Introduction

Zero-day vulnerabilities are often associated with sophisticated nation-state actors or elite security researchers. However, sometimes, a straightforward approach like a basic fuzzer combined with keen observation can unearth critical flaws that evade even the most rigorous security assessments. This post delves into how a simple, custom-built fuzzer exposed a severe zero-day vulnerability in a production web application, highlighting the enduring value of fuzzing in security testing.

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Background

During a red team engagement for a major financial institution, we were tasked with evaluating the security of an internal, business-critical web application. The application had previously undergone multiple security reviews, including both static and dynamic analyses, and was considered secure. Our objective was to validate this assumption through comprehensive testing.

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The Fuzzer Setup

We developed a lightweight HTTP fuzzer in Python, utilizing the requests library. The fuzzer's primary function was to brute-force hidden parameters and mutate form fields using a variety of payload templates, such as:

• Integer overflows
• Excessively long strings
• Encoding tricks (e.g., Unicode, double-encoding)
• Common SSRF and XXE payloads

The fuzzer monitored response codes, content length, and indicators of reflected input to identify anomalies.

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The Discovery

While fuzzing a seemingly innocuous file upload endpoint, we observed that the application returned different HTTP status codes based on subtle changes to the filename parameter. This inconsistency prompted a manual investigation.

By supplying a crafted filename like ../../../../../etc/passwd%00.png, we discovered a full-blown path traversal vulnerability with null-byte injection. The backend logic parsed the filename server-side before enforcing any upload restrictions, allowing us to bypass security checks and access arbitrary files on the host system.

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Impact

The vulnerability had significant implications:

• Arbitrary file read on the host system
• Potential exposure of secrets, API keys, and credential files
• Circumvention of client-side and perimeter security measures

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Why It Was Missed

Several factors contributed to the vulnerability going undetected:

• The endpoint was hidden behind a feature flag, making it inaccessible to standard scanners
• No conventional scanner had visibility into the specific parameter
• Exploitation required precise payload formatting with edge-case encoding

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Lessons Learned

1. Standard scanners have limitations: They often lack the creativity to explore unconventional attack vectors. Custom fuzzers with parameter discovery capabilities can bridge this gap.
2. Edge-case payloads are crucial: Testing with null bytes, various encodings, and path manipulation techniques remains effective in uncovering hidden vulnerabilities.
3. Comprehensive input validation is essential: Secure coding practices must encompass all layers, including backend processing, to prevent such vulnerabilities.

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Remediation

Upon reporting, the vulnerability was addressed within 48 hours:

• File handling logic was rewritten to sanitize input before parsing
• Security testing scopes were expanded to include feature-flagged endpoints

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Leveraging Penzzer for Efficient Fuzzing

Building a custom fuzzer, as we did, requires time and expertise. For teams lacking these resources, Penzzer offers a robust alternative. Penzzer is a fuzzing solution designed to automate the discovery of vulnerabilities in web applications. It provides:

• Automated parameter discovery
• Payload mutation strategies
• Comprehensive monitoring of application responses

By integrating Penzzer into your security testing workflow, you can achieve thorough coverage without the overhead of developing and maintaining custom fuzzing tools.