A Deep Dive into FastAPI’s Security Vulnerability: CVE-2024–24762

CVE-2024–24762, a recently uncovered vulnerability within the FastAPI framework, casts a spotlight on the critical need for vigilant security practices.

This article delves into the intricacies of CVE-2024–24762, exploring its impact, the technical mechanisms at play, and the measures taken to address it.

FastAPI, celebrated for its speed and ease of use in building APIs with Python 3.8+, has encountered a significant security challenge.

At the heart of this issue lies a Regular Expression Denial of Service (ReDoS) vulnerability, intricately linked to the handling of form data within applications​.

CVE-2024–24762 emerges in the context of parsing HTTP Content-Type headers with python-multipart, a dependency of FastAPI for form data handling.

The vulnerability is triggered when an attacker sends a specially crafted Content-Type header that the regular expression (RegEx) parser struggles to process.

This malicious input leads to excessive CPU consumption, indefinitely stalling the main event loop of the application, preventing it from servicing further requests​.

This vulnerability is classified under CWE-400, indicating uncontrolled resource consumption that could lead to the exhaustion of system resources, and has been assigned a CVSS score of 7.5, denoting a high severity level.

The issue specifically affects FastAPI versions up to and including 0.109.0​.

Technical Breakdown and Impact

The essence of the vulnerability lies in the exploitation of the RegEx used by python-multipart to parse form data.

By sending a Content-Type header with a complex and carefully constructed value, an attacker can cause the RegEx parsing to consume disproportionate CPU resources.