Security readout for executives and security teams
Plain-English summary
This vulnerability can make affected Cisco security devices drop normal network traffic. It matters most where those devices sit inline on critical business paths. The issue only applies when Snort3 is configured with specific reset-based blocking rule actions; Snort2 configurations are reported as not vulnerable.
Executive priority
Treat as high priority where Cisco inspection devices protect business-critical network paths. The issue can disrupt availability without authentication, but exposure depends on a specific Snort3 and rule configuration.
Technical view
CVE-2021-40116 is a Snort rule handling flaw in multiple Cisco products, including Cisco Firepower Threat Defense Software. An unauthenticated remote attacker could cause a denial of service by triggering improper handling of Block with Reset or Interactive Block with Reset actions when rules lack proper constraints.
Likely exposure
Exposure is limited to Cisco products using Snort3 with Block with Reset or Interactive Block with Reset rules configured without proper constraints. Snort2 deployments are stated as not vulnerable. Public-facing or inline inspection points carry higher business impact.
Exploitation context
The source describes unauthenticated remote exploitation by crafted IP traffic, with impact limited to availability through dropped through-traffic. The bundle does not show CISA KEV listing or other evidence of active exploitation.
Researcher notes
Key scoping facts are configuration-dependent: Snort3 is required, Snort2 is excluded, and vulnerable rules use reset-based blocking actions without proper constraints. The bundle names Cisco Firepower Threat Defense Software but the title references multiple Cisco products; use Cisco advisory for complete product scope.
Mitigation direction
- Identify Cisco deployments using Snort3 in inline traffic paths.
- Review reset-based blocking rules for proper constraints.
- Check Cisco advisory for supported fixes or workarounds.
- Prioritize remediation for critical-path and internet-facing devices.
- Confirm Snort2-only deployments are out of scope.
Validation and detection
- Inventory Cisco Firepower Threat Defense and related Snort3 deployments.
- Inspect intrusion policies for Block with Reset actions.
- Inspect policies for Interactive Block with Reset actions.
- Document rule constraints for any matching reset-based rules.
- Monitor affected devices for unexpected through-traffic drops.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
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Open ATT&CK lookupCVE-2021-40116 mapping review
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Open ATT&CK lookup- Severity
- High
- CVSS
- 8.6 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H3.94Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
8.6HighVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H
Source materials
- CVE List V5 sourceCVE List V5
- 20211027 Multiple Cisco Products Snort Rule Denial of Service VulnerabilityCVE reference · vendor-advisory, x_refsource_CISCO
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Improper Handling of Unexpected Data Type
Improper Handling of Unexpected Data Type represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.