CVE-2021-38485

Executive Summary

CVE-2021-38485 is an improper input validation vulnerability in Emerson WirelessHART Gateway devices. An authenticated attacker can exploit the restore-file functionality to supply malicious configuration files, enabling arbitrary file replacement on the device's disk. Successful exploitation can lead to complete system compromise, persistent backdoor installation, or denial of service. The vulnerability is patched in firmware version 4.7.105; however, patch confirmation is based on a single source (the CNA), and no public proof-of-concept has been disclosed.

Technical Analysis

The vulnerability resides in the restore-file processing mechanism of Emerson WirelessHART Gateway devices. The affected product fails to perform adequate input validation on configuration files processed during the restore operation. This is a CWE-20 (Improper Input Validation) flaw.

At the vulnerability-class level, the mechanism is as follows:

1. The gateway exposes a restore/configuration-import function, which accepts a configuration bundle — typically a ZIP or similar archive containing configuration data for the device.
2. The device processes the archive's contents without properly validating the paths, filenames, or file types embedded within it.
3. Because path traversal or symlink-style entries are not filtered (or otherwise blocked), an attacker can craft a malicious archive that contains entries pointing to arbitrary filesystem locations on the device (e.g., /etc/passwd, /bin/, /var/, etc.).
4. During the restore extraction process, the device blindly writes these archive entries to disk, overwriting or replacing system files with attacker-controlled content.

The NVD description states: "The affected product is vulnerable to improper input validation in the restore file. This enables an attacker to provide malicious config files to replace any file on disk."

The attack surface is the management/configuration interface of the gateway — a network-accessible service (typically HTTP/HTTPS or a proprietary management protocol). Since the vulnerability requires authentication (the CVSS vector implies a privileged user), the attacker needs valid credentials for the gateway's web interface or management console.

The specific file-format implementation details (zip library used, extraction routine, archive structure) are not disclosed in the NVD description. However, the attack class is well understood: the product fails to sanitize archive entry paths, allowing path traversal (../) or absolute-path entries. This is a common flaw in embedded systems that implement custom restore routines without leveraging secure extraction libraries.

Affected product lines are Emerson WirelessHART Gateways — industrial IoT devices used to bridge WirelessHART field devices (sensors, actuators) into a plant's control network (historically via Modbus, OPC, or similar protocols). These gateways run a restricted embedded Linux environment and are deployed in critical industrial control system (ICS) environments — oil & gas, chemical, food & beverage, water treatment, and discrete manufacturing.

Exploitation Scenarios

Note: No public PoC is available for this CVE. The attack chain is inferred from the vulnerability class (improper input validation in restore operations leading to arbitrary file write) and the NVD description.

Step 1 — Reconnaissance and Target Identification:

An attacker begins by scanning the industrial network for Emerson WirelessHART Gateways. These devices respond to specific application-layer fingerprints — the web management interface typically presents an Emerson-branded login page. Shodan/Censys queries using gateway identification strings (as in the Metadata section) can identify internet-exposed instances. More commonly, the attacker gains initial foothold in the OT network via phishing, VPN compromise, or pivoting from the IT segment, then performs internal network scanning (e.g., using Nmap with service detection) to locate gateway IPs. The attacker then attempts default or weak credentials (a common issue in OT environments) to authenticate to the management interface.

Step 2 — Triggering the Flaw:

The attacker navigates to the gateway's configuration restore/backup-import page. The interface typically prompts for a configuration archive file. The attacker prepares a malicious archive (e.g., a .tar, .zip, or vendor-specific container) that contains:

• Legitimate-looking configuration entries (to pass basic format validation).
• One or more archive entries with path traversal sequences (../../../../etc/passwd) or absolute paths (/usr/sbin/...).

The attacker uploads this archive via the web interface (HTTP POST multipart form) or through a management API. The gateway's restore routine extracts the archive without validating entry paths.

Step 3 — Payload Execution at the Vulnerability-Class Level:

Once extracted, the malicious entries overwrite target files. Depending on the attacker's objective, they may:

• Overwrite /etc/shadow or /etc/passwd to change the administrator password or add a new user with shell access.
• Overwrite /usr/bin/ or /usr/sbin/ binaries (e.g., replacing sshd or a scheduler daemon) with a backdoored variant.
• Overwrite web-root files (e.g., /www/index.html) to deploy a webshell.
• Overwrite /etc/init.d/ scripts to achieve persistence across reboots.
• Overwrite critical system libraries to crash the device (denial of service).

The arbitrary file write is dropped to disk as the root (or equivalent privileged) user, since the restore process runs with elevated privileges.

