CVE-2026-35544Network attack vector

Message display manipulation via !important CSS bypass

Published Apr 3, 2026 · Updated Apr 3, 2026

CSS sanitization bypass in Roundcube Webmail before 1.5.14 and 1.6.14 allows remote attackers to retain fixed-position message content. The sanitize_css_block() method checks whether a position value equals fixed exactly, so fixed !important evades conversion to absolute. A crafted HTML email can therefore keep attacker-controlled content fixed in the recipient viewport; the published impact is limited to message-display integrity.

CVSS severity5.3
Medium
EPSS probability0.37%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

CSS sanitization bypass in Roundcube Webmail before 1.5.14 and 1.6.14 allows remote attackers to retain fixed-position message content. The sanitize_css_block() method checks whether a position value equals fixed exactly, so fixed !important evades conversion to absolute. A crafted HTML email can therefore keep attacker-controlled content fixed in the recipient viewport; the published impact is limited to message-display integrity.

The record

CVE
CVE-2026-35544
Published
Apr 3, 2026
Updated
Apr 3, 2026
Vendor
Roundcube
Product
Roundcube Webmail
Classifications
CWE-669, T1566
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 3, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 3, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <1.5.14
  2. Affected versionversion=1.6.0 <1.6.14

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Roundcube · Roundcube Webmailversion=0 <1.5.14; version=1.6.0 <1.6.14

Published CVSS scores

5.3MITRECVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-669Incorrect Resource Transfer Between Spheres
T1566Phishing

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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