What Is CVE-2025-68645 (Synacor Zimbra Collaboration Suite (ZCS) PHP Remote File Inclusion)?

By NHI Mgmt Group Updated October 17, 2026 Domain: Threats, Abuse & Incident Response

CVE-2025-68645 is a PHP remote file inclusion vulnerability in Synacor Zimbra Collaboration Suite (ZCS). CISA added it to the Known Exploited Vulnerabilities catalogue on 22 January 2026, which means attackers have used it in real attacks. Organisations running the affected product should apply the vendor fix or mitigation as a priority.

What this vulnerability means in practice

remote file inclusion vulnerabilities are dangerous because they can let an attacker make the application load code from an external source. In a webmail and collaboration platform, that can turn a single exposed endpoint into a path for application compromise, especially when the flaw sits in request handling or include logic.

The core issue is not just that the product is vulnerable, but that the application may be tricked into executing attacker-controlled content as though it were trusted application code. That raises the stakes far beyond simple content injection because the result can be server-side execution, data exposure, or a broader foothold inside the environment.

How organisations should assess exposure

The first question is whether the vulnerable Zimbra Collaboration Suite instance exists anywhere in the environment, including forgotten internet-facing deployments, test systems, and secondary mail platforms. Asset inventory matters because remote file inclusion flaws are often exploitable before teams realise the affected service is reachable at all.

Exposure review should also consider whether the instance is reachable from the internet, whether reverse proxies or WAF rules are in place, and whether vendor advisories identify a specific configuration or deployment pattern that changes the risk. For this class of issue, the vendor advisory and the NIST National Vulnerability Database are the right places to confirm scope, while the CVE Program provides the canonical identifier and record structure.

Because this is already in CISA's KEV catalogue, organisations should treat it as active exploitation risk, not as a theoretical application bug. That means prioritising it ahead of routine backlog work and aligning remediation with the asset's business role and external exposure.

Remediation and compensating controls

The preferred response is to apply the vendor fix or mitigation as quickly as change control allows. If patching is delayed, reduce exposure by removing public reachability where possible, tightening access paths, and applying temporary controls that limit the vulnerable code path from being reached.

Compensating controls should be treated as risk reduction, not a substitute for repair. In practice, that means hardening the service boundary, reviewing allowlists and proxy rules, and validating whether any emergency workaround still leaves the vulnerable application callable from untrusted networks.

For broader remediation discipline, the Vulnerability Management Guide is a useful reference for prioritisation, exception handling, and verification after treatment. The same discipline should be applied here: confirm the fix is actually deployed, not merely scheduled.

Detection and post-patch review

Once a vulnerability like this is known to be exploited in the wild, detection should focus on signs that the application was used as an execution path before remediation completed. Review web server logs, application logs, outbound connections, unexpected file writes, and any newly created scripts or modified configuration files around the exposure window.

After patching, organisations should still assume the possibility of post-exploitation activity. A compromised web application can be used to stage additional tooling, probe internal systems, or harvest credentials and session material, so validation should extend beyond the vulnerable endpoint itself.

The right mindset is to treat patching as the start of verification, not the end of the incident review. If compromise is suspected, preserve logs and investigate whether the application was used to establish persistence or pivot to adjacent systems.

Standards & Framework Alignment

This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.

CIS Controls v8, NIST SP 800-53 Rev 5 and OWASP ASVS set the governance and control requirements practitioners need to meet.

Framework	Control / Reference	Relevance
CIS Controls v8	CIS-7 — Continuous Vulnerability Management	Remote file inclusion exploitation is a patch-priority vulnerability management case.
Recommendation — Prioritise, remediate, and verify this KEV-listed flaw within your vulnerability management process.
NIST SP 800-53 Rev 5	RA-5 — Vulnerability Monitoring and Scanning	This CVE requires exposure discovery, prioritisation, and verification of remediation.
	SI-2 — Flaw Remediation	The subject is an exploited software flaw requiring prompt repair and mitigation.
	AU-6 — Audit Record Review, Analysis, and Reporting	Post-patch review depends on log analysis to detect prior exploitation and persistence.
Recommendation — Scan for affected ZCS assets, confirm exposure, and track remediation status until fixed. Apply the vendor fix or approved mitigation and verify the vulnerable condition is removed. Review application and system logs for exploitation indicators before and after remediation.
OWASP ASVS	V15 — Secure Coding and Architecture	Remote file inclusion is an application security flaw tied to unsafe include and request handling.
Recommendation — Review include and file-handling logic to eliminate remote inclusion paths in the application.

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NHIMG Editorial Note
Reviewed and updated by the NHIMG editorial team on October 17, 2026.
NHI Mgmt Group — the #1 independent authority on Non-Human Identity, IAM, and Agentic AI security. nhimg.org

CVE-2025-68645 (Synacor Zimbra Collaboration Suite (ZCS) PHP Remote File Inclusion)