Introduction

Encoding schemes are a crucial yet often overlooked aspect of bypassing security filters in bug bounty hunting. An improperly encoded payload can easily be flagged and blocked by filtering mechanisms, leading to failed exploit attempts and missed vulnerabilities. In this article, we’ll explore the importance of encoding schemes, how they affect payload delivery, and practical tips for effectively using encoding to improve your bug bounty success rate.

Understanding Encoding Schemes and Why They Matter

Encoding schemes are methods of converting data into a specific format to ensure it’s correctly transmitted and interpreted by the receiving system. Common encoding schemes include URL encoding, HTML entity encoding, Base64, UTF-8, and Unicode. Each of these schemes modifies the payload, helping it bypass security filters that rely on detecting specific characters or patterns.

Many security mechanisms implement filters to detect potential attacks, looking for specific keywords or character sequences associated with common exploits. However, these filters often only scan for certain patterns in a specific encoding, such as ASCII or plain text. By encoding a payload in a format the filter doesn’t recognize, you can potentially bypass these filters, allowing your payload to reach the intended target unaltered.

Common Encoding Techniques for Payload Delivery

Here’s a rundown of commonly used encoding schemes in bug bounty hunting, along with examples of how they can help bypass filters.

1. URL Encoding

   • URL encoding replaces special characters with a % sign followed by two hexadecimal digits. For example, "<script>" becomes %3Cscript%3E.
   • Use Case: URL encoding is particularly useful for bypassing filters in query strings, HTTP headers, and URLs, where characters like =, &, or < are often blocked by default.
   • Example: If the filter blocks "<script>", try submitting %3Cscript%3E instead.

2. HTML Entity Encoding

   • HTML entities replace characters with their corresponding HTML representation. For example, "<script>" becomes <script>.
   • Use Case: HTML encoding is useful for bypassing filters in HTML forms, where specific tags or symbols are blocked to prevent Cross-Site Scripting (XSS).
   • Example: If "<script>" is blocked, try <script>.

3. Base64 Encoding

   • Base64 encoding converts data into a text string using a set of 64 characters, making it a common choice for obfuscating strings in URL parameters and HTTP headers.
   • Use Case: Base64 is particularly effective for encoding payloads in HTTP headers, URLs, and cookies where the application expects encoded data.
   • Example: Encode "<script>alert(1);</script>" in Base64 to PHNjcmlwdD5hbGVydCgxKTs8L3NjcmlwdD4=.

4. UTF-8 Encoding and Unicode

   • UTF-8 and Unicode encoding allow characters to be represented in multi-byte sequences, providing a variety of ways to encode text.
   • Use Case: Useful for bypassing filters that may not recognize multi-byte representations of specific characters, such as SQL injection payloads in international applications.
   • Example: If the application blocks ‘ OR 1=1; --, try a UTF-8 encoded variation like \u0027 OR 1=1; --.

Practical Tips for Using Encoding in Bug Bounty Hunting

To effectively use encoding schemes for bypassing filters, follow these best practices:

1. Test Different Encoding Combinations

   • Security filters may block standard payloads but allow encoded variations. Experiment with multiple encoding formats, combining URL encoding with Base64 or HTML entity encoding, to discover combinations that slip past filters.
   • Tip: Create a list of encoded versions for common payloads, such as SQL injection and XSS scripts, and cycle through them during testing.

2. Observe Error Messages

   • Error messages can reveal clues about which encoding types are accepted or blocked. For example, a specific error message may indicate that the system rejects ASCII characters but allows UTF-8.
   • Tip: Adjust your encoding approach based on feedback from error messages to find a working combination faster.

3. Understand the Target’s Context and Limitations

   • Different applications have different filtering rules, often based on their data handling requirements. For example, applications using JSON APIs may parse special characters differently from traditional web forms.
   • Tip: Try encoding the payload in JSON or XML formats when dealing with applications that communicate over these protocols.

4. Use Tools for Encoding Variations

   • Tools like Burp Suite’s Intruder or Repeater, CyberChef, and OWASP ZAP can help automate encoding variations and send multiple encoded requests in a short time.
   • Tip: Configure your tool to rotate through encoding variations of your payload, logging which variations bypass the filter for future reference.

5. Document Successful Encodings

   • Keep a record of encoding schemes that successfully bypass filters on specific platforms. Over time, you’ll build a repository of effective encoding strategies for different target types.
   • Tip: Use tools like Notion or Obsidian to track effective encoding combinations by platform type, target, or vulnerability.

Example: Bypassing a Filter with Encoding

Suppose you’re testing an application’s input field that blocks "<script>" to prevent XSS attacks. Here’s how encoding could help you bypass this filter:

1. Initial Attempt: You input "<script>", but it’s blocked by the filter.
2. URL Encoding: You try %3Cscript%3E, but this is also blocked.
3. HTML Entity Encoding: You try <script>, and the filter allows it.
4. Combination Encoding: In some cases, combining encoding types, such as URL encoding inside Base64, can be effective if the filter doesn’t decode both layers.

In this case, HTML entity encoding succeeds, but each attempt provides insight into how the application processes and blocks input. Repeating this process with various payloads and encoding combinations allows you to fine-tune your approach for future tests.

Conclusion

Encoding schemes are a powerful tool in the bug bounty hunter’s arsenal. By understanding and applying different encoding types, you can bypass filters, reach protected parts of an application, and uncover hidden vulnerabilities. Remember to experiment with various encoding combinations, pay attention to error messages, and keep detailed records of successful encodings for future use. Happy hunting, and remember: sometimes, a simple encoding change is all it takes to make a breakthrough in bug bounty hunting!