CVE-2026-35540: An issue was discovered in Roundcube Webmail 1.6.0 before 1.6.14

Routine

MEDIUM 5.4REMOTEUPGRADE

CVE-2026-35540 is a medium-severity flaw (CVSS 5.4) affecting webmail. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.4 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 68%)
CISA exploited list	No
Weakness	CWE-669
Published	2026-04-03
Last modified	2026-07-24

Publisher (CNA): MITRE · Vendor: roundcube

Last updated 2026-07-24 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-03 05:16 UTCPublished CVE record published
2026-07-24 21:10 UTCUpdated Record last modified

roundcube webmail UPGRADE

Affected: >= 1.6.0 and < 1.6.14

Fixed in: 1.6.14

Safe to downgrade to: a release before 1.6.0

1. Check installed version

apt-cache policy webmail || rpm -q webmail || webmail --version

2. UPGRADE to 1.6.14 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade webmail

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade webmail

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 1.6.0 (Debian / Ubuntu)

apt-cache madison webmail sudo apt install webmail=<VERSION_BEFORE_1.6.0> sudo apt-mark hold webmail

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list webmail sudo dnf downgrade webmail-<VERSION_BEFORE_1.6.0> sudo dnf versionlock add webmail

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-35540?

webmail: upgrade to 1.6.14. Exact commands for each OS are on this page.

Is CVE-2026-35540 being exploited?

CVE-2026-35540 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-35540 affect?

CVE-2026-35540 affects webmail. Severity is medium (CVSS 5.4). 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 ho

How urgent is CVE-2026-35540?

Priority 32/100. Remotely exploitable, no login, no user action

Software pages

Roundcube vulnerabilities

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References

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