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PatchSiren cyber security CVE debrief

CVE-2026-35389 Bulwarkmail CVE debrief

A high-severity vulnerability was discovered in Bulwark Webmail, a self-hosted webmail client for Stalwart Mail Server. The issue, tracked as CVE-2026-35389, allows attackers to bypass S/MIME signature verification, potentially leading to email spoofing and phishing attacks. This vulnerability exists due to the lack of validation of the certificate trust chain (checkChain: false) in the S/MIME signature verification process. The vulnerability can be exploited to bypass S/MIME signature verification, allowing emails signed with self-signed or untrusted certificates to be displayed as having a valid signature.

Vendor
Bulwarkmail
Product
Webmail
CVSS
HIGH 8.7
CISA KEV
Not listed in stored evidence
Original CVE published
2026-04-06
Original CVE updated
2026-07-24
Advisory published
2026-04-06
Advisory updated
2026-07-24

Who should care

Administrators and users of Bulwark Webmail, particularly those using S/MIME encryption, should be aware of this vulnerability and take immediate action to update to version 1.4.11 or later. This vulnerability can be exploited to bypass S/MIME signature verification, potentially leading to email spoofing and phishing attacks. The vulnerability affects Bulwark Webmail, a self-hosted webmail client for Stalwart Mail Server.

Technical summary

The vulnerability exists due to the lack of validation of the certificate trust chain (checkChain: false) in the S/MIME signature verification process. This allows emails signed with self-signed or untrusted certificates to be displayed as having a valid signature, potentially leading to email spoofing and phishing attacks. The issue affects Bulwark Webmail, a self-hosted webmail client for Stalwart Mail Server. The vulnerability can be fixed by updating to version 1.4.11 or later.

Defensive priority

High priority should be given to updating Bulwark Webmail to version 1.4.11 or later, as this vulnerability can be exploited to bypass S/MIME signature verification.

Recommended defensive actions

  • Update Bulwark Webmail to version 1.4.11 or later
  • Verify S/MIME certificate trust chains
  • Monitor email traffic for suspicious activity
  • Review compensating controls for exposed systems while remediation is scheduled and verified
  • Check relevant monitoring, detection, and logs for exposed assets that need extra review
  • Track exceptions, retest remediated assets, and close the item only after evidence is documented
  • Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The CVE record was published on 2026-04-06T21:16:20.580Z and last modified on 2026-07-24T21:10:00.143Z. The NVD entry is currently Analyzed. This information is based on the NVD entry and the CVE record. The vulnerability affects Bulwark Webmail, a self-hosted webmail client for Stalwart Mail Server. The issue allows attackers to bypass S/MIME signature verification, potentially leading to email spoofing and phishing attacks. The CVE record and NVD entry provide further details on the vulnerability.

Official resources

AI-assisted PatchSiren debrief based on the supplied source corpus. The CVE record was published on 2026-04-06T21:16:20.580Z and has not been modified since then. The NVD entry is currently Analyzed.