Multiple Cisco products are affected by a vulnerability in the Snort application detection engine that could allow an unauthenticated, remote attacker to bypass the configured policies on an affected system. The vulnerability is due to a flaw in the detection algorithm. An attacker could exploit this vulnerability by sending crafted packets that would flow through an affected system. A successful exploit could allow the attacker to bypass the configured policies and deliver a malicious payload to the protected network.
Use CWE-670, Cisco vendor hub and Ios Xe product page to widen CVE-2021-1236 into its surrounding weakness, vendor, and product context.
Compare it with CVE-2025-20352, CVE-2025-20363 and CVE-2025-20202 for nearby disclosures in the same product family. Additional editorial context is available in The Weekly Cybersecurity Brief: January 30th, 2026.