CVE-2026-35540: Roundcube Webmail Insufficient CSS Sanitization Vulnerability

MODERATE
Advisory Identifier: GHSA-vxg2-hhgr-37fx
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CVSS 5.4
CWE-669 CWE-918
Published
Advisory Summary

CVE-2026-35540 is a MODERATE (CVSS 5.4) insufficient css sanitization vulnerability (CWE-669: Incorrect Resource Transfer Between Spheres) affecting Roundcube Webmail libraries in the Packagist ecosystem. The issue involves insufficient css sanitization in html e-mail messages. Remediation is available by upgrading affected packages to version 1.7-rc5.

CVE-2026-35540 at a Glance

CVE Identifier CVE-2026-35540
GHSA Identifier GHSA-vxg2-hhgr-37fx
Severity MODERATE
CVSS v3.1 Base Score 5.4
CWE Classification CWE-669: Incorrect Resource Transfer Between Spheres
Affected Product Roundcube Webmail
Package Ecosystem Packagist
Published Date
Last Updated

Security Impact

CVSS: 5.4

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:N).

Attack Vector Network (AV:N)
Attack Complexity High (AC:H)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Changed (S:C)
Confidentiality Low (C:L)
Integrity Low (I:L)
Availability None (A:N)

How to Fix CVE-2026-35540

To remediate this vulnerability, upgrade all affected Roundcube Webmail dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

roundcube/roundcubemail fixed: 1.7-rc5 
Upgrade roundcube/roundcubemail to 1.7-rc5
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

inspect dependencies for roundcube/roundcubemail

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-35540, inspect your project dependencies for Roundcube Webmail:

inspect dependencies for roundcube/roundcubemail

Analyze the terminal output to locate any version of Roundcube Webmail in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 1.7-rc5), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-669: Incorrect Resource Transfer Between Spheres, where the product does not properly transfer a resource/behavior to another sphere, or improperly imports a resource/behavior from another sphere, in a manner that provides unintended control over that resource. In Roundcube Webmail, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

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.

CVE-2026-35540 Affected Roundcube Webmail Versions

1 component(s)
Fixed Versions:
1.7-rc5
Product / Package Ecosystem Affected Versions Fixed Version
roundcube/roundcubemail
 Packagist
≥ 1.7-beta
< 1.7-rc5
1.7-rc5

References

NVD — CVE-2026-35540 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-35540
Security Fix Commit FIX COMMIT
https://github.com/roundcube/roundcubemail/commit/27ec6cc9cb25e1ef8b4d4ef39ce76d619caa6870
Security Fix Commit FIX COMMIT
https://github.com/roundcube/roundcubemail/commit/579b68eff90650a5c782e153debd66c765648942
Roundcube Webmail Source Repository REPOSITORY
https://github.com/roundcube/roundcubemail
Roundcube Webmail Release RELEASE
https://github.com/roundcube/roundcubemail/releases/tag/1.6.14
Roundcube Webmail Release RELEASE
https://github.com/roundcube/roundcubemail/releases/tag/1.7-rc5
Roundcube Webmail Security Information SECURITY
https://roundcube.net/news/2026/03/18/security-updates-1.7-rc5-1.6.14-1.5.14

Frequently Asked Questions

What is CVE-2026-35540?
CVE-2026-35540 is a security vulnerability in Roundcube Webmail involving Insufficient CSS sanitization in HTML e-mail messages under CWE-669: Incorrect Resource Transfer Between Spheres.
Which Roundcube Webmail versions are affected?
Affected versions include releases prior to 1.7-rc5 across affected packages: roundcube/roundcubemail.
What version fixes CVE-2026-35540?
Upgrading to 1.7-rc5 (or the latest patched release for your respective branch) resolves the vulnerability.
Is roundcube/roundcubemail affected?
Yes, roundcube/roundcubemail is affected in releases prior to 1.7-rc5. Upgrade your dependency to 1.7-rc5 or later.
What is the CVSS score of CVE-2026-35540?
CVE-2026-35540 has a CVSS v3.1 base score of 5.4 (MODERATE) with vector string CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:N.
What CWE is associated with CVE-2026-35540?
CVE-2026-35540 is classified under CWE-669: Incorrect Resource Transfer Between Spheres (The product does not properly transfer a resource/behavior to another sphere, or improperly imports a resource/behavior from another sphere, in a manner that provides unintended control over that resource.).
How can I check Packagist dependencies for CVE-2026-35540?
Run the dependency tree command: 'inspect dependencies for roundcube/roundcubemail'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-vxg2-hhgr-37fx",
  "modified" : "2026-04-04T06:53:55Z",
  "published" : "2026-04-03T06:31:32Z",
  "aliases" : [ "CVE-2026-35540" ],
  "summary" : "Roundcube Webmail: Insufficient CSS sanitization in HTML e-mail messages",
  "details" : "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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "roundcube/roundcubemail"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.7-beta"
      }, {
        "fixed" : "1.7-rc5"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-35540"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/roundcube/roundcubemail/commit/27ec6cc9cb25e1ef8b4d4ef39ce76d619caa6870"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/roundcube/roundcubemail/commit/579b68eff90650a5c782e153debd66c765648942"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/roundcube/roundcubemail"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/roundcube/roundcubemail/releases/tag/1.6.14"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/roundcube/roundcubemail/releases/tag/1.7-rc5"
  }, {
    "type" : "WEB",
    "url" : "https://roundcube.net/news/2026/03/18/security-updates-1.7-rc5-1.6.14-1.5.14"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-669", "CWE-918" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-04T06:53:55Z",
    "nvd_published_at" : "2026-04-03T05:16:22Z"
  }
}