CVE-2026-23897 is a vulnerability classified under CWE-1333, related to inefficient regular expression complexity in the Apollo Server GraphQL server. Apollo Server versions from 2.0.0 up to 3.13.0, 4.2.0 up to but not including 4.13.0, and 5.0.0 up to but not including 5.4.0 are affected when using the startStandaloneServer default configuration from the @apollo/server/standalone package. The vulnerability arises because the server processes specially crafted GraphQL request bodies containing exotic character set encodings that cause the server’s regular expression engine to consume excessive CPU resources. This leads to a denial of service (DoS) condition by making the server unresponsive or significantly degraded. Notably, this issue does not affect users who employ Apollo Server as a dependency within integration packages such as @as-integrations/express5 or @as-integrations/next, which use different server startup mechanisms. The vulnerability can be exploited remotely without authentication or user interaction, making it accessible to any attacker who can send crafted requests to the server endpoint. Although no known exploits have been reported in the wild, the vulnerability’s nature and ease of exploitation make it a significant risk. The CVSS v3.1 base score is 7.5, indicating high severity, with the vector AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H, meaning network attack vector, low attack complexity, no privileges or user interaction required, unchanged scope, no confidentiality or integrity impact, but high impact on availability. The vulnerability was published on February 4, 2026, and was reserved on January 16, 2026. No official patches or updates are linked in the provided data, but upgrading to versions beyond the affected ranges or avoiding startStandaloneServer usage is recommended.

For European organizations, this vulnerability poses a significant risk of service disruption, particularly for those relying on Apollo Server in standalone mode to serve GraphQL APIs. Denial of service attacks can lead to downtime, impacting business operations, customer experience, and potentially causing financial losses. Sectors such as finance, e-commerce, telecommunications, and public services that increasingly adopt GraphQL for flexible API management are at risk. The vulnerability’s ease of exploitation without authentication means attackers can launch DoS attacks remotely, potentially from anywhere globally, increasing the threat surface. Additionally, organizations with strict uptime requirements or those providing critical services may face regulatory and reputational consequences if services are disrupted. Since the vulnerability does not affect integration package users, organizations using Apollo Server embedded in frameworks like Express or Next.js are less impacted, but those using the standalone server directly must prioritize mitigation. The lack of known exploits in the wild suggests a window for proactive defense, but the high CVSS score indicates urgency in addressing the issue.

European organizations should immediately audit their use of Apollo Server to determine if they use the startStandaloneServer default configuration in affected versions. If so, they should upgrade Apollo Server to versions later than 3.13.0, 4.13.0, or 5.4.0, where this vulnerability is resolved. If upgrading is not immediately feasible, organizations should consider migrating from standalone server usage to integration packages such as @as-integrations/express5 or @as-integrations/next, which are not affected by this vulnerability. Implementing Web Application Firewalls (WAFs) with rules to detect and block suspicious GraphQL requests containing exotic character set encodings can provide temporary protection. Rate limiting and IP reputation filtering can reduce the risk of DoS attacks. Monitoring server performance metrics and setting up alerts for unusual CPU spikes can help detect exploitation attempts early. Additionally, organizations should review their incident response plans to include scenarios involving GraphQL service disruptions. Finally, keeping dependencies up to date and subscribing to Apollo Server security advisories will help maintain ongoing protection.