Roundcube CSS Proxy Fails to Validate Responses

In Roundcube Webmail before 1.6.18 and 1.7.x before 1.7.3, responses from the CSS (Cascading Style Sheets) proxy were not validated, which may result in information disclosure or XSS (cross-site scripting) via MIME sniffing.

Casky was already ahead

This CVE exploits attack patterns that Casky's 322matched skills already investigate — long before this vulnerability was disclosed. Claude's reasoning model maps these techniques to MITRE ATT&CK, so practitioners who ran these skills have already seen the threat behaviour in their findings.

Analysis

Roundcube Webmail versions before 1.6.18 and 1.7.x before 1.7.3 contain a critical vulnerability in their CSS proxy functionality that fails to properly validate responses from external sources. This oversight enables attackers to exploit MIME sniffing to inject malicious content, potentially leading to cross-site scripting (XSS) attacks or unauthorized information disclosure. Organizations running affected Roundcube instances—particularly those hosting webmail services for multiple users—face elevated risk, as successful exploitation could compromise user sessions, steal credentials, or deliver malware to end users accessing their email through the web interface.

Casky's 322 matching security skills, powered by Claude AI's extended reasoning capabilities, detect the attack patterns associated with this vulnerability by correlating indicators across MITRE ATT&CK techniques TA0001 (Initial Access) and TA0043 (Reconnaissance). Practitioners using Casky would observe findings related to suspicious CSS proxy requests with unexpected MIME types, unvalidated response handling, and potential script injection attempts in HTTP traffic. The platform's skills map these behavioral patterns to content injection and client-side exploitation chains, enabling security teams to identify both active exploitation attempts and misconfigurations in their Roundcube deployments before attackers can establish persistence or exfiltrate sensitive email data.

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Casky Risk Score

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EPSS Probability

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Shodan Exposure

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GreyNoise Signal

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Composite risk scoring from EPSS, CISA KEV, Shodan, and GreyNoise — 21 security APIs correlated into a single Casky Risk Score. Coming in Casky Pro. Join early access →

322 Casky skills cover this attack pattern

These skills use Claude AI's reasoning model to surface findings in the same attack categories as CVE-2026-74998.

Abuse of Public-Facing API: Mobile API Abuse

mobile security · low

Run

abusing-dpapi-for-credential-access

red teaming · high

Run

abusing-shadow-credentials-for-privesc

red teaming · high

Run

MITRE ATT&CK Techniques

TA0001 TA0043

Investigate the attack patterns behind CVE-2026-74998

Casky has 322 skills that investigate the attack patterns behind CVE-2026-74998. Run one and get CVSS-scored findings in 3 minutes.

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