CVE-2026-20011 in Snort 3
Summary
by MITRE • 10/22/2026
Multiple Cisco products are affected by a vulnerability in the Snort 3 SMTP MIME Decoder that could allow an unauthenticated, remote attacker to cause the Snort 3 Detection Engine to restart.
This vulnerability is due to a lack of error checking when the MIME fields of the SMTP header are parsed. An attacker could exploit this vulnerability by sending crafted SMTP packets through an established connection to be parsed by Snort 3. A successful exploit could allow the attacker to cause a denial of service (DoS) condition when the Snort 3 Detection Engine unexpectedly restarts.
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Analysis
by VulDB Data Team • 10/22/2026
The identified security flaw resides within the Simple Mail Transfer Protocol Multipurpose Internet Mail Extensions decoder component of Cisco's implementation of Snort version three, specifically affecting multiple products that integrate this intrusion detection and prevention engine. This vulnerability stems from a fundamental programming error characterized by insufficient validation and boundary checking during the parsing process of MIME fields embedded in SMTP headers. When the Snort 3 Detection Engine processes incoming network traffic, it relies on specialized decoders to interpret protocol-specific data structures. In this instance, the decoder fails to adequately verify the integrity or length of specific MIME header attributes before attempting to access memory locations associated with those fields. This lack of rigorous error checking creates a scenario where malformed input can trigger undefined behavior within the application logic, leading directly to system instability rather than graceful handling of invalid data as intended by secure coding standards.
From an operational perspective, the primary impact of this vulnerability is a denial of service condition resulting from the unexpected restart of the Snort 3 Detection Engine process. Because the flaw allows for remote exploitation without requiring any form of authentication or prior interaction with the system, it presents a significant risk to network security infrastructure that relies on continuous monitoring and packet inspection. An unauthenticated attacker can exploit this weakness by transmitting specially crafted SMTP packets containing malformed MIME fields through an established connection directed at the vulnerable Snort instance. Upon processing these maliciously constructed headers, the decoder encounters conditions it cannot handle safely, causing the detection engine process to crash or terminate abruptly. This restart cycle disrupts the flow of security monitoring capabilities, potentially creating windows of opportunity for attackers to bypass intrusion prevention measures and conduct unauthorized activities undetected while the system is in a degraded state.
This vulnerability aligns with common weakness enumeration classifications related to improper input validation and memory safety issues often found in network protocol parsers. It reflects patterns associated with CWE-20 Improper Input Validation, where the software fails to verify that user-supplied data meets expected criteria before processing. Furthermore, the exploitation technique maps directly to MITRE ATT&CK tactics involving resource exhaustion or service disruption, specifically leveraging application layer protocols to degrade system availability. The ability of an attacker to trigger a process restart remotely underscores the critical importance of robust error handling in network security appliances that operate at high packet rates and must maintain continuous uptime for effective threat detection.
To mitigate this risk, organizations utilizing affected Cisco products should prioritize applying vendor-provided software updates or patches that address the specific parsing logic within the SMTP MIME decoder. These updates typically include enhanced validation routines that ensure all incoming header fields are checked against expected formats and length constraints before being processed by the core engine. In addition to patching, network administrators can implement temporary workarounds such as deploying access control lists on upstream firewalls or routers to restrict direct SMTP traffic from untrusted sources to the Snort sensor interfaces. This reduces the attack surface by limiting exposure to potential exploit attempts until permanent remediation is applied. Regular monitoring of vendor security advisories and maintaining an up-to-date inventory of deployed intrusion detection systems are essential practices for managing such vulnerabilities in enterprise environments where availability and continuous visibility are paramount operational requirements.