External risk intelligence

Cisco Snort 3 MIME Parsing Vulnerability Allows Data Disclosure or Crash

CVE advisorySeverity: CRITICAL (CVSS 9.1)

CVE-2025-20359

Snort is a network intrusion detection and prevention system typically deployed at the edge of networks to inspect incoming internet traffic. Because it is designed to process external traffic directly as a primary security gateway, it is commonly exposed to untrusted network streams in standard deployments.

Information Disclosure

Cisco Snort

3.0.0-233 to before 3.9.3.0

Halo Surface Signal: 4 out of 5 — likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

This vulnerability impacts Cisco Snort 3, a network intrusion detection and prevention system. It could allow an attacker to crash the system or expose sensitive data by sending specially crafted network traffic. The main concern is confirming if these products are in use and if they are exposed to potential threats.

Network security tool can leak data or crash.
Affects critical network defense systems.
Assess exposure and relevance to operations.

Attack Path

How an attacker could exploit the issue

An attacker can exploit this vulnerability by sending specially crafted HTTP packets over a network connection that is being analyzed by Snort 3. The vulnerability resides in how Snort 3 handles MIME fields within HTTP headers, leading to an error in buffer handling. This error, specifically a buffer under-read, can cause the Snort 3 Detection Engine to crash, resulting in a denial-of-service condition, or disclose sensitive information that might be present in the data stream.

Unauthenticated remote access required.
Crafted HTTP packets trigger vulnerability.
Denial of service or data disclosure risk.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability could expose sensitive data within the Snort 3 data stream or cause the detection engine to restart unexpectedly, leading to a denial of service. These outcomes are possible when an attacker sends crafted HTTP packets to a connection being parsed by Snort 3.

Sensitive data in Snort 3 stream.
Crafted HTTP packets sent to connection.
Potential denial of service or data exposure.

Operational Fix

Recommended remediation, mitigation, and detection steps

Teams responsible for network security appliances and intrusion detection systems, likely the Network Security or Security Operations teams, should prioritize investigating this vulnerability. The first practical move is to identify all deployed instances of Snort 3, confirm their exposure to external network traffic, and assess their criticality to business operations. This initial triage will inform the ownership of remediation and the subsequent risk-based action plan.

Network or security teams own the issue.
Verify Snort 3 instances and external reachability.
Plan remediation based on exposure and impact.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is Cisco Snort 3?

Snort 3 is an open-source network intrusion detection and prevention system. It functions as a security sensor that inspects network traffic in real-time, allowing administrators to monitor, analyze, and block malicious activity across their network infrastructure.

What does CWE-127 mean for CVE-2025-20359?

CWE-127 refers to a buffer under-read vulnerability. In this case, the Snort 3 engine makes a mistake when processing MIME fields in HTTP headers. Instead of reading the intended data, it accesses memory before the start of the intended buffer, which can reveal unintended sensitive information or cause the software to crash.

How is this vulnerability triggered?

An unauthenticated attacker triggers this by sending specially crafted HTTP packets to a connection being monitored by Snort 3. The vulnerability is specific to the parsing of HTTP header MIME fields; it is not triggered by standard, non-malicious network traffic.

Why does Halo Surface Signal categorize this as external?

Halo Surface Signal labels this as external because Snort 3 is frequently deployed at network edges to inspect incoming internet traffic. Since it sits as a primary gateway processing untrusted streams, instances are often reachable by attackers outside the internal network.

What should I do if I run Snort 3?

Begin by creating an inventory of all Snort 3 instances within your environment. Verify which of these are exposed to external, untrusted traffic. Once you have identified these assets, assess their criticality to your security posture to prioritize your next steps for mitigation.

References

sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-s…

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.