CVE-2023-28756 is a vulnerability classified as a Regular Expression Denial of Service (ReDoS) affecting the Time component in Ruby versions up to 3.2.1. The root cause lies in the Time parser's mishandling of invalid URLs containing specific characters when converting strings to Time objects. This mishandling causes the underlying regular expression engine to enter a state of excessive backtracking, significantly increasing execution time and CPU consumption. An attacker can exploit this by submitting specially crafted input strings that trigger this pathological regex behavior, leading to denial of service conditions due to resource exhaustion. The vulnerability does not require authentication or user interaction, making it potentially exploitable in any context where untrusted input is parsed as a time string. The fixed versions, starting from 0.2.2, address the issue by correcting the parsing logic to avoid the problematic regex evaluation. Although no known exploits have been reported in the wild, the vulnerability poses a risk to applications relying on vulnerable Ruby versions, especially web services exposed to external input. The absence of a CVSS score necessitates an independent severity assessment based on impact and exploitability factors.

For European organizations, the primary impact of CVE-2023-28756 is the potential for denial of service attacks that can degrade or disrupt services relying on Ruby's Time component for parsing time strings. This can affect web applications, APIs, and backend services that process user-supplied or external data containing time information. The increased CPU usage caused by the ReDoS can lead to service slowdowns, increased operational costs, and potential downtime, impacting business continuity and user experience. Organizations in sectors such as finance, e-commerce, and public services that depend on Ruby-based applications are particularly vulnerable. Additionally, the vulnerability could be leveraged as part of a larger attack chain to distract or exhaust resources during more sophisticated intrusions. While the vulnerability does not directly compromise confidentiality or integrity, the availability impact can have significant operational consequences.

To mitigate CVE-2023-28756, organizations should immediately upgrade Ruby to version 0.2.2 or later where the vulnerability is fixed. Where upgrading is not immediately feasible, implement input validation and sanitization to reject or safely handle malformed or suspicious time strings before parsing. Employ application-layer rate limiting and input size restrictions to reduce the risk of resource exhaustion from repeated malicious requests. Monitor application performance metrics and logs for unusual spikes in CPU usage or parsing errors that may indicate exploitation attempts. Incorporate fuzz testing and static analysis tools in the development lifecycle to detect similar regex-related vulnerabilities. Finally, ensure that incident response plans include procedures for handling denial of service incidents related to input parsing.