Executive Summary

Cisco has released a security advisory addressing two medium-severity vulnerabilities in the Snort 3 detection engine, a core component of many of its security products. The vulnerabilities, CVE-2026-20026 and CVE-2026-20027, reside in the engine's handling of Distributed Computing Environment/Remote Procedure Call (DCE/RPC) traffic. An unauthenticated, remote attacker could exploit these flaws by sending crafted traffic to an affected device. CVE-2026-20026 can cause a denial-of-service (DoS) condition by crashing the inspection engine, while CVE-2026-20027 could allow an attacker to read sensitive data from memory. Cisco has released software updates and hotfixes, but patches for some affected product lines are not yet available, creating a window of risk for some organizations.

Vulnerability Details

The flaws were discovered during internal analysis of the Snort 3 DCE/RPC preprocessor.

• CVE-2026-20026 - Use-After-Free Denial of Service

  • CVSS Score: 5.8 (Medium)
  • Description: This vulnerability is a use-after-free condition in the buffer management code for DCE/RPC traffic. An attacker can trigger it by sending a high volume of specially crafted DCE/RPC packets to an affected device. Successful exploitation causes the Snort 3 process to crash and restart, leading to a temporary DoS condition where network traffic is not inspected.

• CVE-2026-20027 - Out-of-Bounds Read Information Disclosure

  • CVSS Score: 5.3 (Medium)
  • Description: This is an out-of-bounds read vulnerability. By sending crafted DCE/RPC traffic, an attacker could cause the engine to read from memory locations outside of the intended buffer. This could lead to the disclosure of sensitive information from the device's memory, which might include internal network details or fragments of other traffic being inspected.

Exploitation Status: Cisco is not aware of any malicious use of these vulnerabilities in the wild.

Affected Systems

The Snort 3 engine is integrated into a wide range of Cisco's security portfolio. Affected products include:

• Cisco Secure Firewall Threat Defense (FTD) software: Versions 7.0.0 and later where Snort 3 is the default engine.
• Cisco IOS XE Software: When the Unified Threat Defense (UTD) module is enabled.
• Cisco Meraki MX Appliances: Various models are affected.
• Open-source Snort: Version 3 of the open-source detection engine is also affected.

Cisco has stated there are no workarounds for these vulnerabilities. Patching is the only mitigation.

Impact Assessment

• Denial of Service: Exploitation of CVE-2026-20026 would cause the security inspection engine to fail. Depending on the device configuration, this could either cause traffic to be blocked entirely or, more likely, to pass through without inspection (fail-open), leaving the network temporarily unprotected.
• Information Leak: Exploitation of CVE-2026-20027 poses a risk of data leakage. While the attacker cannot control what data is read, repeated exploitation could allow them to piece together sensitive information about the network architecture or other communications.
• Operational Disruption: For organizations relying on these devices for threat prevention, a DoS condition can create significant security gaps and require intervention from security teams to restore service.

Cyber Observables for Detection

Detection Methods

• Vulnerability Scanning: Use a vulnerability scanner with updated plugins to identify affected Cisco devices and software versions in your environment.
• Log Monitoring: Actively monitor the logs of Cisco FTD, IOS XE, and Meraki devices for any entries indicating a crash or restart of the Snort or UTD processes. Correlate these events with inbound traffic patterns to identify potential sources of an attack.
• Network Traffic Analysis: As a form of D3FEND Network Traffic Analysis (D3-NTA), monitor for unusual spikes or malformed DCE/RPC traffic targeting your Cisco security appliances. While this may be difficult to distinguish from legitimate traffic, a significant anomaly could indicate an exploitation attempt.

Remediation Steps

1. Apply Patches: The primary remediation is to update to a fixed software version as recommended in the Cisco security advisory. Fixes are available for open-source Snort (v3.9.6.0) and hotfixes are available for FTD software.
2. Prioritize Patching: Prioritize patching for internet-facing devices and those protecting critical network segments. This is a key principle of D3FEND Software Update (D3-SU).
3. Monitor for Pending Patches: For products where a patch is not yet available (some IOS XE and Meraki versions), organizations should monitor Cisco's advisories closely and be prepared to deploy the updates as soon as they are released in February 2026.
4. Restrict Access (Compensating Control): While not a full workaround, if possible, implement access control lists (ACLs) to restrict DCE/RPC traffic to the affected devices from untrusted networks. This can reduce the attack surface but may impact legitimate services.