CVE-2024-42010

What it actually does

The flaw is in Roundcube's CSS sanitization for rendered email messages, specifically the mod_css_styles handling of CSS token sequences. A crafted HTML email can contain CSS that is not sufficiently filtered before being rendered in the webmail interface. Depending on the payload and browser behavior, this can allow an unauthenticated remote sender to cause the recipient's Roundcube session to leak sensitive information through attacker-controlled CSS-driven resource requests or other unintended rendering side effects. Affected versions include Roundcube through 1.5.7 and 1.6.x through 1.6.7; the upstream Git fix is commit 44ed0d6fb0cefe30c9c7892c61c34a21a7213c56.

Why it matters to the business

Organizations running vulnerable Roundcube webmail may expose mailbox-related sensitive information simply by receiving or viewing maliciously crafted email. Because exploitation can be initiated remotely by email and the EPSS score is elevated, internet-facing webmail deployments should treat this as a high-priority patching item even though it is not currently listed in CISA KEV and no ransomware use is reported. Successful exploitation can lead to disclosure of confidential communications, user identifiers, message metadata, or other data visible in the webmail context, creating privacy, regulatory, and incident-response consequences.

How an attack unfolds

An attacker prepares an HTML email containing CSS token sequences designed to bypass Roundcube's mod_css_styles filtering. The message is delivered to a mailbox served by a vulnerable Roundcube instance. When Roundcube renders the message, the sanitizer fails to remove or neutralize the malicious CSS sequence. The attacker's CSS can then cause the browser or Roundcube rendering flow to expose information through controlled resource loads, style-based inference, or unintended access to rendered content context. The attacker monitors their external infrastructure for callbacks or leaked values and uses the resulting information for mailbox reconnaissance, targeted phishing, or follow-on compromise. Public reporting associates this issue with Roundcube webmail client-side rendering risk rather than authenticated server takeover.

How to fix it

Upgrade Roundcube to a fixed release or apply the upstream fix containing commit 44ed0d6fb0cefe30c9c7892c61c34a21a7213c56. Specifically, ensure deployments are no longer running Roundcube 1.5.7 or earlier in the 1.5 branch, or 1.6.7 or earlier in the 1.6 branch. If immediate upgrade is not possible, reduce exposure by disabling or restricting HTML email rendering where operationally acceptable, blocking automatic loading of remote content in email, enforcing strict Content Security Policy controls for Roundcube, and limiting outbound web requests from user browsers or webmail contexts to untrusted domains where feasible. Also review custom plugins or local patches that alter message sanitization, HTML rendering, or remote-content behavior, because these can reintroduce similar exposure even after an upstream update.

How to confirm the fix worked

Confirm the installed Roundcube version is newer than the affected ranges or that the codebase includes upstream commit 44ed0d6fb0cefe30c9c7892c61c34a21a7213c56. Review package-manager metadata, Roundcube's version file, deployment artifacts, container images, and vendor changelogs rather than relying only on the web UI. Validate that HTML email sanitization behavior has changed by testing with a benign proof-of-concept message containing CSS constructs such as external url() references, @import rules, and unusual escaped token sequences, and confirm they are removed, neutralized, or prevented from causing outbound requests. Verify browser developer tools or proxy logs show no unexpected external fetches when viewing the test message. Re-run external exposure scans to identify any internet-facing Roundcube instances still on vulnerable versions.

Typical attack path

1. Identify an organization using Roundcube webmail with a vulnerable version through 1.5.7 or 1.6.x through 1.6.7.
2. Craft an HTML email containing malicious CSS token sequences intended to survive Roundcube's CSS sanitizer.
3. Send the message to one or more target mailboxes handled by the vulnerable Roundcube deployment.
4. Wait for the message to be rendered in the Roundcube web interface, such as during preview or message open.
5. Trigger attacker-controlled external resource requests or rendering side effects from the crafted CSS.
6. Collect leaked sensitive information or behavioral signals from attacker-controlled infrastructure.
7. Use the disclosed information for account targeting, phishing, mailbox reconnaissance, or additional intrusion attempts.

How to detect it

To assess possible prior exploitation, review mail stores, message quarantine systems, secure email gateway logs, Roundcube access logs, web server logs, browser proxy logs, DNS logs, and outbound network telemetry. Focus on HTML emails received before patching that contain style blocks or inline styles with external resource references, obfuscated CSS escapes, @import rules, or repeated CSS selectors intended to cause conditional loading of attacker-controlled URLs. Correlate the delivery time of suspicious messages with Roundcube web access events for mail preview or message display, followed by outbound HTTP, HTTPS, or DNS requests to unusual domains from user browsers, web proxies, or webmail-related network paths. In web server logs, look for users opening or previewing suspicious messages around the same time external callbacks occurred. In mail archives, search MIME HTML parts for CSS constructs that Roundcube normally should sanitize, especially if they point to newly registered, low-reputation, or attacker-controlled domains. Because no universal exploit string or campaign-specific infrastructure is inherent to this CVE, detection should be behavior-based and content-based rather than relying solely on fixed IOCs.

• <style
• </style>
• @import
• url(
• background:url(
• background-image:url(
• list-style-image:url(
• cursor:url(
• image-set(
• -webkit-image-set(
• _task=mail&_action=show
• _task=mail&_action=preview
• _task=mail&_action=get
• Content-Type: text/html
• Content-Type: multipart/alternative

What your security team should do

• Inventory all Roundcube deployments, including packaged installations, container images, source checkouts, and appliances bundling Roundcube.
• Prioritize upgrades for internet-facing Roundcube instances and shared hosting environments because the vulnerability is remotely reachable through email content and has a high EPSS score.
• Upgrade to a fixed Roundcube version or apply upstream commit 44ed0d6fb0cefe30c9c7892c61c34a21a7213c56, then restart relevant web and PHP services as required.
• Temporarily disable HTML email rendering or remote content loading if patching cannot be completed quickly.
• Add or tighten Content Security Policy controls for the Roundcube application to limit image, style, font, and connect destinations to trusted sources.
• Search recent mail stores and web access logs for suspicious HTML email containing CSS external resource loads or obfuscated CSS token sequences.
• Review outbound proxy, DNS, and web logs for unusual requests generated while users accessed Roundcube mail views.
• Educate helpdesk and mail administrators that exploitation may arrive as ordinary email and may not require attachments or credential entry.
• Monitor vendor advisories for Roundcube 1.5 and 1.6 maintenance releases and ensure downstream packages include the sanitizer fix.