UnratedActively exploited

Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability

CVE-2025-27915

Attackers are using this one right now. CISA added it to its Known Exploited Vulnerabilities list on 7 Oct 2025, and US federal agencies had to fix it by 28 Oct 2025. Treat it as urgent.

How serious is it

  • Severity: not rated yet.
  • Exploit prediction (EPSS): 4.1% chance of exploitation in the next 30 days, higher than 91% of all vulnerabilities.
  • Confirmed exploited in the wild (CISA KEV).
  • No public exploit in Exploit-DB.

Is your app affected by CVE-2025-27915?

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Who is affected

  • Synacor Zimbra Collaboration Suite (ZCS)

How to check

  1. Find out whether you run the affected product and which version, then compare it with the versions above.

How to fix

Copies a prompt for Claude Code, Cursor or Codex that asks it to check whether you run this product.
  1. Apply the vendor's update or mitigation from the references below. If there is none yet, limit exposure of the affected service until there is.
  2. If a vulnerable version was live, check your logs and rotate the secrets that server could read.

The advisory

Synacor Zimbra Collaboration Suite (ZCS) contains a cross-site scripting vulnerability that 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 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.

Check your whole app

Dependencies are only half of it. Free and read-only: before you push, run npx shippable-dev scan localhost:3000 in your project folder, no account needed. For your live app, you prove it's yours with a one-line tag first.

References

  • No references listed.

Merged from CISA KEV, FIRST EPSS. Last updated 24 Oct 2026. All vulnerabilities · RSS · JSON

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