CVE-2020-7796
Synacor Zimbra Collaboration Suite (ZCS) Server-Side Request Forgery Vulnerability
Description
CVE-2020-7796 is a critical server-side request forgery (SSRF) vulnerability in Zimbra Collaboration Suite (ZCS) before version 8.8.15 Patch 7. The flaw exists in the ProxyServlet component and allows an unauthenticated attacker to send crafted requests that are forwarded by the Zimbra server to internal services, potentially enabling access to sensitive internal resources and arbitrary code execution. The vulnerability is rated CVSS v3.1 base score 9.8 (Critical severity) with a vector of CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H, reflecting trivial remote exploitation with maximum impact on confidentiality, integrity, and availability. Zimbra has addressed this issue in ZCS 8.8.15 Patch 7 and ZCS 9.0.0 Patch 2. This vulnerability is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog with a remediation deadline of March 10, 2026, and has an EPSS score of 0.66284 (98.44th percentile), indicating extremely high exploitation probability.
KEV Information
CVSS Score
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:HOpen in CalculatorAffected Products
| Vendor | Product | Version |
|---|---|---|
| synacor | zimbra collaboration suite | < 8.8.15; 8.8.15 |
Multiple CVSS Assessments
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
References
- https://wiki.zimbra.com/wiki/Zimbra_Releases/8.8.15/P7(Release Notes, Vendor Advisory)
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2020-7796(US Government Resource)
Weakness Type
CWE-918: Server-Side Request Forgery (SSRF)
CVE-2020-7796 is classified under CWE-918, which describes a vulnerability where an application can be manipulated to issue requests to arbitrary destinations on behalf of the server. In this case, the Zimbra Collaboration Suite's ProxyServlet component accepts user-controlled input that specifies the destination of proxied HTTP requests without adequate validation. An unauthenticated attacker can craft requests that cause the Zimbra server to connect to internal network resources, cloud metadata services, or other backend systems that should not be directly accessible from the internet. The SSRF in ProxyServlet is particularly dangerous because Zimbra servers typically have extensive network access to mail stores, LDAP directories, and other internal infrastructure components.
Impact Analysis
Confidentiality, Integrity, and Availability are all maximally impacted according to the CVSS assessment (C:H/I:H/A:H). Unlike many SSRF vulnerabilities that primarily affect confidentiality, CVE-2020-7796 can lead to arbitrary code execution on the server, which is reflected in the critical severity rating and the high impact across all three dimensions.
Confidentiality Impact is severe because the SSRF allows access to internal services reachable from the Zimbra server. This includes email data stores containing potentially sensitive corporate communications, LDAP directories with user credentials and organizational information, internal APIs, and cloud provider metadata endpoints. Zimbra servers are frequently deployed with privileged network positions due to their need to communicate with multiple backend services.
Integrity Impact is high because the vulnerability can potentially be chained with other internal service vulnerabilities to achieve remote code execution. An attacker who gains code execution can modify email data, inject malicious content into messages, alter server configurations, and install persistent backdoors. The low attack complexity (AC:L) and lack of required privileges (PR:N) mean these attacks can be executed without any prior foothold.
Availability Impact is equally critical. Exploitation can lead to denial of service through resource exhaustion, server crashes caused by malicious requests to internal services, or intentional disruption by an attacker who achieves code execution. For organizations using Zimbra as their primary email and collaboration platform, loss of availability directly impacts business operations.
Enterprise-Wide Risk is substantial given Zimbra's role as a communication platform. The EPSS score of 0.66284 (98.44th percentile) indicates that this vulnerability is among the most actively targeted in the entire CVE database. Organizations running unpatched Zimbra instances face a near-certain probability of exploitation attempts.
Exploit Maturity
CVE-2020-7796 has confirmed active exploitation as indicated by its inclusion in CISA's Known Exploited Vulnerabilities (KEV) catalog with a remediation deadline of March 10, 2026. The EPSS score of 0.66284 places it in the 98.44th percentile, meaning exploitation is more probable than for approximately 98% of all known vulnerabilities.
Public Exploit Availability is supported by detailed technical information available through the Zimbra bug tracker at bugzilla.zimbra.com and the Zimbra Security Advisories. The SSRF in ProxyServlet has been well-analyzed by the security research community, and exploitation techniques are broadly understood.
Exploitation Simplicity is a key factor. The CVSS vector indicates low attack complexity (AC:L), no required privileges (PR:N), and no user interaction (UI:N). An attacker needs only to send crafted HTTP requests to the Zimbra server's ProxyServlet endpoint to trigger the vulnerability. No authentication, social engineering, or special environmental conditions are required.
Zimbra as a High-Value Target amplifies the exploitation risk. Email servers are frequently targeted by advanced persistent threat (APT) groups and cybercriminal organizations because they contain sensitive communications, credentials, and organizational intelligence. Multiple Zimbra vulnerabilities have been actively exploited in recent years, and threat actors maintain active scanning for vulnerable Zimbra deployments. CISA's ransomware association for this CVE is listed as "Unknown."
