CVE-2019-1955 in Email Security Appliance
Summary
by MITRE
A vulnerability in the Sender Policy Framework (SPF) functionality of Cisco AsyncOS Software for Cisco Email Security Appliances (ESA) could allow an unauthenticated, remote attacker to bypass configured user filters on the device. The vulnerability is due to incomplete input and validation checking mechanisms for certain SPF messages that are sent to an affected device. An attacker could exploit this vulnerability by sending a customized SPF packet to an affected device. A successful exploit could allow the attacker to bypass the header filters that are configured for the affected device, which could allow malicious content to pass through the device.
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Analysis
by VulDB Data Team • 11/21/2023
The vulnerability identified as CVE-2019-1955 represents a critical weakness in the Sender Policy Framework implementation within Cisco AsyncOS Software for Email Security Appliances. This flaw exists within the email security infrastructure that organizations rely upon to filter and control incoming email traffic. The vulnerability stems from insufficient validation mechanisms that process SPF messages, creating an exploitable gap in the security controls designed to protect against malicious email content. The issue specifically affects Cisco Email Security Appliances where the software fails to properly validate SPF records, allowing attackers to manipulate the filtering process through crafted SPF packets.
The technical exploitation of this vulnerability occurs through the manipulation of SPF message structures that are normally used to validate email sender authenticity. When an attacker crafts and sends specially formatted SPF packets to the affected device, the incomplete input validation allows these malicious messages to bypass the configured header filters. This bypass mechanism operates at the protocol level where SPF records are processed, effectively undermining the security controls that should prevent unauthorized email content from passing through the appliance. The vulnerability is particularly concerning because it requires no authentication credentials from the attacker, making it a remote exploit that can be executed from any location with network access to the target device.
The operational impact of this vulnerability extends beyond simple bypass of email filters, potentially allowing attackers to deliver malicious content through the email security appliance. This includes phishing emails, malware distribution, and other malicious payloads that would normally be blocked by the configured security policies. Organizations using affected Cisco ESA appliances face the risk of increased security incidents, potential data breaches, and compromised email infrastructure. The vulnerability essentially creates a backdoor that allows attackers to circumvent the very protections that make email security appliances valuable for enterprise security programs. This weakness can lead to significant business disruption and regulatory compliance issues, particularly in environments with strict email governance requirements.
Cisco has released patches and updates to address this vulnerability, which should be implemented immediately across all affected appliances. Organizations should conduct thorough vulnerability assessments to identify all instances of the affected software versions and apply the necessary security updates. Network segmentation and monitoring should be enhanced to detect potential exploitation attempts, while security teams should review existing email filtering policies to ensure they remain effective against such attacks. The vulnerability aligns with CWE-20, which describes "Improper Input Validation," and represents a clear example of how inadequate validation can create security weaknesses in network infrastructure components. From an ATT&CK perspective, this vulnerability maps to techniques involving bypassing security controls and privilege escalation through protocol manipulation, making it a significant concern for organizations implementing email security measures.