CVE-2025-20360: Snort 3 HTTP Decoder DoS Vulnerability

CVE-2025-20360 Overview

CVE-2025-20360 affects the Snort 3 HTTP Decoder embedded in multiple Cisco products. The flaw stems from incomplete error checking during parsing of Multipurpose Internet Mail Extensions (MIME) fields in HTTP headers. An unauthenticated, remote attacker can send crafted HTTP packets over an established connection to trigger the Snort 3 Detection Engine to restart. The result is a denial-of-service (DoS) condition that disrupts inspection while the engine recovers. The weakness is classified as [CWE-805] Buffer Access with Incorrect Length Value. Cisco published the advisory on October 15, 2025.

Critical Impact
Remote attackers can force the Snort 3 Detection Engine to restart without authentication, temporarily suspending traffic inspection on affected Cisco security devices.

Affected Products

Cisco products running vulnerable versions of the Snort 3 Detection Engine
Cisco security appliances that use Snort 3 for HTTP inspection
Refer to the Cisco Security Advisory for the complete list of affected platforms and fixed versions

Discovery Timeline

2025-10-15 - CVE-2025-20360 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-20360

Vulnerability Analysis

The vulnerability resides in the HTTP Decoder component of the Snort 3 Detection Engine. When Snort 3 processes HTTP traffic containing MIME headers, the decoder fails to perform complete error checking on parsed MIME fields. Malformed or unexpected MIME field values cause the decoder to access memory outside intended boundaries, which triggers an abnormal termination of the detection engine. When the engine restarts, in-flight inspection is interrupted, and traffic policy enforcement is degraded until the process resumes normal operation.

Cisco categorizes this as an availability issue with no impact to confidentiality or integrity. Because the flaw sits in inline traffic inspection logic, an attacker who can reach an existing HTTP flow through the device can repeatedly trigger the restart condition, sustaining a DoS state.

Root Cause

The root cause is incomplete input validation during MIME header parsing, aligning with [CWE-805] Buffer Access with Incorrect Length Value. The decoder trusts length or delimiter values derived from attacker-controlled MIME fields without enforcing complete bounds checks. Specially crafted values push the parser into an unsafe state, causing the Snort 3 process to abort.

Attack Vector

Exploitation requires network access to a device inspecting HTTP traffic with Snort 3. The attacker does not need authentication or user interaction. The attacker sends crafted HTTP packets containing malformed MIME headers through an established connection that traverses the inspection path. Successful exploitation restarts the Snort 3 Detection Engine and produces a DoS condition. The Exploit Prediction Scoring System (EPSS) currently rates this issue at 0.359%.

No verified public proof-of-concept code is available. See the Cisco Security Advisory for vendor technical details.

Detection Methods for CVE-2025-20360

Indicators of Compromise

Unexpected restarts of the Snort 3 Detection Engine process on affected Cisco devices
Gaps in HTTP inspection logs corresponding to engine restart events
Repeated crash entries referencing the HTTP Decoder or MIME parsing routines in device diagnostic output
Bursts of malformed HTTP requests containing anomalous MIME headers arriving from a single source

Detection Strategies

Monitor Cisco device syslog for Snort 3 engine restart messages and correlate with inbound HTTP flows
Alert on traffic patterns showing repeated HTTP POST requests with malformed or oversized Content-Type and MIME boundary fields
Baseline normal Snort 3 process uptime and flag deviations that align with external HTTP activity

Monitoring Recommendations

Enable detailed logging for the Snort 3 Detection Engine and forward events to a centralized analytics platform
Track HTTP inspection availability metrics to detect coverage gaps caused by engine restarts
Correlate Snort 3 restart events with source IP telemetry to identify attacker infrastructure

How to Mitigate CVE-2025-20360

Immediate Actions Required

Review the Cisco Security Advisory to confirm exposure across all Cisco security appliances in your environment
Apply the fixed software releases published by Cisco to all affected devices
Restrict untrusted HTTP traffic reaching devices running Snort 3 until patches are deployed
Enable device monitoring to detect and respond to repeated Snort 3 restarts

Patch Information

Cisco has published fixed software versions in the vendor advisory cisco-sa-snort3-mime-vulns-tTL8PgVH. Administrators should upgrade to a release that includes the corrected Snort 3 HTTP Decoder with complete MIME field error checking. Consult the advisory for release-specific upgrade paths.

Workarounds

Cisco has not published a documented workaround; upgrading to a fixed release is the recommended remediation
Where immediate patching is not possible, use upstream access controls to limit exposure of the inspection path to untrusted networks
Consider switching HTTP inspection to a fallback engine or policy that does not rely on the vulnerable Snort 3 MIME parser, if supported by the platform

bash

# Verify current Snort 3 version on a Cisco device before and after upgrade
show snort3 version
show version | include Snort