CVE-2025-53645: Zimbra Collaboration DoS Vulnerability

CVE-2025-53645 Overview

CVE-2025-53645 is a denial of service vulnerability in Zimbra Collaboration Suite (ZCS). The flaw resides in the Admin Console, which improperly handles excessive comma-separated path segments in HTTP GET requests. An unauthenticated remote attacker can send specially crafted requests that trigger redundant processing and inflated responses. This leads to uncontrolled resource consumption [CWE-400] on the targeted server.

The vulnerability affects Zimbra Collaboration 9.0.0 before Patch 46, 10.0.x before 10.0.15, and 10.1.x before 10.1.9. Zimbra has released patched versions across all supported branches.

Critical Impact

Unauthenticated attackers can exhaust server resources remotely over the network, rendering the Zimbra Admin Console and associated services unavailable to legitimate administrators.

Affected Products

• Zimbra Collaboration (ZCS) 9.0.0 prior to Patch 46
• Zimbra Collaboration (ZCS) 10.0.x prior to 10.0.15
• Zimbra Collaboration (ZCS) 10.1.x prior to 10.1.9

Discovery Timeline

• 2025-07-09 - CVE-2025-53645 published to NVD
• 2026-04-15 - Last updated in NVD database

Technical Details for CVE-2025-53645

Vulnerability Analysis

The vulnerability stems from how the Zimbra Admin Console parses HTTP GET requests containing path segments separated by commas. When the server receives a request with an excessive number of comma-separated segments, the request handler performs redundant processing for each segment. The server then generates inflated response payloads proportional to the segment count.

This design flaw enables an attacker to amplify the cost of a single request. Each malicious request consumes disproportionate CPU and memory on the server. Sustained sending of such requests exhausts available resources and prevents legitimate administrative access.

The issue is categorized under [CWE-400: Uncontrolled Resource Consumption]. No authentication is required, and no user interaction is needed to trigger the condition.

Root Cause

The root cause is missing input validation on the length and structure of comma-separated path segments processed by the Admin Console. The application does not enforce upper bounds on the number of segments before entering processing loops. This allows attackers to control the cost of request handling directly through input size.

Attack Vector

The attack vector is the network. An unauthenticated remote attacker sends crafted HTTP GET requests to the Admin Console endpoint. Because the Admin Console is often exposed to internal networks or, in misconfigured deployments, to the internet, the attack surface can be substantial. See the Zimbra Security Advisories for upstream technical details.

Detection Methods for CVE-2025-53645

Indicators of Compromise

• HTTP GET requests to Admin Console URIs containing unusually long, comma-separated path segments.
• Spikes in CPU and memory utilization on the Zimbra mailbox or proxy node coinciding with Admin Console traffic.
• Elevated response sizes or response times for Admin Console endpoints in access logs.
• Repeated requests from a single source IP targeting the Admin Console port (typically 7071).

Detection Strategies

• Inspect web server and proxy access logs for GET requests with abnormal path lengths or high comma counts in the URI.
• Correlate Admin Console traffic patterns with system resource metrics to identify amplification behavior.
• Deploy WAF or reverse proxy rules that count delimiter occurrences in request URIs and flag outliers.

Monitoring Recommendations

• Monitor Zimbra mailboxd JVM heap and CPU consumption for sudden, sustained increases.
• Alert on bursts of Admin Console requests from non-administrative source IPs.
• Track HTTP 5xx error rates and request latency on port 7071 as early indicators of resource exhaustion.

How to Mitigate CVE-2025-53645

Immediate Actions Required

• Upgrade Zimbra Collaboration to a patched release: 9.0.0 P46, 10.0.15, or 10.1.9.
• Restrict network access to the Admin Console (port 7071) to trusted administrative networks only.
• Place the Admin Console behind a reverse proxy or WAF that can enforce request size and structure limits.

Patch Information

Zimbra has released fixed versions across all supported branches. Refer to the official release notes for upgrade procedures and security fix details: Zimbra Release 9.0.0 P46 Fixes, Zimbra Release 10.0.15 Fixes, and Zimbra Release 10.1.9 Fixes.

Workarounds

• Block external access to the Admin Console at the network perimeter using firewall rules.
• Configure upstream proxies or load balancers to drop requests containing excessive commas or oversized URIs.
• Apply rate limiting on Admin Console endpoints to reduce the impact of repeated abuse.
• Monitor system health and be prepared to restart mailboxd if resource exhaustion occurs before patching.

# Example: restrict access to the Zimbra Admin Console with iptables
iptables -A INPUT -p tcp --dport 7071 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 7071 -j DROP