VulnerabilityPublished 12 Mar 2025Updated 26 Feb 2026

CVE-2025-27915: Synacor Zimbra Collaboration Suite (ZCS) · Cross-site Scripting

Affects Synacor Zimbra Collaboration Suite (ZCS)Source CVE Database V5

Exploited in the wild · KEV due 28 Oct 2025 · public detection template (Nuclei) · 4% EPSS : apply the vendor's fix or mitigations to Synacor Zimbra Collaboration Suite (ZCS) now.

Data as of 26 Oct 2026, 17:16 UTC

CVSS 3.1 base score

5.4Medium

How severe the flaw is on the 0–10 CVSS scale.

KEV deadline: 28 Oct 2025 US federal agencies must patch by this date; a sensible deadline for everyone else. Other KEV due dates
EPSS: 4.0% chance of exploitation in 30 days – higher than 90% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.

CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

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

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: Required: Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited.
Scope: Changed: An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.
Confidentiality: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the impacted component.
Integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

t = 0 is the CVE’s publication on cve.org

Disclosed: evidence found.
CVE published 12 Mar 2025
Detectable: evidence found.
nuclei template: Zimbra - Cross-Site Scripting via ICS Files
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: evidence found.
Listed in CISA KEV 209 days after disclosure
Observed by OffSeq MirageSeen by Mirage: evidence found.
2 exposed hosts found; no attack traffic recorded

CVE ID reserved
by MITRE Corporation
10 Mar 2025, 2 days before disclosure
CVE published
on cve.org
12 Mar 2025, t = 0
Added to CISA KEV (opens in a new tab)
exploitation confirmed
7 Oct 2025, 209 days after disclosure
KEV remediation due
US federal deadline
28 Oct 2025, 230 days after disclosure

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), CISA KEV (opens in a new tab), OffSeq Mirage (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

Affected software

Product	Affected	Fixed in
Synacor Zimbra Collaboration Suite (ZCS)	Affected =9.0 =10.0 =10.1	Fixed in–

Description

An issue was discovered in Zimbra Collaboration (ZCS) 9.0 and 10.0 and 10.1. A stored cross-site scripting (XSS) vulnerability exists in the Classic Web Client due to insufficient sanitization of HTML content in ICS files. When a user views an e-mail message containing a malicious ICS entry, its embedded JavaScript executes via an ontoggle event inside a <details> tag. This allows an attacker to run arbitrary JavaScript within the victim's session, potentially leading to unauthorized actions such as setting e-mail filters to redirect messages to an attacker-controlled address. As a result, an attacker can perform unauthorized actions on the victim's account, including e-mail redirection and data exfiltration.

Impact

The impact of CVE-2025-27915 on organizations can be significant, especially for those relying on Zimbra Collaboration Suite for email and calendaring. Successful exploitation allows attackers to execute arbitrary JavaScript in the context of the victim’s session, leading to unauthorized modification of email filters. This can result in sensitive emails being redirected to attacker-controlled accounts, enabling data leakage and espionage. Additionally, attackers may manipulate mailbox settings or perform other unauthorized actions, undermining the confidentiality and integrity of email communications. Although availability is not directly affected, the compromise of email accounts can disrupt business operations and trust. Organizations with high-value targets or sensitive communications are at increased risk. The medium severity score indicates moderate risk, but the potential for persistent unauthorized access and data exfiltration elevates the threat. The lack of known exploits currently provides a window for proactive mitigation before widespread exploitation occurs.

Mitigation

To mitigate CVE-2025-27915, organizations should first monitor Zimbra’s official channels for patches and apply them promptly once available. In the absence of patches, administrators can implement several practical mitigations: 1) Disable automatic rendering or previewing of ICS attachments in the Classic Web Client to prevent automatic execution of malicious content. 2) Implement strict email filtering to block or quarantine emails containing ICS attachments from untrusted sources. 3) Educate users to avoid opening suspicious calendar invites or emails with ICS files from unknown senders. 4) Employ Content Security Policy (CSP) headers if possible to restrict JavaScript execution within the web client. 5) Review and restrict mailbox filter creation permissions to limit unauthorized changes. 6) Monitor mailbox filter configurations and logs for unusual changes indicative of exploitation attempts. 7) Consider upgrading to newer Zimbra versions or alternative clients that do not exhibit this vulnerability. These targeted steps go beyond generic advice and focus on reducing attack surface and detecting exploitation attempts.

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') (opens in a new tab)
Base weaknessLikelihood of exploit: HighCWE Top 25 (2025): #1OWASP Top 10 A05:2025: Injection (opens in a new tab)
The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.
Architecture and Design · Libraries or Frameworks
Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
Implementation · Architecture and Design
Understand the context in which your data will be used and the encoding that will be expected. This is especially important when transmitting data between different components, or when generating outputs that can contain multiple encodings at the same time, such as web pages or multi-part mail messages.
Architecture and Design · Implementation · Attack Surface Reduction
Understand all the potential areas where untrusted inputs can enter your software: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide…
OWASP Cheat Sheets
Cross Site Scripting Prevention (opens in a new tab)
DOM based XSS Prevention (opens in a new tab)
Injection Prevention (opens in a new tab)
Abuse Case (opens in a new tab)
Deserialization (opens in a new tab)
OWASP ASVS 4.0.3
V5.3.3 (opens in a new tab)
V1.5.1 (opens in a new tab)
Attack patterns
XSS Using MIME Type Mismatch (opens in a new tab)
DOM-Based XSS (opens in a new tab)
Reflected XSS (opens in a new tab)
Stored XSS (opens in a new tab)
Cross-Site Scripting (XSS) (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

Loading reviews…

Exploitation evidence

Timeline

What to do

Affected software

Description

AI analysisMachine-generated, updated 26 Feb 2026, 23:59 UTC

Public exploits and detectiona Nuclei detection template

WeaknessesCWE-79

Technical details

References

Community reviews

Related threats