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A vulnerability in the TCP Normalizer of Cisco Adaptive Security Appliance (ASA) Software and Firepower Threat Defense (FTD) Software operating in transparent mode could allow an unauthenticated, remote attacker to poison MAC address tables, resulting in a denial of service (DoS) vulnerability. This vulnerability is due to incorrect handling of certain TCP segments when the affected device is operating in transparent mode. An attacker could exploit this vulnerability by sending a crafted TCP segment through an affected device. A successful exploit could allow the attacker to poison the MAC address tables in adjacent devices, resulting in network disruption.
Use CWE-924, Cisco vendor hub and Adaptive Security Appliance product page to widen CVE-2021-34793 into its surrounding weakness, vendor, and product context.
Compare it with CVE-2021-1573, CVE-2021-40118 and CVE-2021-40117 for nearby disclosures in the same product family. Additional editorial context is available in Cybersecurity Weekly Roundup: April 27, 2026 — Critical Zero-Days and Framework Failures.