CVE-2020-17407

Executive Summary

CVE-2020-17407 is a critical unauthenticated remote code execution vulnerability affecting Microhard Bullet-LTE devices prior to version 1.2.0-r1112. The flaw resides in the handling of authentication headers, where user-supplied data is copied to a fixed-length stack-based buffer without proper length validation, resulting in a classic stack buffer overflow. With a CVSS score of 9.8 and no authentication requirement, any network-reachable device is exposed to complete compromise, allowing attackers to execute arbitrary code with root privileges. The vulnerability has an EPSS score of 0.06812, placing it in the 93.4th percentile of exploited CVEs, indicating elevated real-world exploitation interest despite no CISA KEV listing.

Technical Analysis

The vulnerability exists in the authentication header parsing component of Microhard Bullet-LTE firmware. The flaw is a stack-based buffer overflow (CWE-121) triggered during the processing of authentication headers in incoming HTTP requests. The affected code path copies user-controlled data from the authentication header into a fixed-length stack buffer without first verifying that the input length does not exceed the buffer capacity.

The attack mechanism is straightforward: when the device receives an HTTP request containing an authentication header with a payload exceeding the stack buffer's allocated size, the overflow corrupts adjacent stack memory. This corruption can overwrite saved return addresses, frame pointers, or other critical stack data. Because the stack region is executable in typical embedded firmware environments (or because the attacker can leverage return-oriented programming techniques), the overflow can redirect execution flow to attacker-controlled code placed within the overflow payload.

Authentication is not required to reach the vulnerable code path, as the authentication header is parsed before any authentication check succeeds. This means the overflow is reachable by any unauthenticated network actor capable of sending HTTP requests to the device's management interface or any exposed service that processes these headers.

The lack of proper bounds checking in the length validation is the root cause. The NVD description explicitly identifies "the lack of proper validation of the length of user-supplied data prior to copying it to a fixed-length stack-based buffer" as the vulnerability mechanism. The impact is arbitrary code execution in the context of root, indicating the vulnerable process runs with the highest privilege level on the device.

Insufficient technical detail is available regarding the specific function names, buffer sizes, or exact header parameter affected. The report is therefore based on the vulnerability class and public CVE metadata. The vulnerability was disclosed via the Zero Day Initiative as ZDI-CAN-10596, indicating coordinated disclosure through an established research program.

Exploitation Scenarios

Step 1: Reconnaissance and Target Identification

An attacker first identifies exposed Microhard Bullet-LTE devices. These devices are LTE routers/gateways used in industrial, remote, and vehicular deployments. Attackers can use Shodan or similar internet-wide scanning services with queries such as microhard bullet-lte or by fingerprinting HTTP responses from the device's web management interface. Because the devices are often deployed in remote locations with static IPs or exposed management ports for operational convenience, they present attractive targets.

Step 2: Triggering the Vulnerability

Once a target is identified, the attacker crafts an HTTP request containing an oversized authentication header. The request does not need valid credentials, as the vulnerable parsing occurs before authentication. The authentication header value is embedded with a malformed, overly long payload designed to exceed the stack buffer size. The request is sent directly to the device's HTTP management interface (typically on port 80 or 443).

Step 3: Payload Execution

The oversized header is copied to the stack-based buffer without length validation, causing a stack overflow. The attacker's payload then overwrites the saved instruction pointer (return address) on the stack. The payload can be structured for direct shellcode execution or return-oriented programming (ROP) if stack execution prevention (NX) is present. Since the device is embedded firmware running Linux (typical for such LTE gateways), the attacker can craft shellcode to spawn a reverse shell, inject a backdoor binary, or modify device configuration.

Step 4: Post-Exploitation

With code execution in the root context, the attacker gains complete control of the device. This enables:

• Persistent backdoor installation (e.g., adding an SSH server or modifying existing services)
• Network traffic interception and rerouting through the LTE gateway
• Configuring the device to exfiltrate data or serve as a pivot point into the industrial network
• Disabling logging to hide forensic evidence

No public PoC is available; the attack chain is inferred from the vulnerability class and NVD description.

Impact Assessment

The CVSS score of 9.8 (Critical) reflects the severity: network-exploitable without authentication (AV:N/AC:L/PR:N/UI:N) with complete confidentiality, integrity, and availability impact (C:H/I:H/A:H). The compromise of a Bullet-LTE device has cascading consequences:

• Network Pivot: These devices are typically deployed at the edge of industrial or vehicular networks. A compromised device grants the attacker a foothold inside the trusted network boundary.
• Data Interception: LTE gateways carry sensitive operational data, SCADA communications, and potentially location/tracking information in mobile deployments. Full compromise means all traffic can be intercepted or modified.
• Availability Disruption: Attackers can brick the device or reconfigure it to disrupt operations, causing denial-of-service for all dependent systems.
• Persistence: Embedded devices often lack robust endpoint security controls and remote logging, making persistent compromise difficult to detect.

Because the devices are deployed in remote, hard-to-reach locations (vehicles, industrial sites, remote monitoring stations), physical recovery and manual patching may be logistically challenging, extending the exposure window.

Detection Guidance

For all detection rules below, adjust threshold values based on your environment's baseline traffic to minimize false positives.

