Threats / Synacor / CVE-2025-27915
CVE-2025-27915
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-08-04
Synacor Zimbra Collaboration Suite (ZCS) vulnerability
Synacor Zimbra Collaboration Suite contains a cross-site scripting vulnerability in the Classic Web Client that allows arbitrary JavaScript execution through malicious ICS file entries, enabling unauthorized account actions.
Verdict
Today item — known-exploited.
An attacker can craft a malicious ICS file embedded in an email that executes JavaScript in a victim's browser session when viewed, allowing unauthorized actions such as email filter manipulation and data exfiltration from the victim's account.
CISA KEV Yes · 2025-10-074EPSS 0.04344 (verify live)5CVSS 5.4 (CISA)3CISA SSVC Exploitation: active · Automatable: no · Impact: total3Exploit Public PoC / detection · nuclei template12
01
Is it exploitable?
— the evidence, ranked above the scoreReported exploitation
6 independent public reports of in-the-wild exploitation are cataloged.Distinct reporting sources (vendor, incident response, government); open them for the underlying claims.
Exploited in the wild
Listed in the CISA Known Exploited Vulnerabilities catalog (added 2025-10-07).
Probability (EPSS)
EPSS 0.04344 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 5.4 (CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N) · Affected: Synacor, Zimbra Collaboration Suite (ZCS). Confirm exact fixed builds in the vendor advisory.CVSS — CISA-assigned via the ADP container (CVE.org), version 3.1; the CNA left no metrics, so CISA's own score is used ahead of NVD.
Weakness (CWE)
Mapped to CWE-79 Cross-site Scripting (XSS) — weakness family: Web / client.CWE assignment from the public NVD record (may be stale); the weakness class drives how the flaw is exploited.
CISA SSVC (triage disposition)
Exploitation: active · Automatable: no · Technical Impact: totalCISA's own stakeholder-specific triage inputs, published in the CVE record (CISA Coordinator role). NOTE: this is CISA's three decision points; the final Act/Attend/Track decision also requires a Mission & Well-being input not published here, so no overall decision is asserted.
Currency of the record
CISA added this to its KEV catalog 209 days after the CVE record was published.KEV lag = CISA-KEV dateAdded − CVE.org datePublished; a floor on how long the known-exploited flaw sat in the public record before the federal catalog listed it. Severity/weakness here is CNA- or CISA-assigned (current at disclosure), independent of NVD's analysis backlog.
Public exploit available
Public exploit tooling is catalogued for this CVE (a public proof-of-concept / detection template). Catalog entries: nuclei template. We link the catalog page(s) only — no payload or code is hosted or restated here.Link, never host — the catalog page is the source; basicsecurity.net does not redistribute payloads.
02
Who’s exploiting it?
— attribution turns risk into urgencyAttribution not established
No confirmed (advisory-backed) threat-actor attribution is established for this record. Absence of a named actor is not absence of compromise — see Coverage & confidence.
03
Why it matters
— the attack path, told twice: adversary, then board1
Front door — unauthenticated access narrative 1
Attacker
I craft an email containing a malicious ICS calendar entry with embedded JavaScript in an ontoggle event handler.
Business
Email security controls fail to prevent delivery of the malicious message to end users.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I send the email to a target user who opens and views the message in Zimbra's Classic Web Client.
Business
The vulnerable client renders the ICS content without proper sanitization, triggering script execution in the user's session context.
3
Lateral reach — past segmentation narrative 3
Attacker
I execute arbitrary JavaScript within the victim's authenticated session to modify email filters or redirect messages.
Business
Attacker gains unauthorized access to sensitive emails and can exfiltrate confidential data without the user's knowledge or consent.
04
What to do
— defensible action- Remediate per the vendor advisory — confirm the fixed build for your version and verify exposure.1
Say it to the boardA vulnerability with this evidence profile is a defensible budget line, not a backlog ticket — fund the change against the proof above.
05