CVE-2026-35540Network attack vector

Internal resource disclosure via stylesheet proxy SSRF

Published Apr 3, 2026 · Updated Apr 3, 2026

SSRF in Roundcube Webmail 1.6.0 before 1.6.14 allows remote attackers to reach local-network hosts via HTML email stylesheet links. The washtml_link_callback path rewrites HTTP(S) stylesheet hrefs to the modcss proxy, which performs a server-side GET without rejecting local, loopback, or link-local destinations. A crafted HTML email is required, and proxied CSS content can disclose resources reachable from the webmail server.

CVSS severity5.4
Medium
EPSS probability0.39%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

SSRF in Roundcube Webmail 1.6.0 before 1.6.14 allows remote attackers to reach local-network hosts via HTML email stylesheet links. The washtml_link_callback path rewrites HTTP(S) stylesheet hrefs to the modcss proxy, which performs a server-side GET without rejecting local, loopback, or link-local destinations. A crafted HTML email is required, and proxied CSS content can disclose resources reachable from the webmail server.

The record

CVE
CVE-2026-35540
Published
Apr 3, 2026
Updated
Apr 3, 2026
Vendor
Roundcube
Product
Roundcube Webmail
Classifications
CWE-918, CWE-669, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 3, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 3, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=1.6.0 <1.6.14

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Roundcube · Roundcube Webmailversion=1.6.0 <1.6.14

Published CVSS scores

5.4GitHubCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-918Server-Side Request Forgery (SSRF)
CWE-669Incorrect Resource Transfer Between Spheres
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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