Cisco ESA mail Bypass

Vulnerability

A vulnerability in the email filtering mechanism of Cisco Secure Email Gateway (AsyncOS) allows an unauthenticated remote attacker to bypass configured rules and allow emails that should have been denied to flow through an affected device [1]. The issue is due to improper handling of email passing through the device. Affected versions include all release trains before the first fixed release: 14.2 and earlier, 15.0, and 16.0 prior to 16-0-0-054 [1].

Exploitation

An attacker can exploit this vulnerability by sending a crafted email through the affected device [1]. The attacker does not require any prior authentication, local access, or special network position beyond the ability to send email to a target system protected by the gateway. The specific crafting technique that triggers the bypass has not been disclosed in the available references.

Impact

Successful exploitation allows the attacker to bypass email filters on the affected device [1]. This defeats the configured security policies, potentially enabling delivery of malicious or unwanted email (spam, phishing, malware attachments) that the administrator intended to block, leading to a breach of confidentiality and integrity on downstream mail recipients.

Mitigation

Cisco has released fixed software: version 16-0-0-054 for the 16.0 release train [1]. Customers running 14.2 and earlier or 15.0 must migrate to a fixed release. Upgrades can be performed via the System Administration > System Upgrade interface. No workarounds are documented in the advisory; upgrading to the fixed release is the recommended mitigation [1].