{"id":"CVE-2026-20221","kind":"vulnerability","aliases":["CVE-2026-20221"],"related":[],"title":"Cisco Snort 3: crafted nested GRE traffic restarts the detection engine","description":"Multiple Cisco products are affected by a vulnerability in the generic routing encapsulation (GRE) tunnel decapsulation feature of the Snort 3 Detection Engine that could allow an unauthenticated, remote attacker to cause an unexpected restart of the Snort 3 Detection Engine.\r\n\r\nThis vulnerability is due to improper handling of nested GRE traffic. An attacker could exploit this vulnerability by sending a crafted GRE payload through an affected device. A successful exploit could allow the attacker to trigger an unexpected reload of the Snort 3 Detection Engine, resulting in either a bypass or denial of service (DoS) condition, depending on device configuration. For more information, see the  section of this advisory. The Snort 3 Detection Engine will restart automatically. No manual intervention is required.","summary":"Cisco's Snort 3 Detection Engine mishandles nested GRE tunnel traffic, so a crafted GRE packet sent through an affected device makes Snort restart. The restart can cause a denial of service or let traffic bypass inspection, depending on device configuration.","severity":"medium","cvss":5.8,"epss":0.00454,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["cisco","infrastructure","network-appliance","snort-3"],"not_affected_if":["no-gre-tunnels"],"how_to_check":"- unknown","fix":"- unknown","status":"open","published_at":"2026-10-21T17:18:27+00:00","first_seen_at":"2026-10-23T21:57:29+00:00","triaged_at":"2026-10-25T04:10:01+00:00","updated_at":"2026-10-25T04:10:01+00:00","category":"infrastructure","confidence":0.6,"triage":{"at":"2026-10-25T04:10:01Z","model":"claude-haiku-5-5","output":{"fix":"- unknown","reach":"network","title":"Cisco Snort 3: crafted nested GRE traffic restarts the detection engine","impact":"medium","summary":"Cisco's Snort 3 Detection Engine mishandles nested GRE tunnel traffic, so a crafted GRE packet sent through an affected device makes Snort restart. The restart can cause a denial of service or let traffic bypass inspection, depending on device configuration.","category":"infrastructure","nicknames":[],"who_cares":["cisco","snort-3","network-appliance"],"confidence":0.6,"needs_auth":"no","worst_case":"Snort 3 restarts repeatedly, causing denial of service or inspection bypass","analyst_note":"Find the Cisco advisory for CVE-2026-20221 to list affected products, fixed versions and the device configuration that decides between bypass and denial of service.","how_to_check":"- unknown","needs_analyst":true,"not_affected_if":["no-gre-tunnels"],"who_is_affected":"Cisco products that run the Snort 3 Detection Engine with GRE tunnel decapsulation are affected, and the sources do not list which product versions. Devices that do not run Snort 3 or do not process GRE tunnels are likely outside the issue, but the sources do not confirm this.","user_interaction":"no","needs_special_setup":"unknown","severity_if_unknown":"medium"},"cost_usd":0.000596,"input_tokens":3530,"output_tokens":485,"severity_before":"medium"},"analyst_status":null,"analyst_note":null,"search":"'-20221':14A,17A '-2026':13A,16A '-3':24B '3':3A,28B,86C,104C,149C,178C 'advisori':175C 'affect':44B,70C,132C 'allow':91C,138C 'applianc':22B 'attack':95C,119C,140C 'automat':183C 'bypass':60B,156C 'caus':52B,97C 'cisco':1A,18B,25B,67C 'condit':162C 'configur':65B,166C 'could':90C,120C,137C 'craft':4A,38B,127C 'cve':12A,15A 'decapsul':81C 'denial':54B,158C 'depend':62B,163C 'detect':10A,29B,87C,105C,150C,179C 'devic':45B,64B,133C,165C 'dos':161C 'due':110C 'either':154C 'encapsul':78C 'engin':11A,30B,88C,106C,151C,180C 'exploit':121C,136C 'featur':82C 'generic':76C 'gre':6A,33B,39B,79C,116C,128C 'handl':113C 'improp':112C 'inform':169C 'infrastructur':19B 'inspect':61B 'intervent':186C 'let':58B 'make':46B 'manual':185C 'mishandl':31B 'multipl':66C 'nest':5A,32B,115C 'network':21B 'network-appli':20B 'packet':40B 'payload':129C 'product':68C 'reload':145C 'remot':94C 'requir':188C 'restart':8A,48B,50B,100C,182C 'result':152C 'rout':77C 'section':172C 'see':170C 'send':125C 'sent':41B 'servic':56B,160C 'snort':2A,23B,27B,47B,85C,103C,148C,177C 'success':135C 'traffic':7A,35B,59B,117C 'trigger':142C 'tunnel':34B,80C 'unauthent':93C 'unexpect':99C,144C 'vulner':73C,108C,123C","nicknames":[],"mitigations":[],"who_affected":"Cisco products that run the Snort 3 Detection Engine with GRE tunnel decapsulation are affected, and the sources do not list which product versions. Devices that do not run Snort 3 or do not process GRE tunnels are likely outside the issue, but the sources do not confirm this.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-21T17:18:27+00:00","kev":null,"cvss_vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:L","cvss_by":"psirt@cisco.com","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-20221","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-20221","severity":null,"published_at":"2026-10-21T17:18:27+00:00","modified_at":"2026-10-22T14:17:40+00:00","first_seen_at":"2026-10-23T21:57:29+00:00","fetched_at":"2026-10-23T22:29:01+00:00","note":null},{"source":"epss","external_id":"CVE-2026-20221","url":"https://api.first.org/data/v1/epss?cve=CVE-2026-20221","severity":null,"published_at":null,"modified_at":null,"first_seen_at":"2026-10-24T13:15:54+00:00","fetched_at":"2026-10-24T13:15:54+00:00","note":null}],"documents":[],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}