Cyber Resilience

CVE-2025-66376

XSS in Synacor Zimbra Collaboration Suite 10.0.0 – 10.0.18

CISA KEVActive ExploitationEUVD ExploitedXSS
Published
05 January 2026
Modified
18 March 2026
KEV Added
18 March 2026
Patch / advisory
View advisory
CVSS Score v3.1 7.2
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N
EPSS Score 0.22 97th percentile
Risk Priority 80 floored blend · peak EPSS

Summary

CVE-2025-66376 is a high-severity Cross-site Scripting (CWE-79) vulnerability in Synacor Zimbra Collaboration Suite. Its CVSS base score is 7.2 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Browser Session Hijacking (T1185); ranked in the top 3% of CVEs by exploit likelihood; CISA has added it to the Known Exploited Vulnerabilities catalog.

The strongest mitigations our analysis identified map to SA-11 (Developer Testing and Evaluation) and SI-10 (Information Input Validation) — see the control section below for these in your framework.

Deeper analysis AI-assisted summary

Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.

CVE-2025-66376 is a stored cross-site scripting (XSS) vulnerability (CWE-79) in the Classic UI of Zimbra Collaboration Suite (ZCS) versions 10 before 10.0.18 and 10.1 before 10.1.13. It arises from the improper handling of Cascading Style Sheets (CSS) @import directives embedded in HTML email messages, allowing malicious payloads to be stored and executed when rendered in the Classic UI.

Unauthenticated attackers (PR:N) can exploit this vulnerability over the network (AV:N) with low complexity (AC:L) and no user interaction required (UI:N), as indicated by its CVSS v3.1 base score of 7.2 (S:C/C:L/I:L/A:N). By sending a crafted HTML email containing a malicious CSS @import directive, the attacker can store the payload server-side. When victims access the email via the affected Classic UI, the XSS executes in the context of the Zimbra application, potentially enabling session hijacking, data theft, or further compromise with low confidentiality and integrity impacts due to the changed scope.

Zimbra's security advisories and release notes for versions 10.0.18 and 10.1.13 document fixes for this issue, recommending immediate upgrades to these patched releases. Additional guidance is available in the Zimbra Security Center, Security Advisories, and Responsible Disclosure Policy on their wiki.

OWASP Top 10 for Web (2025)

A05:2025 Injection

EU & UK References

Vulnerability Data

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.

CWE(s)
CWE-79
KEV Date Added
18 March 2026

Related Threats

MITRE ATT&CK Enterprise Techniques

T1185 Browser Session Hijacking Collection
Adversaries may take advantage of security vulnerabilities and inherent functionality in browser software to change content, modify user-behaviors, and intercept information as part of various browser session hijacking techniques.
attack.mitre.org →
T1539 Steal Web Session Cookie Credential Access
An adversary may steal web application or service session cookies and use them to gain access to web applications or Internet services as an authenticated user without needing credentials.
attack.mitre.org →
T1659 Content Injection Initial Access
Adversaries may gain access and continuously communicate with victims by injecting malicious content into systems through online network traffic.
attack.mitre.org →
T1189 Drive-by Compromise Initial Access
Adversaries may gain access to a system through a user visiting a website over the normal course of browsing.
attack.mitre.org →
T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2018-6882Same product: Synacor Zimbra Collaboration Suiteboth on KEV
CVE-2022-27926Same product: Synacor Zimbra Collaboration Suiteboth on KEV
CVE-2023-34192Same product: Synacor Zimbra Collaboration Suiteboth on KEV
CVE-2023-37580Same product: Synacor Zimbra Collaboration Suiteboth on KEV
CVE-2023-5631Same product class: email / collaborationboth on KEV
CVE-2023-43770Same product class: email / collaborationboth on KEV
CVE-2020-35730Same product class: email / collaborationboth on KEV
CVE-2020-13965Same product class: email / collaborationboth on KEV
CVE-2023-47272Same product class: email / collaboration
CVE-2023-43102Same product class: email / collaboration

Affected Assets

synacor
zimbra collaboration suite
10.0.0 — 10.0.18 · 10.1.0 — 10.1.13

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)
  • V1.1.2
  • V1.3.2

Mitigating Controls (NIST 800-53 r5) AI

SA-11 Developer Testing and Evaluation good match

Developer testing and evaluation can discover missing or incorrect input neutralization through targeted web-application tests.

SI-10 Information Input Validation good match

Input validation directly enforces neutralization of untrusted data before it reaches web output generation.

SI-15 Information Output Filtering good match

Output filtering can catch or sanitize unneutralized script content before it is served to users.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 mostly match
prevents

Secure SDLC practices directly target introduction of XSS via coding standards/testing (mostly), yet the single broad outcome leaves many specific neutralization vectors unaddressed (partial).

PR.PS-02 partial match
prevents

Patching and EOL replacement can remediate known XSS instances in libraries or frameworks (partial) but do nothing to enforce input neutralization in application code (none).

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.29 Security testing in development and acceptance mostly match
finds

Secure-coding testing and automated code-analysis tools are applied to detect improper neutralization of script-related content during web-page generation.

A.5.6 Contact with special interest groups partial match
prevents

Knowledge exchange on emerging attack techniques and patches reduces the likelihood that cross-site scripting flaws remain unaddressed in deployed applications.

A.5.7 Threat intelligence partial match
prevents

Operational indicators of compromise for web-application attacks can be incorporated into WAF or input-filtering rules, lowering the likelihood that unsanitized data reaches the browser.

A.8.25 Secure development life cycle partial match
prevents

Requiring language-specific secure-coding standards and automated scanning during the SDLC catches missing output encoding or improper neutralization of untrusted data before the software reaches production.

A.8.28 Secure coding partial match
prevents

Secure-coding standards, SAST scans and removal of insecure code samples together eliminate the failure to neutralize script content that produces cross-site scripting flaws.

A.8.7 Protection against malware none match
none

Webpage malware scanning and block-listing of known malicious sites reduce the likelihood that reflected or stored script payloads reach a user’s browser.

References