CVE-2026-20355 in Secure Email
Summary
by MITRE • 09/02/2026
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.
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Analysis
by VulDB Data Team • 09/02/2026
The identified security flaws reside within the Secure/Multipurpose Internet Mail Extensions decryption subsystem of Cisco Secure Email, presenting a critical risk to organizations relying on this platform for secure communications. These vulnerabilities stem from insufficient validation of message integrity during the decryption process, which fundamentally undermines the confidentiality guarantees that S/MIME is designed to provide. The core technical deficiency lies in the application's failure to rigorously verify cryptographic signatures or authentication tags before processing decrypted content. This oversight creates a scenario where the system accepts modified ciphertext as valid input, allowing an adversary to manipulate encrypted data without detection by the decryption engine itself.
An unauthenticated, remote attacker can exploit these weaknesses through a machine-in-the-middle attack vector. By intercepting traffic between email gateways or within the transmission path of S/MIME messages, the attacker can alter the encrypted payload before it reaches its intended destination for decryption. Because the system does not adequately validate that the decrypted output corresponds to an unmodified original message, the application proceeds with processing the tampered data. This lack of integrity verification means that even if encryption keys remain secure and uncompromised, the semantic meaning and content of the messages can be altered or extracted by a sophisticated adversary who controls the network path between endpoints.
The operational impact of these vulnerabilities is severe, as they directly compromise the confidentiality and potentially the integrity of sensitive communications protected by S/MIME. A successful exploit allows an attacker to recover plain text from encrypted email messages that were presumed secure. This exposure can lead to significant data breaches, revealing proprietary information, personal identifiable information, or other confidential business details. Furthermore, if the vulnerability permits modification rather than just observation, it could facilitate message injection attacks where false instructions are inserted into legitimate communication channels, potentially leading to fraudulent transactions or social engineering successes that rely on trusted email sources.
From a classification perspective, these issues align with CWE-347 Improper Verification of Cryptographic Signature and CWE-208 Observable Timing Discrepancy if the validation failure allows for side-channel analysis, though primarily they represent a fundamental breakdown in integrity checks mandated by secure coding practices. In terms of adversary tactics, this exploitation method maps to MITRE ATT&CK technique T1557 Adversary-in-the-Middle, where attackers position themselves between two communicating parties to intercept and potentially alter communications. The failure also reflects CWE-327 Use of a Broken or Risky Cryptographic Algorithm if the specific implementation deviates from standard S/MIME profiles that require strict integrity verification mechanisms such as detached signatures or authenticated encryption modes like AES-GCM without proper tag validation.
To mitigate these risks, organizations must ensure their Cisco Secure Email appliances are updated to versions where the decryption logic has been hardened to enforce rigorous message integrity checks before any plaintext is exposed or processed. Administrators should verify that all S/MIME configurations strictly adhere to industry standards for authenticated encryption and signature verification. Additionally, implementing network-level protections such as TLS inspection with proper certificate validation can help detect anomalies in traffic patterns indicative of man-in-the-middle attempts. Regular security audits focusing on cryptographic implementation details are essential to identify similar weaknesses in other components of the email infrastructure that may rely on S/MIME or similar protocols for securing data in transit.