CVE-2026-20057: Cisco Snort 3 VBA DoS Vulnerability

CVE-2026-20057 Overview

Multiple Cisco products are affected by a vulnerability in the Snort 3 Visual Basic for Applications (VBA) feature which could allow an unauthenticated, remote attacker to cause the Snort 3 Detection Engine to crash. This vulnerability is due to a lack of proper error checking when decompressing VBA data. An attacker could exploit this vulnerability by sending crafted VBA data to the Snort 3 Detection Engine on the targeted device. A successful exploit could allow the attacker to cause the Snort 3 Detection Engine to unexpectedly restart, resulting in a denial of service (DoS) condition.

Critical Impact

Successful exploitation allows unauthenticated remote attackers to crash the Snort 3 Detection Engine, potentially disrupting network security monitoring and intrusion detection capabilities across affected Cisco security appliances.

Affected Products

• Cisco Products with Snort 3 Detection Engine
• Cisco Firepower Threat Defense (FTD)
• Cisco Security Appliances utilizing Snort 3 VBA inspection

Discovery Timeline

• 2026-03-04 - CVE-2026-20057 published to NVD
• 2026-03-05 - Last updated in NVD database

Technical Details for CVE-2026-20057

Vulnerability Analysis

This vulnerability (CWE-369: Divide By Zero) exists within the Snort 3 Detection Engine's VBA decompression functionality. The core issue stems from inadequate error handling during the decompression process of VBA data streams. When the Snort 3 engine processes malformed or specially crafted VBA content, the lack of proper validation and error checking can trigger a division by zero condition, causing the detection engine to crash.

The attack can be initiated remotely without authentication, meaning any attacker with network access to the affected device can potentially trigger the vulnerability. The scope is changed (affects resources beyond the vulnerable component), as crashing the Snort 3 engine impacts the overall security posture of the network by disabling intrusion detection capabilities.

Root Cause

The root cause of CVE-2026-20057 is improper error handling (CWE-369) in the VBA decompression module of Snort 3. Specifically, the vulnerability arises when the engine fails to validate certain parameters or edge cases during VBA data decompression operations. This can result in a divide-by-zero condition when processing malicious input, leading to an unhandled exception that crashes the detection engine.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Crafting malicious VBA data designed to trigger the decompression flaw
2. Sending the crafted payload through network traffic that will be inspected by the Snort 3 Detection Engine
3. When the Snort 3 engine attempts to decompress and analyze the VBA data, the lack of proper error checking causes a crash
4. The detection engine restarts, creating a temporary gap in network security monitoring

The vulnerability allows attackers to repeatedly trigger the DoS condition, potentially maintaining persistent disruption of security monitoring capabilities. Since the attack requires no privileges and can be executed remotely, it presents a significant risk to organizations relying on Cisco security products with Snort 3 for network protection.

Detection Methods for CVE-2026-20057

Indicators of Compromise

• Unexpected Snort 3 Detection Engine crashes or restarts
• Gaps in network security monitoring logs corresponding to engine restart events
• Increased volume of network traffic containing VBA content from suspicious sources
• System logs indicating divide-by-zero errors or unhandled exceptions in Snort 3 processes

Detection Strategies

• Monitor system logs for repeated Snort 3 Detection Engine crashes or restart events
• Implement network traffic analysis to identify anomalous VBA content in network streams
• Configure alerting for Snort 3 service availability degradation or unexpected downtime
• Review Cisco security appliance health metrics for patterns indicating exploitation attempts

Monitoring Recommendations

• Enable detailed logging for Snort 3 Detection Engine health and status events
• Set up automated alerts for detection engine restarts exceeding normal operational thresholds
• Monitor network traffic patterns for unusual volumes of VBA-containing content
• Implement high-availability configurations to detect single-node failures indicative of exploitation

How to Mitigate CVE-2026-20057

Immediate Actions Required

• Review the Cisco Security Advisory for specific affected product versions and remediation guidance
• Apply vendor-provided patches or software updates as soon as they become available
• Consider temporarily disabling VBA inspection if operationally feasible until patches are applied
• Implement network segmentation to limit exposure of affected security appliances

Patch Information

Cisco has released a security advisory addressing this vulnerability. Administrators should consult the Cisco Security Advisory for detailed information on affected software versions and available patches. Organizations should prioritize updating their Cisco security appliances running Snort 3 to the latest patched versions recommended by Cisco.

Workarounds

• If VBA inspection is not business-critical, consider disabling the Snort 3 VBA inspection feature as a temporary mitigation
• Deploy additional network monitoring solutions to maintain visibility during potential exploitation attempts
• Implement rate limiting or traffic filtering for content likely to contain VBA data from untrusted sources
• Configure high-availability failover to minimize impact of detection engine crashes

# Check Snort 3 Detection Engine status (example command)
show snort status
# Review system logs for crash events
show logging | include snort