates sirf designated administrators hi access karein, developers nahi. Yeh supply chain attacks aur insider threats ka risk kam karta hai.

2️⃣ Inputs Ko Sanitize Karna

✅ Input sanitization SSTI ka risk kaafi had tak kam kar sakta hai.
✅ Templates me expected inputs ko validate karna chahiye aur unwanted elements remove karne chahiye.
✅ Allowlist approach best hoti hai—jo inputs expected hain sirf unhi ko allow karein, baki sab deny ho jaye.
✅ Regular expressions ka use karke allowed input patterns define kiye ja sakte hain.

⚠️ Limitation – Attackers sanitization bypass karne ke naye tareeke dhundh sakte hain, isliye 100% protection guarantee nahi hoti.

3️⃣ Sandboxing Ka Use Karna

✅ Sandboxing sanitization se zyada secure hota hai.
✅ Yeh ek safe aur isolated environment create karta hai jisme dangerous functions aur modules disable hote hain.
✅ Iska fayda yeh hai ki agar vulnerability exploit ho bhi jaye, toh damage limited rahta hai.

⚠️ Limitation – Sandboxing implement karna mushkil hota hai, aur misconfigurations ka fayda utha ke attackers sandbox bypass kar sakte hain.

4️⃣ Logic-Less Templates Use Karna

✅ Sabse secure approach logic-less templates ka use karna hai.
✅ Yeh code execution ko visual representation se completely alag kar dete hain.
✅ Mustache ek example hai logic-less template engine ka.
✅ Control flow statements ensure karte hain ki templates sirf data-driven ho, aur RCE ka risk bilkul minimal ho jata hai.

Conclusion

SSTI attacks ko prevent karne ke liye access control, input sanitization, sandboxing, aur logic-less templates ka combination best approach hai. Logic-less templates sabse secure option hai, lekin sandboxing aur access restrictions bhi zaroori hote hain.

Application Security with Imperva

Imperva Runtime Application Self-Protection (RASP) provide karta hai, jo ki real-time attack detection aur prevention karta hai aapke application ke runtime environment me, chahe aapke applications kahin bhi ho. RASP external attacks aur injections ko rokta hai aur aapke vulnerability backlog ko reduce karta hai.

RASP ke alawa, Imperva aapke applications, APIs, aur microservices ke liye comprehensive protection provide karta hai:

✅ Web Application Firewall – Aapke applications ke web traffic ka world-class analysis karke attacks prevent karta hai.

✅ API Security – Automated API protection ensure karta hai ki jaise hi aap APIs publish karte hain, wo protected rahein, taki koi bhi unka exploitation na kar sake.

✅ Advanced Bot Protection – Websites, mobile apps aur APIs ke har access point se business logic attacks ko prevent karta hai. Bot traffic pe complete visibility aur control deta hai taaki online fraud jaise account takeover ya competitive price scraping ko roka ja sake.

✅ DDoS Protection – Attack traffic ko edge par hi block karke business continuity ensure karta hai, bina kisi performance impact ke. Chahe aapke assets AWS, Microsoft Azure ya Google Public Cloud me ho, ye on-premises aur cloud-based assets dono ko secure karta hai.

✅ Attack Analytics – Machine learning aur domain expertise ke saath complete visibility provide karta hai, taki application security stack me noise ke beech patterns detect kiye ja sakein aur application attacks isolate aur prevent kiye ja sakein.

✅ Client-Side Protection – Third-party JavaScript code par visibility aur control deta hai, taaki supply chain fraud, data breaches aur client-side attacks ko roka ja sake.