Cisco Secure Email Secure/Multipurpose Internet Mail Extensions Ciphertext Decryption Vulnerabilities
Multiple vulnerabilities in the Secure/Multipurpose Internet Mail Extensions (S/MIME) decryption functionality of Cisco Secure Email could allow an unauthenticated, remote attacker to recover plain text from encrypted email messages. These vulnerabilities are due to insufficient validation of message integrity. An attacker could exploit these vulnerabilities by using a machine-in-the-middle technique to intercept and modify traffic between email gateways. A successful exploit could allow the attacker to obtain plaintext content from the encrypted communication. There are no workarounds that address these vulnerabilities. This advisory is available at the following link: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-esa-smime-disc-dzw4rEdY Security Impact Rating: Medium CVE: CVE-2026-20354,CVE-2026-20355
CSIRTS triage
- What
- S/MIME decryption functionality has insufficient message integrity validation, allowing plaintext recovery from encrypted email via man-in-the-middle modification.
- Who is affected
- Cisco Secure Email deployments processing encrypted messages.
- Urgency
- Medium; requires active man-in-the-middle positioning but exposes encrypted email content.
- Action
- Apply the security update for Cisco Secure Email addressing CVE-2026-20354 and CVE-2026-20355.
AI-assisted analysis generated from the source advisory — verify against the original.
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Details
Referenced CVEs
| CVE | CSIRTS overview | External |
|---|---|---|
| CVE-2026-20354 | coverage & exploitation status | NVD · CVE.org |
| CVE-2026-20355 | coverage & exploitation status | NVD · CVE.org |
Same CVEs, other sources
How other CERTs, PSIRTs and databases cover the vulnerabilities in this advisory.
- medium[NEW] [medium] Cisco Secure Email Gateway: Multiple vulnerabilities enable bypass of security measurescert-bund
- mediumCVE-2026-20355: Multiple vulnerabilities in the Secure/Multipurpose Internet Mail Extensions (S/MIME) decrypti…nvd
- mediumCVE-2026-20354: Multiple vulnerabilities in the Secure/Multipurpose Internet Mail Extensions (S/MIME) decrypti…nvd
- criticalCisco Advance Notification for Publication of September 2, 2026, Security Advisoriescisco-psirt
Recent advisories for Cisco Secure Email
A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.
- medium[NEW] [medium] Cisco Secure Email Gateway: Multiple vulnerabilities enable bypass of security measurescert-bund · 2026-09-03
- mediumCVE-2026-20355: Multiple vulnerabilities in the Secure/Multipurpose Internet Mail Extensions (S/MIME) decrypti…nvd · 2026-09-02
- mediumCVE-2026-20354: Multiple vulnerabilities in the Secure/Multipurpose Internet Mail Extensions (S/MIME) decrypti…nvd · 2026-09-02
- criticalexploited2025-042: Critical Vulnerability in Cisco Secure Email and Web Managercert-eu · 2025-12-18
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