Affects widely used Cisco Secure Email/ESA; allows recovery of S/MIME plaintext — broad data exposure for organizations using Cisco email gateways.
CVE-2026-20355
S/MIME decryption flaw in Cisco Secure Email allows MITM recovery of encrypted email plaintext.
Is CVE-2026-20355 being exploited?
Not confirmed. CVE-2026-20355 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.15% probability of exploitation in the next 30 days.
How severe is CVE-2026-20355?
CVE-2026-20355 is rated Critical with a CVSS score of 10.0. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-20355?
Yes. A fix has been recorded for CVE-2026-20355. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-20355 affect?
CVE-2026-20355 affects Cisco Secure Email, Cisco Email Security Appliance (ESA), Email gateways performing S/MIME decryption. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-20355?
Apply Cisco security update immediately and harden gateway TLS/mitigation controls.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Unauthenticated remote attacker (MITM) can obtain plaintext from S/MIME-encrypted email messages, exposing confidential data.
Immediate action required
- affected14.0.0-698
- affected13.5.1-277
- affected13.0.0-392
- affected14.2.0-620
- affected13.0.5-007
- affected13.5.4-038
- affected14.2.1-020
- affected14.3.0-032
- affected15.0.0-104
- affected15.0.1-030
- affected15.5.0-048
- affected15.5.1-055
- affected15.5.2-018
- affected16.0.0-050
- affected15.0.3-002
- affected16.0.0-054
- affected15.5.3-022
- affected16.0.1-017
- affected15.5.4-012
- affected16.0.4-016
- affected15.0.5-016
- affected16.0.2-112
- affected16.0.3-044
- affected16.5.0-780
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.