U.S. CISA adds Microsoft SharePoint and Zimbra flaws to its Known Exploited Vulnerabilities catalog

Zimbra Collaboration Suite (ZCS) 10 before 10.0.18 and 10.1 before 10.1.13 is vulnerable to stored cross-site scripting through its Classic webmail interface. An attacker sends an HTML e-mail containing a Cascading Style Sheets (CSS) @import directive, and when a recipient opens that message in Classic UI, the injected content executes as script in the victim's browser session. Successful exploitation lets an attacker run arbitrary JavaScript in the Zimbra webmail context, potentially hijacking the session, reading mail, or acting as the user, consistent with the cross-scope impact reflected in the 6.1 CVSS score. Only deployments running the affected ZCS 10/10.1 versions with the Classic UI enabled are exposed; organizations on patched releases or not using Classic UI are not impacted. CISA added the flaw to the Known Exploited Vulnerabilities catalog on 2026-03-18, confirming exploitation in the wild, and recent reporting describes Zimbra flaws being used by Russian-aligned espionage actors against Western and Ukrainian targets.

Do: Upgrade ZCS to 10.0.18 or 10.1.13 (or later) following vendor instructions, as required by CISA's BOD 22-01 KEV guidance; federal agencies and critical infrastructure should prioritize this by the catalog deadline. Until patched, filter or sanitize HTML mail containing CSS @import directives and consider restricting or disabling the Classic UI. Check mailboxes and webmail access logs for suspicious HTML messages and unexplained session activity, which may indicate exploitation.

CVE-2026-20963 is a deserialization of untrusted data flaw (CWE-502) in Microsoft SharePoint that allows an unauthorized attacker to execute code remotely over the network. The flaw is triggered when SharePoint processes maliciously crafted serialized data without validating it, enabling an attacker to run arbitrary code in the context of the SharePoint service. Successful exploitation gives the attacker code execution on the affected SharePoint server, a foothold that typically supports further lateral movement and data access within the environment. Organizations running affected SharePoint deployments are in scope, though affected version ranges have not yet been published in the available data. The vulnerability was added to CISA's Known Exploited Vulnerabilities catalog on 2026-03-18, confirming active exploitation, and its EPSS of 31.6% (98th percentile) indicates a high near-term probability of exploitation; no public proof-of-concept is known and CVSS scoring is not yet available.

Do: Inventory all SharePoint deployments, prioritize any internet-facing SharePoint Server instances, and apply Microsoft's security updates or vendor-specified mitigations as soon as they are available; federal agencies must follow BOD 22-01 (including cloud services) with its standard remediation timeline, and others should treat KEV inclusion as a patch-now signal despite the absence of a public PoC. Until patched, restrict network exposure of SharePoint and review server logs for signs of untrusted serialized data being processed leading to unexpected code execution.