CVE-2024-2757 is a denial-of-service vulnerability identified in the PHP Group's PHP language, specifically affecting versions 8.3.* before 8.3.5. The root cause lies in the mb_encode_mimeheader() function, which is designed to encode MIME headers for multibyte character sets. When this function processes input strings that contain long sequences of non-space characters followed by a space, it can enter an infinite loop, causing the PHP process to hang indefinitely. This behavior can be exploited by a remote attacker who sends crafted input data to an application using this function, leading to resource exhaustion and denial of service. The vulnerability does not affect confidentiality or integrity but severely impacts availability. It requires no authentication or user interaction, making it remotely exploitable over the network. The CVSS v3.1 base score is 7.5 (High), with vector AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H, indicating network attack vector, low attack complexity, no privileges or user interaction required, unchanged scope, and high impact on availability. Although no known exploits are reported in the wild yet, the vulnerability is significant due to the widespread use of PHP in web applications. The CWE classification is CWE-400 (Uncontrolled Resource Consumption), highlighting the risk of resource exhaustion attacks. The vulnerability was reserved in March 2024 and published in April 2024, with no official patch links yet, implying that users should monitor for updates and apply patches promptly once released.

For European organizations, this vulnerability poses a significant risk to the availability of web services running PHP 8.3.*. Many European enterprises, government agencies, and service providers rely on PHP for web applications, including content management systems, e-commerce platforms, and internal tools. An attacker exploiting this flaw can cause denial of service by sending specially crafted requests, potentially leading to downtime, degraded user experience, and loss of business continuity. Critical sectors such as finance, healthcare, and public administration could face operational disruptions. Additionally, the vulnerability could be leveraged as part of a larger attack chain or to distract security teams while other attacks are carried out. Since no authentication or user interaction is needed, the attack surface is broad, affecting any exposed PHP-based service. The absence of known exploits currently provides a window for proactive mitigation, but the high CVSS score indicates that exploitation could be straightforward once exploit code becomes available. The impact on confidentiality and integrity is negligible, but the availability impact is high, which is critical for service reliability in European markets.

1. Upgrade PHP to version 8.3.5 or later as soon as it is released by the PHP Group, since this will contain the official fix for the vulnerability. 2. In the interim, implement input validation and sanitization to detect and block unusually long strings of non-space characters followed by a space in inputs processed by mb_encode_mimeheader(). 3. Employ web application firewalls (WAFs) with custom rules to detect and mitigate suspicious payloads targeting this function. 4. Monitor application logs and network traffic for signs of repeated or anomalous requests that could indicate exploitation attempts. 5. Limit exposure of PHP-based services to the internet where possible, using network segmentation and access controls. 6. Conduct code reviews to identify any direct or indirect usage of mb_encode_mimeheader() and assess if alternative encoding methods can be used temporarily. 7. Prepare incident response plans for potential DoS attacks, including resource scaling and failover strategies. 8. Stay informed via official PHP security announcements and CVE databases to apply patches promptly.