CVERadar
MEDIUM · CVSS 3.1 6.1CISA KEV · added 2025-05-19EPSS 24.61% (p98)Threat level 53

CVE-2024-27443: Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting (XSS) Vulnerability

Synacor · Zimbra Collaboration Suite (ZCS) · NVD published 2024-08-12 · posted 2025-05-19 · last modified 2026-06-17

What it is

An issue was discovered in Zimbra Collaboration (ZCS) 9.0 and 10.0. A Cross-Site Scripting (XSS) vulnerability exists in the CalendarInvite feature of the Zimbra webmail classic user interface, because of improper input validation in the handling of the calendar header. An attacker can exploit this via an email message containing a crafted calendar header with an embedded XSS payload. When a victim views this message in the Zimbra webmail classic interface, the payload is executed in the context of the victim's session, potentially leading to execution of arbitrary JavaScript code.

WeaknessCWE-79
CVSS vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N (source: [email protected])
CISA due date2025-06-09 (for US federal agencies under BOD; a good deadline for everyone)
ATT&CK tacticsInitial Access → Execution → Collection → Exfiltration

Exploitation evidence

First exploit evidence seen: 2025-05-19 (earliest date across the sources below).

Exploited in the wild

  • CISA KEV: listed since 2025-05-19. Source

Sources: CISA KEV.

Defend the Chain

The attack chain for CVE-2024-27443 laid across the MITRE ATT&CK Enterprise tactics. Solid cards come from the MITRE CTID KEV mappings; dashed cards are inferred by CVERadar from the CWE or description. Correct defences are real ATT&CK mitigations for that technique.

Threat level 53/100
Defender level 1 · 0 XP
Chain links broken: 0/4

Reconnaissance

Resource Development

Initial Access

Execution

Persistence

Privilege Escalation

Stealth

Defense Impairment

Credential Access

Discovery

Lateral Movement

Collection

Command and Control

Exfiltration

Impact

Pick a red technique card on the board, then choose the defence that stops it.

Defend against T1566.002 Spearphishing Link CTID: exploitation technique

Attackers can send a malicious email with a specially crafted calendar header in order to execute arbitrary JavaScript code in the browser. This can lead to email collection, which can then be exfiltrated.

How to fix it

CISA required action

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

Fixed versions

  • zimbra collaboration: upgrade to 10.0.7 or later (affected from 10.0.0)

Derived from the NVD CPE configuration (versionEndExcluding / versionEndIncluding). Always confirm with the vendor advisory.

Vendor advisories and patches

MITRE ATT&CK mitigations for this chain

  • M1054 Software Configuration (for T1566.002) : Use anti-spoofing and email authentication mechanisms to filter messages based on validity checks of the sender domain (using SPF) and integrity of messages (using DKIM). Enabling these mechanisms within an organization (through policies such as DMARC) may enable recipients (intra-org and cross domain) to perform simil
  • M1021 Restrict Web-Based Content (for T1566.002) : Determine if certain websites that can be used for spearphishing are necessary for business operations and consider blocking access if activity cannot be monitored well or if it poses a significant risk.
  • M1047 Audit (for T1566.002, T1114) : Audit applications and their permissions to ensure access to data and resources are limited based upon necessity and principle of least privilege.
  • M1018 User Account Management (for T1566.002) : Azure AD Administrators apply limitations upon the ability for users to grant consent to unfamiliar or unverified third-party applications.
  • M1017 User Training (for T1566.002) : Users can be trained to identify social engineering techniques and spearphishing emails with malicious links which includes phishing for consent with OAuth 2.0. Additionally, users may perform visual checks of the domains they visit; however, homographs in ASCII and in IDN domains and URL schema obfuscation may render
  • M1038 Execution Prevention (for T1059.004) : Use application control where appropriate. On ESXi hosts, the `execInstalledOnly` feature prevents binaries from being run unless they have been packaged and signed as part of a vSphere installation bundle (VIB).
  • M1032 Multi-factor Authentication (for T1114) : Use of multi-factor authentication for public-facing webmail servers is a recommended best practice to minimize the usefulness of usernames and passwords to adversaries.
  • M1060 Out-of-Band Communications Channel (for T1114) : Use secure out-of-band authentication methods to verify the authenticity of critical actions initiated via email, such as password resets, financial transactions, or access requests. For highly sensitive information, utilize out-of-band communication channels instead of relying solely on email to prevent adversaries fr
  • M1041 Encrypt Sensitive Information (for T1114) : Use of encryption provides an added layer of security to sensitive information sent over email. Encryption using public key cryptography requires the adversary to obtain the private certificate along with an encryption key to decrypt messages.
  • M1031 Network Intrusion Prevention (for T1041) : Network intrusion detection and prevention systems that use network signatures to identify traffic for specific adversary malware can be used to mitigate activity at the network level. Signatures are often for unique indicators within protocols and may be based on the specific obfuscation technique used by a particular

Detection Generic guidance

  • Initial Access: Alert on unusual requests to the affected service (odd paths, oversized payloads, exploit strings) in WAF, reverse-proxy and application logs.
  • Execution: Watch for the service process spawning shells or script interpreters (cmd.exe, powershell, sh, bash, python) via EDR process-tree rules.
  • Collection: Watch for bulk reads of databases, file shares or mailboxes by the affected service.
  • Exfiltration: Watch for large or unusual outbound transfers from the affected host.

Response checklist Generic guidance

  1. Inventory: find every instance of the affected product and version (include internet-facing and forgotten ones).
  2. Patch or mitigate per the vendor advisory; if you cannot patch, remove internet exposure or apply the vendor workaround.
  3. Assume compromise if the system was exposed before patching: hunt for the indicators in the advisory and review logs back to the disclosure date.
  4. Rotate credentials and secrets stored on or used by the affected system if compromise cannot be ruled out.

Sources