CVE-2025-20359 Overview
CVE-2025-20359 is a buffer under-read vulnerability [CWE-127] in the Snort 3 HTTP Decoder that affects multiple Cisco products. The flaw resides in the buffer handling logic when parsing Multipurpose Internet Mail Extensions (MIME) fields within HTTP headers. An unauthenticated, remote attacker can exploit this issue by sending crafted HTTP packets through an established connection processed by Snort 3. Successful exploitation results in either a denial of service (DoS) through unexpected restart of the Snort 3 Detection Engine, or disclosure of sensitive information from the Snort 3 data stream. Cisco published the advisory on October 15, 2025.
Critical Impact
Unauthenticated remote attackers can crash the Snort 3 Detection Engine or read sensitive memory contents from adjacent buffers over the network.
Affected Products
- Multiple Cisco products using the Snort 3 Detection Engine
- Cisco security appliances leveraging Snort 3 HTTP inspection
- Refer to the Cisco Security Advisory for the definitive product list
Discovery Timeline
- 2025-10-15 - CVE-2025-20359 published to NVD
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2025-20359
Vulnerability Analysis
The vulnerability originates in the Snort 3 HTTP Decoder component responsible for parsing MIME fields within HTTP headers. When the decoder processes MIME field boundaries, incorrect buffer handling logic permits a read operation that references memory positioned before the intended buffer start. This buffer under-read condition [CWE-127] falls within the broader class of out-of-bounds read vulnerabilities.
Exploitation produces one of two outcomes. The first is a crash of the Snort 3 Detection Engine, disrupting inspection services and creating a denial-of-service condition on the affected Cisco device. The second is unintended disclosure of memory contents adjacent to the intended buffer, which may include data that is not part of the current connection stream.
The attack requires no authentication and no user interaction. The attacker only needs to send crafted HTTP traffic over an established TCP connection traversing a device that inspects with Snort 3.
Root Cause
The root cause is a boundary calculation error in the MIME header parsing routine. The decoder does not correctly validate the lower bound of the buffer pointer during field extraction, allowing reads from memory locations before the buffer.
Attack Vector
The attack vector is network-based. An attacker sends specially crafted HTTP packets with malformed MIME header fields through any established connection that Snort 3 inspects. Because Snort typically operates inline or as a passive sensor, any traffic passing through the sensor can trigger the flaw.
No verified public proof-of-concept exploit code is available. Refer to the Cisco Security Advisory for authoritative technical details.
Detection Methods for CVE-2025-20359
Indicators of Compromise
- Unexpected restarts or crash logs from the Snort 3 Detection Engine process
- Gaps in HTTP inspection telemetry corresponding to malformed MIME requests
- Anomalous inbound HTTP traffic containing malformed or truncated Content-Disposition, Content-Type, or MIME boundary headers
- Repeated HTTP requests targeting the same session with unusual MIME framing
Detection Strategies
- Monitor Snort 3 process health and restart counters on affected Cisco appliances
- Alert on HTTP traffic containing malformed MIME multipart headers or truncated boundary declarations
- Correlate detection engine restarts with recent HTTP flows to identify triggering sources
Monitoring Recommendations
- Forward Cisco device syslog and Snort operational logs to a centralized analytics platform for anomaly identification
- Track baseline HTTP inspection throughput and flag deviations that align with engine restarts
- Review network telemetry for repeated crafted HTTP requests originating from single sources
How to Mitigate CVE-2025-20359
Immediate Actions Required
- Review the Cisco Security Advisory to identify affected products and fixed software versions in your environment
- Apply Cisco-provided software updates to devices running the Snort 3 Detection Engine
- Prioritize internet-facing devices and inline inspection sensors for patching
Patch Information
Cisco has published fixed software releases through the security advisory cisco-sa-snort3-mime-vulns-tTL8PgVH. Consult the advisory for specific fixed version numbers corresponding to each affected product line. No workaround eliminates the underlying buffer under-read, so applying vendor updates is the required remediation.
Workarounds
- No official workarounds are provided by Cisco for this vulnerability
- Consider temporarily switching to Snort 2 on supported platforms if immediate patching is not feasible, subject to Cisco guidance
- Restrict untrusted HTTP traffic through affected inspection points until patches are applied
# Verify Snort 3 version on Cisco Firepower / FTD devices
show version
show snort3 status
# After upgrade, confirm the fixed release is active
show running-config | include snort
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
