CVE-2025-20360 is a vulnerability in Cisco Secure Firewall Threat Defense (FTD) software versions 7.3.0 through 7.7.0, specifically in the Snort 3 HTTP Decoder component. The flaw stems from insufficient error checking when parsing MIME fields within HTTP headers. An attacker can exploit this by sending crafted HTTP packets through an established connection, triggering the Snort 3 Detection Engine to restart unexpectedly. This restart causes a denial-of-service (DoS) condition by temporarily disabling the detection engine, thereby reducing the firewall's ability to inspect and block malicious traffic. The vulnerability is remotely exploitable without authentication or user interaction, increasing its risk profile. The CVSS v3.1 base score is 5.8 (medium), reflecting network attack vector, low complexity, no privileges required, no user interaction, and impact limited to availability. There is no impact on confidentiality or integrity. Although no public exploits are known, the critical role of Cisco FTD in network security makes this a significant concern. The vulnerability affects multiple widely deployed versions, indicating a broad attack surface. The lack of complete error checking in MIME parsing suggests that crafted HTTP headers can trigger unexpected behavior in the detection engine, highlighting the importance of robust input validation in security software.

For European organizations, the primary impact is a denial-of-service condition on Cisco Secure Firewall Threat Defense devices, which are widely used for perimeter and internal network security. The temporary loss of the Snort 3 Detection Engine reduces the firewall's ability to detect and block threats, potentially allowing malicious traffic to pass uninspected during the downtime. This can increase the risk of successful cyberattacks, data breaches, or lateral movement within networks. Critical infrastructure sectors such as finance, energy, healthcare, and government agencies that rely on Cisco FTD for threat detection are particularly vulnerable. The medium severity score reflects no direct data compromise but highlights availability risks that can disrupt business operations and incident response capabilities. The ease of remote exploitation without authentication increases the threat level, especially in environments exposed to untrusted networks or the internet. Although no known exploits exist yet, the vulnerability could be weaponized by threat actors to cause targeted disruptions or as part of multi-stage attacks.

1. Apply Cisco's security patches or updates for affected FTD versions as soon as they become available to address the vulnerability directly. 2. Implement network-level filtering to block or restrict suspicious HTTP traffic that could contain malformed MIME headers, especially from untrusted sources. 3. Monitor Snort 3 Detection Engine logs and system stability closely to detect unexpected restarts or anomalies indicative of exploitation attempts. 4. Employ redundant or failover firewall configurations to maintain security coverage during potential DoS conditions. 5. Limit exposure of Cisco FTD management and inspection interfaces to trusted networks only, reducing the attack surface. 6. Conduct regular security assessments and penetration testing focusing on firewall resilience against crafted HTTP traffic. 7. Educate network security teams about this vulnerability to ensure rapid detection and response to potential exploitation attempts. 8. Consider deploying additional intrusion detection/prevention layers to complement Cisco FTD capabilities during patching windows.