CVE-2024-31309 is a vulnerability classified under CWE-20 (Improper Input Validation) found in Apache Traffic Server (ATS), a widely used open-source caching proxy server. The flaw arises from ATS's handling of HTTP/2 CONTINUATION frames, which are used to continue header blocks in HTTP/2. Specifically, ATS versions 8.0.0 through 8.1.9 and 9.0.0 through 9.2.3 do not adequately limit the number of CONTINUATION frames processed per minute. An attacker can exploit this by sending an excessive number of CONTINUATION frames, causing ATS to consume excessive server resources such as memory and CPU, leading to a denial-of-service (DoS) condition. This resource exhaustion can degrade or completely disrupt service availability. The vulnerability does not affect confidentiality or integrity of data. No privileges or user interaction are required, and the attack can be launched remotely over the network. To mitigate this, Apache introduced a new configuration setting, proxy.config.http2.max_continuation_frames_per_minute, allowing administrators to limit the rate of CONTINUATION frames. Additionally, fixed versions 8.1.10 and 9.2.4 have been released that incorporate these protections by default. Organizations running vulnerable ATS versions should upgrade promptly to avoid potential DoS attacks. No known exploits are currently reported in the wild, but the ease of exploitation and impact warrant urgent attention.

For European organizations, this vulnerability poses a significant risk to service availability, especially for those relying on Apache Traffic Server in high-traffic or critical environments such as ISPs, content delivery networks, and large enterprise proxy infrastructures. A successful exploitation could lead to denial-of-service conditions, resulting in downtime, degraded user experience, and potential loss of business continuity. This is particularly critical for sectors like finance, telecommunications, and government services where uninterrupted access is essential. While confidentiality and integrity are not directly impacted, the availability disruption can indirectly affect operational security and trust. The vulnerability's network-exploitable nature means attackers can launch DoS attacks remotely without authentication, increasing the threat surface. European organizations with strict uptime and service-level agreements (SLAs) may face regulatory and reputational consequences if affected. The lack of known exploits in the wild currently reduces immediate risk but does not eliminate the threat, as attackers may develop exploits following public disclosure.

European organizations should immediately assess their Apache Traffic Server deployments to identify affected versions (8.0.0 through 8.1.9 and 9.0.0 through 9.2.3). The primary mitigation is to upgrade to Apache Traffic Server versions 8.1.10 or 9.2.4, which include fixes for this vulnerability. If immediate upgrading is not feasible, administrators should configure the proxy.config.http2.max_continuation_frames_per_minute setting to a conservative limit to restrict the number of CONTINUATION frames processed per minute, thereby mitigating resource exhaustion. Network-level protections such as rate limiting and anomaly detection on HTTP/2 traffic can also help detect and block abnormal frame rates indicative of an attack. Monitoring ATS logs for unusual HTTP/2 frame patterns and resource usage spikes is recommended to identify potential exploitation attempts. Additionally, organizations should ensure that their incident response and business continuity plans account for potential DoS scenarios involving ATS. Regular patch management and vulnerability scanning should be enforced to prevent exploitation of this and similar vulnerabilities.