CVE-2026-20179 in Open Source Snort3
Summary
by MITRE • 10/21/2026
Multiple Cisco products are affected by a vulnerability in the Snort 3 Generic Routing Encapsulation (GRE) Detection Engine that could allow an unauthenticated, remote attacker to cause the Snort 3 Detection Engine to restart, resulting in an interruption of packet inspection.
This vulnerability is due to incomplete error checking when Snort 3 parses GRE packets. An attacker could exploit this vulnerability by sending a crafted GRE packet to a targeted device to be parsed by Snort. A successful exploit could allow the attacker to cause the Snort 3 Detection Engine to restart unexpectedly, resulting in a DoS condition.
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Analysis
by VulDB Data Team • 10/21/2026
The Cisco Intrusion Prevention System and related security appliances rely heavily on the Snort 3 detection engine for deep packet inspection and threat analysis. Within this architecture, the Generic Routing Encapsulation protocol handler plays a critical role in processing tunneled traffic that traverses network boundaries. A significant flaw has been identified within the GRE Detection Engine component of Snort 3, specifically concerning how it validates incoming packets before further processing. This vulnerability stems from incomplete error checking during the parsing phase of GRE packet headers. When an attacker crafts a maliciously formatted GRE packet and sends it to a targeted device running affected Cisco software, the detection engine fails to properly validate specific header fields or payload structures. Instead of rejecting the malformed input gracefully, the system encounters an unexpected state that triggers a critical failure within the Snort 3 process itself.
The operational impact of this flaw is severe for network security operations because it results in a denial-of-service condition against the inspection mechanism rather than the underlying host operating system. When the vulnerability is successfully exploited by sending a crafted GRE packet, the Snort 3 Detection Engine restarts unexpectedly. This restart causes an immediate interruption in packet inspection capabilities. During this window of downtime, which can last from several seconds to minutes depending on service recovery times, network traffic flows through the device without being analyzed for malicious content or policy violations. Attackers can leverage this gap to bypass security controls, exfiltrate data, or launch other attacks that would normally be detected and blocked by the intrusion prevention system. The ability of an unauthenticated remote attacker to trigger this restart means that no prior access credentials are required, significantly lowering the barrier for exploitation in exposed environments.
From a classification perspective, this vulnerability aligns with CWE-20 Improper Input Validation, as the core issue is the failure to adequately sanitize or verify input data before processing it within the application logic. Furthermore, the attack vector and effect map directly to MITRE ATT&CK technique T1499 Endpoint Denial of Service, specifically under sub-techniques involving service exhaustion via resource consumption or process crashes. The remote nature of the exploit places it in the Initial Access or Impact phase depending on whether the restart is used as a distraction for other attacks or as an end goal to degrade security posture. Organizations must recognize that while the device remains online and forwarding traffic, its primary defensive function is compromised during each exploitation event.
Mitigation strategies should prioritize immediate application of vendor-provided patches if available, as this addresses the root cause within the Snort 3 codebase by implementing robust input validation checks for GRE packet structures. In scenarios where patching cannot be applied immediately due to operational constraints or compatibility concerns, network segmentation and access control lists can help restrict exposure. Administrators should ensure that GRE traffic is only permitted from trusted sources and consider deploying rate-limiting rules on edge devices to mitigate the frequency of crafted packets reaching the inspection engine. Additionally, monitoring for frequent restarts of security services in system logs can serve as an early warning indicator of ongoing exploitation attempts, allowing incident response teams to investigate potential breaches during periods when detection capabilities were impaired.