Executive Summary

[{'type': 'paragraph', 'content': 'CVE-2026-3419 is a moderate severity vulnerability in the Fastify web framework affecting versions from 5.7.2 up to 5.8.0. The issue arises because Fastify incorrectly accepts malformed Content-Type headers that contain trailing invalid characters after the subtype token, such as "application/json garbage". This behavior violates the HTTP specification RFC 9110 §8.3.1, which requires strict validation of Content-Type headers.'}, {'type': 'paragraph', 'content': 'Due to a missing end anchor in the regular expression used to validate Content-Type headers, these malformed headers pass validation and are processed normally instead of being rejected with a 415 Unsupported Media Type error. When regex-based content-type parsers are used, the entire malformed string, including the trailing garbage, is matched against registered parsers. This can cause requests with invalid Content-Type headers to be routed to and processed by inappropriate parsers.'}] [1, 3]

Useful 0 Not Useful 0

Impact Analysis

An attacker can exploit this vulnerability by sending requests with malformed Content-Type headers that bypass the usual validation checks. These requests, which should normally be rejected, are instead accepted and processed by the server.

This can lead to unintended processing of malformed or malicious data by content-type parsers that were not designed to handle such inputs, potentially causing unexpected behavior or low integrity impact on the server. Although there is no direct confidentiality or availability impact, the integrity of the processing can be compromised.

A workaround to mitigate this risk is to deploy a Web Application Firewall (WAF) rule that blocks malformed Content-Type headers until the official fix is applied.

Useful 0 Not Useful 0

Compliance Impact

I don't know

Useful 0 Not Useful 0

Detection Guidance

[{'type': 'paragraph', 'content': 'This vulnerability can be detected by monitoring HTTP requests for malformed Content-Type headers that contain trailing invalid characters after the subtype token, such as "application/json garbage". Such headers violate RFC 9110 §8.3.1 and should normally be rejected with a 415 Unsupported Media Type response but are accepted due to the flaw.'}, {'type': 'paragraph', 'content': 'To detect this on your network or system, you can capture and inspect HTTP request headers for Content-Type values that do not strictly conform to the expected media type format. For example, you can use tools like tcpdump, Wireshark, or command-line utilities like curl combined with grep or awk to filter suspicious Content-Type headers.'}, {'type': 'list_item', 'content': "Use tcpdump to capture HTTP traffic and filter for Content-Type headers with trailing garbage: tcpdump -A -s 0 'tcp port 80' | grep -i 'Content-Type: '"}, {'type': 'list_item', 'content': "Use curl to send test requests with malformed Content-Type headers and observe server responses: curl -v -H 'Content-Type: application/json garbage' http://yourserver/"}, {'type': 'list_item', 'content': 'Use log analysis tools or scripts to scan web server logs for requests containing malformed Content-Type headers.'}] [1, 3]

Useful 0 Not Useful 0

Mitigation Strategies

Immediate mitigation steps include deploying a Web Application Firewall (WAF) rule to block or reject HTTP requests containing malformed Content-Type headers with trailing invalid characters. This can prevent such requests from reaching the vulnerable Fastify server.

Additionally, upgrading Fastify to version 5.8.1 or later is the definitive fix, as this version includes corrected regular expressions and enhanced validation logic that properly rejects malformed Content-Type headers in compliance with RFC 9110.

Useful 0 Not Useful 0