CVE-2026-20067: Cisco Snort 3 Engine DoS Vulnerability

CVE-2026-20067 Overview

Multiple Cisco products are affected by a vulnerability in the Snort 3 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 parsing the Multicast DNS fields of the HTTP header. An attacker could exploit this vulnerability by sending crafted HTTP packets through an established connection to be parsed by Snort 3. A successful exploit could allow the attacker to cause a DoS condition when the Snort 3 Detection Engine unexpectedly restarts.

Critical Impact

Successful exploitation allows unauthenticated remote attackers to disrupt network security monitoring by causing repeated Snort 3 Detection Engine restarts, potentially enabling malicious traffic to pass uninspected during restart windows.

Affected Products

• Cisco products running Snort 3 Detection Engine
• Cisco Firepower Threat Defense (FTD) with Snort 3
• Cisco network security appliances utilizing Snort 3 for packet inspection

Discovery Timeline

• 2026-03-04 - CVE CVE-2026-20067 published to NVD
• 2026-03-05 - Last updated in NVD database

Technical Details for CVE-2026-20067

Vulnerability Analysis

This vulnerability stems from an out-of-bounds write condition (CWE-787) within the Snort 3 Detection Engine's HTTP header parsing functionality. The flaw specifically affects the processing of Multicast DNS (mDNS) fields embedded within HTTP headers.

The Snort 3 Detection Engine performs deep packet inspection on network traffic, including HTTP communications. When the engine encounters HTTP packets containing specially crafted mDNS field values, incomplete error checking allows malformed data to trigger memory corruption. This corruption causes the detection engine process to crash and restart.

The vulnerability is network-accessible without authentication, meaning any attacker who can route traffic through a protected network segment can potentially trigger the exploit. The impact is limited to availability—there is no evidence of data exfiltration or code execution capabilities associated with this vulnerability.

Root Cause

The root cause is incomplete error checking when parsing Multicast DNS fields within HTTP headers. The Snort 3 Detection Engine's parser does not properly validate boundary conditions before writing parsed field data to memory, resulting in an out-of-bounds write condition (CWE-787). When malformed mDNS field data exceeds expected bounds, the write operation corrupts adjacent memory regions, causing engine instability and subsequent restart.

Attack Vector

The attack is conducted remotely over the network by sending specially crafted HTTP packets through an established connection that passes through a device running the vulnerable Snort 3 Detection Engine. The attacker does not require authentication or prior access to the target system.

The attack flow involves:

1. The attacker establishes or hijacks an HTTP connection that traverses the Snort 3-protected network segment
2. Crafted HTTP packets containing malicious mDNS field values in the headers are transmitted
3. The Snort 3 Detection Engine attempts to parse these fields for inspection
4. Incomplete error checking leads to an out-of-bounds write condition
5. The detection engine crashes and restarts, creating a window where packet inspection is interrupted

For detailed technical information about exploitation, refer to the Cisco Security Advisory.

Detection Methods for CVE-2026-20067

Indicators of Compromise

• Unexpected Snort 3 Detection Engine restarts or crashes in system logs
• HTTP traffic containing anomalous or malformed mDNS field values in headers
• Gaps in packet inspection logs corresponding to engine restart periods
• Elevated crash dump generation from Snort 3 processes

Detection Strategies

• Monitor Snort 3 process stability and restart frequency using system health monitoring tools
• Implement log correlation rules to detect patterns of repeated engine restarts within short time windows
• Deploy network traffic analysis to identify HTTP packets with unusual mDNS header fields
• Configure SIEM alerts for Snort 3 crash events paired with suspicious network activity

Monitoring Recommendations

• Enable verbose logging for Snort 3 Detection Engine process events
• Set up automated alerts for any unscheduled Snort 3 service restarts
• Monitor network traffic patterns during Snort 3 restart windows for potential attack exploitation
• Track HTTP connections that precede engine crashes for forensic analysis

How to Mitigate CVE-2026-20067

Immediate Actions Required

• Review the Cisco Security Advisory for specific patch and version guidance
• Assess all Cisco products in your environment that utilize Snort 3 Detection Engine
• Prioritize patching for perimeter and critical network security devices
• Consider implementing compensating controls until patches can be applied

Patch Information

Cisco has released security updates to address this vulnerability. Administrators should consult the Cisco Security Advisory for specific fixed software versions and upgrade instructions applicable to their deployed products. Apply patches according to your organization's change management procedures, prioritizing internet-facing and critical infrastructure devices.

Workarounds

• Implement upstream filtering to detect and block HTTP traffic with malformed mDNS header fields
• Configure redundant Snort 3 detection engines where possible to maintain inspection during restarts
• Enable fail-close mode if available to prevent uninspected traffic during engine restarts
• Consider rate limiting HTTP connections from untrusted sources as a temporary measure

# Example: Monitor Snort 3 process health and log restarts
# Add to cron or systemd timer for regular monitoring
journalctl -u snort3 --since "1 hour ago" | grep -i "restart\|crash\|error"