Cisco warned its customers about two critical security holes that affect almost every product the company makes. The vulnerabilities could be used by malicious hackers to create so-called 'denial of service' (DoS) attacks, causing Cisco products to abruptly restart or drop active connections with other devices.Cisco\u00a0warned its customers about two critical security holes that affect almost every product the company makes. The vulnerabilities could be used by malicious hackers to create so-called "denial of service" (DoS) attacks, causing Cisco products to abruptly restart or drop active connections with other devices.Cisco issued advisories on Tuesday, revealing the impact on the company's products of\u00a0a security hole in TCP\u00a0(for\u00a0IOS devices\u00a0and\u00a0non-IOS devices) and another serious\u00a0vulnerability in the company's IOS that affects SNMP. The advisories are just the latest in a string of security warnings from the San Jose network equipment maker.Following warnings yesterday from the U.K.'s\u00a0National Infrastructure Security Co-Ordination Centre\u00a0(NISCC) and the U.S.\u00a0Computer Emergency Readiness Team\u00a0(US-CERT), Cisco issued two advisories regarding a security vulnerability in the standard implementation of TCP.Cisco is just one of a large number of\u00a0software and hardware makers that are affected by the TCP hole.The TCP hole is found in all implementations of TCP that comply with the Internet Engineering Task Force's TCP specification. By exploiting the holes, malicious hackers could cause TCP sessions to end prematurely, creating a DoS attack. The TCP vulnerability could also disrupt communications among Internet routers by interrupting BGP (Border Gateway Protocol) sessions that use TCP, NISCC said Tuesday.In one\u00a0advisory, Cisco published software updates for more than 47 of the company's products that contain the TCP vulnerability but do not use the IOS operating system.Cisco issued\u00a0a separate advisory\u00a0listing updates for scores of versions of the IOS operating system that are also affected by the TCP hole and provided workaround instructions for customers who are unable to update their operating system.In\u00a0a third advisory, Cisco said that it patched a flaw in the way certain versions of IOS process\u00a0SNMP traffic. The software vulnerability, which was introduced by a coding error to fix an earlier IOS problem, could cause memory on the Cisco devices running IOS to be corrupted, forcing the affected device to restart unexpectedly, Cisco said.The company said it fixed the\u00a0SNMP problem and published information on updating IOS with new versions of the operating system.US-CERT also issued a\u00a0warning\u00a0about the Cisco SNMP hole and advised Cisco customers to upgrade their devices that use affected versions of IOS.The warnings are just the latest from Cisco, which has disclosed a number of serious vulnerabilities in recent weeks, including a\u00a0hole in Cisco VPN hardware and software\u00a0and in\u00a0two products\u00a0used to manage wireless LANs and e-business services in corporate data centers.