Sigma Rule

title: Microhard Bullet-LTE Stack Overflow in Authentication Header
id: 3c8f4a12-abcd-4e10-9f01-2b42d1c7e9ab
status: experimental
description: Detects HTTP requests with oversized authentication headers targeting Microhard Bullet-LTE devices
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2020-17407
    - https://www.zerodayinitiative.com/advisories/ZDI-CAN-10596/
author: Valtersit Security Team
date: 2024/01/15
logsource:
    category: webserver
    product: microhard
detection:
    selection:
        http.response.body|contains: 'Bullet-LTE'
    condition: selection
falsepositives:
    - Legitimate administrative access with large authentication tokens
level: critical

YARA Rule

rule Microhard_BulletLTE_StackOverflow
{
    meta:
        author = "Valtersit Security Team"
        description = "Detects payloads targeting CVE-2020-17407 stack overflow in authentication headers"
        cve = "CVE-2020-17407"
        date = "2024-01-15"
    strings:
        $auth_hdr = "Authorization:" nocase
        $bullethdr = "Bullet-LTE" nocase
        $stack_overflow = {41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41}
    condition:
        $auth_hdr and $bullethdr and #stack_overflow > 1
}

Suricata IDS Rule

alert http any any -> any any (msg:"ET EXPLOIT Microhard Bullet-LTE Stack Overflow Attempt (CVE-2020-17407)"; flow:to_server,established; http.header_names; content:"authorization"; http.header_value; content:"AAAA"; threshold:type=both, track=by_src, count=1, seconds=60; metadata:created_at 2024_01_15, updated_at 2024_01_15, former_category EXPLOIT; reference:cve, CVE-2020-17407; reference:url, nvd.nist.gov/vuln/detail/CVE-2020-17407; sid:5000111; rev:1;)

Elastic Detection

sequence by source.ip
  [network where destination.port == 80 or destination.port == 443 and
   event.action == "connection_established"]
  [http where
   http.request.headers.authorization != null and
   length(http.request.headers.authorization) > 200 and
   http.response.status_code == 200]

Splunk SPL Query

sourcetype="stream:http" OR sourcetype="pan:traffic" OR sourcetype="cisco:asa"
| where like(http_request_headers, "%authorization%")
| eval auth_len = len(replace(http_request_headers, ".*[Aa]uthorization[:\\s]+([^\\r\\n]+).*", "\\1"))
| where auth_len > 200
| table _time, src_ip, dest_ip, dest_port, uri, http_method, auth_len
| sort - _time

Wazuh OSSEC Rule

<group name="microhard_exploit,">
  <rule id="100511" level="12">
    <if_sid>30600</if_sid>
    <field name="url">\.*Bullet-LTE.*</field>
    <description>Potential CVE-2020-17407 stack overflow attempt against Microhard Bullet-LTE</description>
    <group>attack_exploitation,</group>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence: 0/4 — UNPATCHED. No patch evidence from any tracked source. The vendor has not publicly released a fixed version, and the only known reference to remediation is the version number v1.2.0-r1112 being mentioned as "prior to" in the advisory — implying this version may include a fix, but no official patch advisory has been confirmed.

Immediate Mitigations (until a patch is verified):

1. Network Segmentation: Isolate Bullet-LTE devices on a dedicated management VLAN. Do not expose the management interface to the internet or untrusted networks.
2. Access Control: Implement strict firewall rules that restrict HTTP access to the device to only trusted administrative IP addresses.
3. Disable Remote Management: If remote management is not operationally required, disable the HTTP/HTTPS management interface entirely.
4. VPN Enforcement: Require VPN access for any administrative interface access, ensuring the authentication header parsing cannot be reached by unauthenticated network paths.
5. Monitoring: Deploy the detection rules above at network perimeters to alert on oversized authentication headers.

Patch/Upgrade Steps:

1. Contact Microhard support or check vendor portals weekly for a firmware release containing a fix. The version v1.2.0-r1112 may include remediation — verify with the vendor before considering the device patched.
2. Before applying any firmware update, ensure device configuration is backed up, and test the update in a lab environment if possible.

Long-Term Hardening:

1. Firmware Update Program: Establish a routine schedule for checking and applying firmware updates from Microhard.
2. Network Architecture: If these devices are deployed in vehicles or remote sites, implement a zero-trust network architecture where devices are not directly reachable from the internet.
3. Physical Security: Ensure devices are physically secured to prevent direct serial or console access that could bypass network controls.
4. Logging and Telemetry: Configure syslog forwarding to a central SIEM where device logs are retained for at least 90 days for forensic analysis.
5. Inventory Management: Maintain an accurate inventory of all Bullet-LTE deployments, including firmware versions, to track which units remain vulnerable.
6. Alternative Device Evaluation: For new deployments, consider whether alternative LTE gateway products with better security track records should be evaluated.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2020-17407
2. Zero Day Initiative Advisory: https://www.zerodayinitiative.com/advisories/ZDI-CAN-10596
3. Microhard Product Page (for firmware updates): https://www.microhardcorp.com/Bullet-LTE.php
4. CISA Known Exploited Vulnerabilities Catalog (CVE-2020-17407 not listed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No evidence of active exploitation in the wild or threat actor campaigns targeting this vulnerability has been observed in public reporting as of the writing of this report. However, the high EPSS percentile (93.4%) suggests the vulnerability is of significant interest to exploit developers and potential attackers, and the lack of a confirmed patch makes this a high-priority risk to address.