CVE-2026-93633 — Zimbra Collaboration Suite Briefcase CSS Sanitization Bypass via Non-ASCII Stylesheet Characters
A CSS sanitization vulnerability in Zimbra Collaboration Suite (ZCS) allows an authenticated user with Briefcase access to publish a document that displays attacker-controlled content under the Zimbra origin. When processing a stylesheet containing non-ASCII characters, DefangFilter appends the final ASCII segment without sanitizing it. HTML stored as application/x-zimbra-doc can therefore retain CSS URL references that the normal sanitizer removes. An attacker can share a crafted document that loads external resources and displays a credential-collection form using Zimbra's own visual assets. Exploitation requires the victim to open the document in a browser that renders the content type inline; credential theft additionally requires form submission. ZCS 10.1.16 and 10.1.20 are affected.
Who exploits it and how urgent it is
A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.
How the score was calculated
- EPSS 0.2% (30-day exploitation probability)+2
- Technical severity (medium)+3
The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).
Triage
Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.
CISA SSVC assessment CISA
Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.
Remediation
Fixes and mitigations documented by the sources — linked to the original advisory.
No fix documented by the sources we poll. That does not mean none exists: check the vendor advisory.
First 24 hours
Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.
- Identify exposed assets running the affected vendor/product/version combinations (Synacor Zimbra Collaboration Suite, Zimbra Zimbra Collaboration Suite).
- No fix documented in the sources: reduce exposure and monitor the vendor advisory.
- Exploitation requires user interaction: tighten email/web filtering and warn affected users.
- Search available logs for exploit probes matching the vulnerability class (cross-site scripting), errors and authentication anomalies.
Affected scope
Vendor, product and versions as the sources word them.
| Vendor | Product | Versions | Status | Source |
|---|---|---|---|---|
| Synacor | Zimbra Collaboration Suite | ≤ 10.1.20 | Vulnerable | NIST National Vulnerability Database |
| Zimbra | Zimbra Collaboration Suite | ≤ 10.1.20 | Vulnerable | CVE Program (cve.org) e CISA ADP |
Attack characteristics
The CVSS vector, decoded. It describes the attack, not your exposure to it.
Every base score collected
Sources score independently and may disagree; each row says who scored it and under which version.
| Score | Version | Severity | Expl. | Impact | Scored by |
|---|---|---|---|---|---|
| 5.4 | CVSS 3.1 | Medium | 2.3 | 2.7 | cve@rapid7.com via NIST National Vulnerability Database |
| 5.4 | CVSS 3.1 | Medium | — | — | GitHub, Inc. via GitHub Security Advisories |
| 5.4 | CVSS 3.1 | Medium | — | — | rapid7 via CVE Program (cve.org) e CISA ADP |
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N (score shown at the top: cve@rapid7.com) Weakness MITRE
Weakness class according to MITRE’s CWE catalog.
EPSS over time FIRST
Oct 24, 2026 → Oct 30, 2026 · 7 reading(s) · max 0.24%
Detection
Read off the CVSS vector and the weakness class. Starting points, not tested rules.
- Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
- Monitor CSP violation reports and script payloads in reflected parameters.
Timeline
What happened to this CVE, newest first.
- 2026
- Affected products added · synacor/zimbra collaboration suite
- Source status changed · Undergoing Analysis → Analyzed
- SSVC assessment updated
- Source status changed · Awaiting Analysis → Undergoing Analysis
- Source status changed · Received → Awaiting Analysis
- Affected products added · zimbra/zimbra collaboration suite
- Record published
- CVSS changed · 5.4
- Source status changed · → Received
- Record created in this database
- CVE published
Source activity
Repeated readings are counted rather than listed.
- 2× References added · Oct 23, 2026 – Oct 28, 2026
- 1× Official description updated · Oct 23, 2026 – Oct 23, 2026
References
5 on the record
Elsewhere on this site
Related CVEs (same products)
- High CVE-2026-93640 Zimbra Collaboration Suite Local Privilege Escalation via Unrestricted zmunbound Sudo Configuration 7d
- Medium CVE-2026-93634 Zimbra Collaboration Suite Cross-Account Authorization Bypass via Forged Shared-File Records 7d
- High CVE-2026-93631 Zimbra Collaboration Suite OAuth Extension Cross-Site Request Forgery Enables Account Linking 7d
- Medium CVE-2026-93638 Zimbra Collaboration Suite Pre-Authentication Reflected Cross-Site Scripting via Login Page zrememberme Parameter 7d
- Medium CVE-2026-93636 Zimbra Collaboration Suite LDAP Filter Injection via Calendar Resource Attribute Name 7d
- High CVE-2026-93639 Zimbra Collaboration Suite Missing Authorization in ConfigureZimlet Enables Delegated Administrator Privilege Escalation 7d
Not in any source we poll
Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.
- No fixed version, patch or mitigation documented.
- No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
- No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
- No exploit packet captures, log samples, or incident case IDs supplied.
Answered from this record
Text assembled only from the fields above — no additional claims.
What should defenders know first?
CVE-2026-93633 is "Zimbra Collaboration Suite Briefcase CSS Sanitization Bypass via Non-ASCII Stylesheet Characters", rated Medium. It affects Zimbra Collaboration Suite (Synacor); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 5.4 describes technical severity. EPSS 0.23% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.
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