Actively Exploited — CISA KEV

CVE-2025-66376: Synacor Zimbra Collaboration Suite (ZCS) High-Severity Vulnerability

Severity: High | CVSS 3.1: 7.2 | Published: 2026-01-05 | KEV Added: 2026-03-18

What is CVE-2025-66376?

Zimbra Collaboration (ZCS) 10 before 10.0.18 and 10.1 before 10.1.13 allows Classic UI stored XSS via Cascading Style Sheets (CSS) @import directives in an HTML e-mail message.

Technical Analysis

Weakness Classification

This vulnerability is classified as CWE-79 — Cross-site Scripting (XSS). Malicious scripts can be injected into web pages viewed by other users, enabling session hijacking, credential theft, or defacement.

Attack Vector Breakdown

  • Attack Vector: Network
  • Attack Complexity: Low
  • Privileges Required: None
  • User Interaction: None
  • Scope: Changed
  • Confidentiality: Low
  • Integrity: Low
  • Availability: None

Impact Analysis

This vulnerability is exploitable over the network, meaning any system reachable via the internet or internal network is potentially at risk without physical access. The attack complexity is low, requires no special privileges, and needs no user interaction — making it straightforward to exploit at scale. Notably, the vulnerability's scope is changed, meaning exploitation can affect resources beyond the vulnerable component — for example, escaping a sandbox or container to impact the host system. Malicious scripts can be injected into web pages viewed by other users, enabling session hijacking, credential theft, or defacement.

Exploitation Probability (EPSS): 10.9% chance of exploitation in the next 30 days (94th percentile). This is a significantly elevated exploitation probability — prioritize patching.

Is CVE-2025-66376 being exploited?

Yes. CVE-2025-66376 is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, restricted to CVEs with confirmed active exploitation in the wild. CISA added this vulnerability on 2026-03-18. Inclusion in KEV triggers the patching mandate under BOD 22-01 for U.S. federal civilian agencies, with a remediation deadline of 2026-04-01.

How to fix CVE-2025-66376

Apply the security update issued by Synacor for Zimbra Collaboration Suite (ZCS). If an immediate patch is not possible, consult the vendor advisory for mitigation guidance, restrict network exposure of the affected service, and monitor logs for indicators of exploitation. CISA's Known Exploited Vulnerabilities catalog requires U.S. federal agencies to remediate this vulnerability by the due date shown below under Binding Operational Directive 22-01. Private organizations should treat KEV entries as priority-one patches because active exploitation has been confirmed in the wild.

Related Vulnerabilities

Other actively exploited vulnerabilities affecting Zimbra Collaboration Suite (ZCS) or sharing the same weakness (CWE-79):

  • CVE-2025-48700 (Medium 6.1) [same product + same CWE] — An issue was discovered in Zimbra Collaboration (ZCS) 8.8.15 and 9.0 and 10.0…
  • CVE-2025-68645 (High 8.8) [same product] — A Local File Inclusion (LFI) vulnerability exists in the Webmail Classic UI of…
  • CVE-2025-27915 (Unknown N/A) [same product] — Synacor Zimbra Collaboration Suite (ZCS) contains a cross-site scripting…
  • CVE-2019-9621 (High 7.5) [same product] — Zimbra Collaboration Suite before 8.6 patch 13, 8.7.x before 8.7.11 patch 10…
  • CVE-2024-27443 (Unknown N/A) [same product] — Zimbra Collaboration contains a cross-site scripting (XSS) vulnerability in…

Frequently asked questions

Is CVE-2025-66376 critical?

CVE-2025-66376 has a CVSS 3.1 base score of 7.2 — High severity. It should be treated as a priority patch, especially given the active exploitation confirmed by CISA.

Is CVE-2025-66376 being actively exploited?

Yes — CISA added CVE-2025-66376 to the Known Exploited Vulnerabilities catalog on 2026-03-18. Inclusion in KEV means CISA has confirmed in-the-wild exploitation.

How do I patch CVE-2025-66376?

Apply the security update from Synacor for Zimbra Collaboration Suite (ZCS). Federal agencies must remediate by 2026-04-01 under BOD 22-01.

What type of vulnerability is CVE-2025-66376?

CVE-2025-66376 is classified under CWE-79 (Cross-site Scripting (XSS)). Malicious scripts can be injected into web pages viewed by other users, enabling session hijacking, credential theft, or defacement.

What is the CVSS score for CVE-2025-66376?

CVE-2025-66376 has a CVSS 3.1 base score of 7.2, classifying it as High severity. The attack vector is Network, attack complexity is Low, and no privileges are required.