Remediation
Apply Patches Immediately. Upgrade Zimbra Collaboration Suite to version 8.8.15 Patch 7 or later for the 8.x branch, or to version 9.0.0 Patch 2 or later for the 9.x branch. The patches address the SSRF vulnerability in the ProxyServlet component. Consult the Zimbra Security Advisories for complete patch details and installation guidance.
Restrict External Access to the Zimbra server as a temporary mitigation. Use a web application firewall (WAF) or reverse proxy to filter requests targeting the ProxyServlet endpoint. Limit network access to only necessary client-facing services and block direct access to administrative and proxy endpoints from untrusted networks.
Implement Network Segmentation to limit the impact of SSRF exploitation. Restrict the Zimbra server's ability to reach internal services that are not required for its operation. Apply egress filtering rules to prevent the server from connecting to internal metadata endpoints, administrative interfaces, and other sensitive resources.
Monitor for Exploitation Indicators by reviewing web server access logs for suspicious requests to the ProxyServlet path. Look for requests containing internal IP addresses, localhost references, or cloud metadata endpoint URLs. Analyze outbound connection logs from the Zimbra server for connections to unusual internal destinations.
Conduct Post-Patch Security Audit after applying the update. Given the critical severity and high exploitation probability, assume that unpatched instances may have already been compromised. Review email data integrity, check for unauthorized mailbox access, audit user accounts for suspicious changes, and inspect the server filesystem for web shells or other persistence mechanisms.
Technical Details
CVE-2020-7796 targets the ProxyServlet component in Zimbra Collaboration Suite, which is responsible for proxying HTTP requests on behalf of users and internal services. The vulnerability exists because ProxyServlet does not properly validate or restrict the destination of proxied requests, allowing an attacker to specify arbitrary target URLs.
Vulnerability Mechanics: The ProxyServlet accepts HTTP requests containing destination parameters and forwards them to the specified URL from the server side. By manipulating these parameters, an attacker can direct the Zimbra server to issue requests to internal network addresses, loopback interfaces, or cloud metadata services. The server then returns the response to the attacker, effectively acting as an open proxy to the internal network.
Affected Versions: Zimbra Collaboration Suite versions before 8.8.15 Patch 7 and versions 9.0.0 before Patch 2 are vulnerable. Both the open-source and commercial editions are affected, as the ProxyServlet component is present in the core Zimbra platform.
Attack Surface: Zimbra deployments typically expose HTTP/HTTPS services to the internet for webmail, CalDAV, CardDAV, and ActiveSync access. The ProxyServlet endpoint is accessible through these same network-facing services, making it directly reachable by remote attackers. No authentication is required to reach the vulnerable code path.
Exploitation Chain to RCE: While the primary vulnerability is SSRF, the critical severity rating and high integrity/availability impact suggest that exploitation can be chained with access to internal services to achieve remote code execution. The Zimbra server communicates with multiple backend services including the mailbox server, LDAP directory, and various internal APIs. SSRF access to these services may enable further exploitation, such as LDAP injection, internal API abuse, or access to administrative endpoints that permit code execution.
Network Context: Zimbra servers are typically positioned in network architectures where they can reach LDAP servers, mail stores (often on the same host or adjacent hosts), anti-virus/anti-spam services, and potentially cloud provider APIs. This network position makes the SSRF vulnerability particularly impactful, as the range of reachable internal targets is typically broad.
Frequently Asked Questions
How critical is CVE-2020-7796?
CVE-2020-7796 is rated 9.8 Critical on the CVSS v3.1 scale. It allows an unauthenticated remote attacker to exploit an SSRF vulnerability in Zimbra's ProxyServlet, potentially leading to access to internal services and arbitrary code execution on the server.
What versions of Zimbra are affected by CVE-2020-7796?
Zimbra Collaboration Suite versions before 8.8.15 Patch 7 and ZCS 9.0.0 versions before Patch 2 are affected. Both open-source and commercial editions contain the vulnerable ProxyServlet component.
Can CVE-2020-7796 be exploited without authentication?
Yes. The vulnerability requires no authentication (PR:N), no user interaction (UI:N), and has low attack complexity (AC:L). An attacker only needs network access to the Zimbra server's web interface to exploit the SSRF in ProxyServlet.
Is CVE-2020-7796 being actively exploited?
Yes. CISA has added this vulnerability to its Known Exploited Vulnerabilities (KEV) catalog, and it has an EPSS score of 0.66284 (98.44th percentile), indicating that exploitation is extremely widespread. Multiple Zimbra vulnerabilities have been actively targeted by threat actors in recent years.
What should I do if my Zimbra server was unpatched?
If your Zimbra instance was running an unpatched version, assume potential compromise. After applying the patch, conduct a thorough security audit including checking for web shells, reviewing mailbox access logs, auditing user account changes, and inspecting server filesystem integrity. Consider engaging incident response support for critical environments.
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