CVE-2025-53645 is a high-severity denial of service (DoS) vulnerability affecting Zimbra Collaboration Suite (ZCS) versions prior to 9.0.0 Patch 46, 10.0.x versions before 10.0.15, and 10.1.x versions before 10.1.9. The vulnerability arises from improper handling of excessive, comma-separated path segments in the Zimbra Admin Console's HTTP GET requests. Specifically, an unauthenticated remote attacker can craft GET requests with an abnormally large number of comma-separated path segments. This malformed input triggers redundant processing within the server, causing inflated response sizes and excessive consumption of server resources such as CPU and memory. The uncontrolled resource consumption ultimately leads to denial of service, rendering the Admin Console and potentially the entire Zimbra service unavailable to legitimate users. The vulnerability is classified under CWE-400 (Uncontrolled Resource Consumption), indicating that the root cause is the failure to limit resource usage in response to crafted input. The CVSS v3.1 base score is 7.5, reflecting a high severity due to the vulnerability's network attack vector (no authentication or user interaction required), and its impact on availability without affecting confidentiality or integrity. No known exploits are reported in the wild as of the publication date, but the ease of exploitation and unauthenticated access make this a significant threat to organizations relying on vulnerable Zimbra versions for email and collaboration services.

For European organizations, the impact of this vulnerability can be substantial. Zimbra Collaboration Suite is widely used by enterprises, educational institutions, and government agencies across Europe for email, calendaring, and collaboration. A successful DoS attack exploiting this vulnerability could disrupt critical communication channels, impair administrative access to the mail system, and degrade overall productivity. In sectors such as finance, healthcare, and public administration, where timely and reliable communication is essential, such outages could lead to operational delays, compliance issues, and reputational damage. Additionally, denial of service on the Admin Console could delay incident response and remediation efforts during an attack, exacerbating the impact. Given that the attack requires no authentication and can be launched remotely, threat actors could easily target vulnerable Zimbra servers to cause widespread disruption. This risk is heightened in environments where patching cycles are slow or where legacy Zimbra versions remain in use.

To mitigate this vulnerability effectively, European organizations should prioritize the following actions: 1) Immediate patching: Upgrade Zimbra Collaboration Suite to versions 9.0.0 Patch 46, 10.0.15, 10.1.9 or later, as these contain fixes addressing the improper handling of excessive path segments. 2) Network-level protections: Implement rate limiting and request size restrictions on firewalls or reverse proxies in front of the Zimbra Admin Console to detect and block abnormal GET requests with excessive comma-separated path segments. 3) Access controls: Restrict access to the Admin Console interface to trusted IP addresses or VPNs to reduce exposure to unauthenticated remote attackers. 4) Monitoring and alerting: Deploy monitoring tools to detect unusual spikes in resource consumption or anomalous HTTP request patterns targeting the Admin Console. 5) Incident response readiness: Prepare playbooks for DoS scenarios affecting Zimbra services to enable rapid mitigation and recovery. 6) Vendor communication: Stay informed via Zimbra security advisories and subscribe to vulnerability notifications to ensure timely awareness of patches and emerging threats. These targeted mitigations go beyond generic advice by focusing on both patch management and proactive network-level defenses tailored to the nature of this vulnerability.