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": "2026-09-02T17:17:34.027Z",
  "pubdate": "2026-09-02T17:17:34.027Z",
  "executiveSummary": "This advisory pertains to multiple vulnerabilities within the S/MIME decryption functionality of Cisco Secure Email, specifically concerning insufficient message integrity validation.\nThe identified flaws reside in the cryptographic processing layer, enabling an unauthenticated, remote attacker to perform a plaintext recovery attack against encrypted email traffic.\nThe attack vector relies on a machine-in-the-middle (MITM) technique, where an adversary intercepts and manipulates encrypted communication packets between email gateways.\nBy bypassing integrity checks, the attacker can force the decryption module to output plaintext content, effectively nullifying the confidentiality guarantees provided by S/MIME encryption.\nThe risk is critical for organizations relying on Cisco Secure Email to maintain the privacy of inter-gateway communication, as successful exploitation results in the exposure of sensitive message data.\nExploitation requires the attacker to position themselves within the network path of the encrypted traffic to perform active modification of the data stream.",
  "technicalDetails": "The root cause of these vulnerabilities is the failure of the S/MIME decryption engine to properly validate the integrity of the encrypted message structure prior to or during the decryption process.\nS/MIME implementations typically rely on robust Message Authentication Codes (MACs) or Authenticated Encryption with Associated Data (AEAD) to ensure that the ciphertext has not been tampered with; the lack of sufficient validation allows an attacker to manipulate the encrypted payload without triggering a decryption failure.\nThe attack flow follows a machine-in-the-middle pattern: the attacker intercepts the transmission of an S/MIME-encrypted email between two Cisco Secure Email gateways. Because the decryption component does not verify the message integrity, the attacker can selectively modify specific fields or blocks within the encrypted envelope.\nBy crafting specific modifications to the ciphertext, the attacker leverages the oracle properties of the decryption process—specifically, by observing how the system responds to different malformed or altered inputs. Through repeated interactions with the gateway as an oracle, the attacker can incrementally recover the plaintext content of the message.\nThis vulnerability is strictly remote and does not require pre-existing authentication, as the gateway processes the incoming traffic as part of its standard mail transport workflow. The exposure is confined to the network segment where the email transit occurs, necessitating the capability to perform man-in-the-middle interception (e.g., ARP poisoning, BGP hijacking, or physical network access).\nPost-exploitation impact involves the total loss of confidentiality for the intercepted messages. Once the attacker has established the decryption oracle, they can automate the recovery of plaintext for any intercepted S/MIME traffic subject to the same validation flaw, potentially leading to widespread data leakage of sensitive organizational communications.\nThe vulnerable component is the S/MIME decryption module integrated into Cisco Secure Email's mail transfer agent (MTA) logic. Because the integrity check failure occurs during the parsing of the PKCS#7 or CMS (Cryptographic Message Syntax) containers, the gateway effectively treats the attacker-modified payload as legitimate during the decryption attempt."
}