Description

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.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.9",
  "pubDate": "2026-09-02T17:17:33.897Z",
  "pubdate": "2026-09-02T17:17:33.897Z",
  "executiveSummary": "This advisory pertains to multiple vulnerabilities within the Secure/Multipurpose Internet Mail Extensions (S/MIME) decryption functionality of Cisco Secure Email. These flaws stem from inadequate validation of message integrity, which compromises the security posture of encrypted email communications.\nThe vulnerability allows an unauthenticated, remote attacker to perform man-in-the-middle (MitM) attacks to intercept and modify traffic between email gateways. By exploiting the lack of robust integrity verification, an attacker can manipulate the encrypted data stream to recover plaintext content from previously secure messages.\nThe impact is significant, as it leads to the unauthorized disclosure of confidential communications, undermining the primary purpose of S/MIME encryption. Successful exploitation requires an attacker to possess the capability to position themselves between network entities to intercept and manipulate traffic. Given the nature of these vulnerabilities, the risk implications are high for organizations relying on Cisco Secure Email to maintain data confidentiality for sensitive correspondence.",
  "technicalDetails": "The core issue resides in the S/MIME decryption logic within Cisco Secure Email, specifically concerning its handling of cryptographic message integrity verification. S/MIME is designed to provide both encryption and authentication/integrity via digital signatures; however, in this implementation, the system fails to sufficiently validate the integrity of the message before or during the decryption process.\nThe vulnerability is primarily triggered when an attacker executes a man-in-the-middle (MitM) attack between two email gateways. By intercepting the communication stream, the attacker can alter the ciphertext or its associated metadata. Because the receiving Cisco Secure Email gateway performs insufficient integrity checks, it proceeds to process the modified ciphertext through its decryption routine.\nThe exploitation flow typically follows these steps: First, the attacker intercepts an encrypted email transit between gateways. Second, the attacker performs targeted modifications to the encrypted message packets. Third, the receiving Cisco Secure Email gateway, failing to detect these manipulations due to the identified integrity validation flaw, utilizes its internal decryption mechanisms to process the altered data. Finally, through the analysis of the resulting output or error responses generated by the decryption process, the attacker can reconstruct or directly recover the original plaintext content of the encrypted email.\nThis vulnerability is classified as remote and unauthenticated, meaning no prior access or credentials are required for an attacker to initiate the exploitation. The exposure is limited to the network path of the email traffic. The post-exploitation impact is a complete loss of confidentiality regarding the intercepted encrypted communications, exposing potentially sensitive organizational intelligence, credentials, or personal identifiable information (PII) that were intended to be protected by S/MIME standards.\nWhile specific vulnerable version numbers were not provided in the input, the issue is systemic to the Cisco Secure Email S/MIME decryption component. The failure of the system to enforce strict adherence to cryptographic integrity standards allows for adaptive chosen-ciphertext style manipulations that bypass expected security controls."
}