Infinite Loop in facil.io and iodine JSON Parser Causes High CPU Usage

Executive Summary

CVE-2026-41146 is a high-severity denial-of-service vulnerability in the facil.io C micro-framework and the downstream iodine Ruby gem, both of which share the same JSON parser implementation.

The vulnerability occurs because the JSON parser enters an infinite loop when it encounters a nested JSON value starting with the character 'i' or 'I'. Specifically, the parser tries to parse these tokens as integers but fails to advance the parsing position, causing the parser to repeatedly process the same input without progress.

This infinite loop results in the process consuming 100% CPU on one core indefinitely, effectively causing a denial-of-service condition.

The issue arises from the parser's numeral handling logic, where zero-character consumption during numeric parsing is not treated as a failure, allowing invalid tokens like bare 'i' or 'I' to be accepted erroneously.

Minimal JSON payloads that trigger this bug include malformed inputs such as `[i`, `{"a":i`, `[""i`, and `{"a":""i`.

A patch was applied to fix this by adding checks to ensure the parser does not continue if the parsing position does not advance after attempting to parse a number.

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Detection Guidance

This vulnerability can be detected by monitoring for processes using facil.io or iodine that exhibit unusually high CPU usage, specifically pegging one CPU core at 100% during JSON parsing operations.

A practical detection method is to send crafted JSON payloads that trigger the infinite loop, such as minimal JSON inputs containing bare 'i' or 'I' tokens inside arrays or objects, for example: `[i`, `{"a":i`, `[""i`, or `{"a":""i`.

You can test the vulnerability by sending these payloads via HTTP POST requests with Content-Type: application/json to services using facil.io or iodine JSON parsers and observing if the process hangs or CPU usage spikes.

Example command using curl to test a web service endpoint (replace <url> with target):

• curl -X POST -H "Content-Type: application/json" -d '[i' <url>
• curl -X POST -H "Content-Type: application/json" -d '{"a":i' <url>

Additionally, monitoring system tools like top, htop, or ps can help identify processes stuck in the infinite loop by their CPU usage.

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Impact Analysis

This vulnerability can cause a denial-of-service (DoS) condition by making the affected application consume 100% CPU on one core indefinitely when parsing crafted JSON input containing invalid 'i' or 'I' tokens.

Attackers can exploit this by sending specially crafted JSON payloads to applications using facil.io or iodine that parse attacker-controlled JSON, causing the application to hang and become unresponsive.

The impact is limited to availability; there are no observed impacts on confidentiality or integrity.

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Compliance Impact

CVE-2026-41146 is a denial-of-service vulnerability that causes high CPU usage and process hangs due to an infinite loop in JSON parsing. The vulnerability impacts availability but does not affect confidentiality or integrity of data.

Since the vulnerability does not lead to unauthorized data access, modification, or disclosure, it does not directly compromise data privacy or security controls required by standards such as GDPR or HIPAA.

However, the denial-of-service impact could affect system availability, which is a component of some compliance frameworks that require maintaining service availability and resilience.

Organizations relying on facil.io or iodine for JSON parsing should consider this vulnerability in their risk assessments and apply the patch to maintain compliance with availability requirements.

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Mitigation Strategies

The immediate mitigation step is to update facil.io and iodine to versions that include the patch fixing this vulnerability.

The patch modifies the JSON parser to detect and reject zero-character consumption during numeric parsing, preventing the infinite loop caused by invalid tokens like bare 'i' or 'I'.

Specifically, upgrade facil.io to versions including commit 5128747363055201d3ecf0e29bf0a961703c9fa0 or later, and similarly update iodine to version 0.7.58 or later where the fix is applied.

If immediate upgrading is not possible, consider implementing input validation or filtering to reject JSON inputs containing bare 'i' or 'I' tokens in arrays or objects before parsing.

Additionally, monitor system processes for abnormal CPU usage patterns that may indicate exploitation attempts.

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