Step 4 — Post-Exploitation Impact:

The attacker now has controlled content on the device's filesystem. Immediate consequences include:

• Remote Code Execution: Replacing a system-level binary or adding a cron entry executes attacker commands with full device privileges.
• Persistent Backdoor: A modified startup script re-esablishes the attacker's presence after every reboot.
• Data Exfiltration: Malicious software can parse and exfiltrate process data collected by the gateway (e.g., tank-level, temperature, pressure readings from WirelessHART field devices).
• Denial of Service: Replacing/core-dumping critical files causes the gateway to fail, disrupting data flow between field devices and the control system.
• Lateral Movement: The compromised gateway becomes a pivot point into the plant's control network (e.g., reaching PLCs, DCS, or historians).

Impact Assessment

CVSS Score: 8.0 (High)

• Confidentiality Impact (High): Attacker can overwrite files to extract sensitive configuration, operational data, and process variables; may also gain access to credential stores.
• Integrity Impact (High): Arbitrary file replacement allows complete control over the device's software state — the attacker can modify system binaries, configuration, or data.
• Availability Impact (High): File replacement can crash the device or render it inoperable; a failed restore can also brick the gateway.
• Attack Vector (Network): Exploitable remotely over the network.
• Attack Complexity (Low): The restore routine is a standard feature — no exotic condition is required.
• Privileges Required (Low/Medium): Requires authentication, but default/weak credentials are common in ICS deployments.
• User Interaction (Required/Not Required): Dependent on exact interface — but the attack can be fully automated once authenticated.

Business Impact:

The WirelessHART Gateway is a critical communication bridge in industrial deployments. It aggregates data from WirelessHART field transmitters (which often measure safety-relevant parameters) and forwards them to the control system. Compromise of this device can:

• Corrupt or fabricate process variable readings, potentially causing operators to make incorrect safety decisions.
• Disrupt real-time monitoring, affecting production throughput or quality.
• Provide attackers with a network pivot from the OT network into the IT network (or vice versa).
• Violate regulatory compliance (NERC CIP, IEC 62443, GDPR or local data protection laws) if the device stores personal or security-sensitive data.

Blast Radius:

• Direct: Compromised gateway device. Full system control achieved.
• Downstream: Field devices (transmitters) — their data can be intercepted/modified. Control system (DCS/SCADA) — the gateway is a conduit; manipulated data impacts operator decisions. Plant network — the gateway acts as a bridge between OT and possibly corporate networks; lateral movement opportunity.
• Upstream: Historians and enterprise systems receiving data from the gateway can be fed malicious data.

Detection Guidance

Note on Detection Paradigm: This is a file-write vulnerability via an authenticated web-based restore function. Detection focuses on: (a) anomalous archive uploads, (b) unusual file modification patterns on the gateway filesystem, (c) suspicious process execution post-upload, and (d) network-level signatures of malicious archive content.

Sigma Rule

title: Emerson WirelessHART Gateway - Suspicious Configuration Restore Upload
id: 8f5e464b-8b0a-4b63-8d60-9a98a9e13c21
status: experimental
description: Detects attempts to upload configuration files to an Emerson WirelessHART Gateway that may trigger CVE-2021-38485 (improper input validation in restore file allowing arbitrary file replacement).
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2021-38485
author: Valtersit Security
date: 2024/01/15
tags:
    - attack.impact
    - attack.t1213
    - cve.2021-38485
logsource:
    category: webserver
    product: emerson_gateway
detection:
    selection:
        http.method: 'POST'
        http.uri|contains:
            - '/restore'
            - '/config'
            - '/import'
    filter_normal:
        http.file_ext:
            - '.bin'
            - '.bak'
            - '.tar.gz'
    condition: selection and not filter_normal
level: high
falsepositives:
    - Legitimate configuration restore by authorized administrators
    - Scheduled firmware/configuration backups

YARA Rule

rule CVE_2021_38485_WirelessHART_Gateway_Restore_File_Check
{
    meta:
        author = "Valtersit Security"
        description = "Detects suspicious archive files that may be used to exploit CVE-2021-38485 by path traversal in configuration restore of Emerson WirelessHART Gateway"
        date = "2024-01-15"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2021-38485"
        cve = "CVE-2021-38485"
        severity = "high"

    strings:
        $path_traversal = /(\.\.[\/\\]|\.\.\/)/ 
        $zip_magic = { 50 4B 03 04 }          // ZIP file magic
        $tar_magic = { 75 73 74 61 72 }       // tar file identifier
        $system_path = /etc\/(passwd|shadow|init\.d|crond)/
        $root_write = { 72 6F 6F 74 }

    condition:
        ($zip_magic or $tar_magic) and $path_traversal and ($system_path or $root_write)
}

Suricata IDS Rule

alert http any any -> $EMERSON_GATEWAY any (msg:"CVE-2021-38485 Emerson WirelessHART Gateway - Suspicious Restore File Upload Attempt"; flow:to_server; http.method:POST; http.uri; content:"restore"; nocase; http.request_body; content:"../../../"; http.client_body; pcre:"/\.\.\//"; classtype:attempted-admin; sid:10038485; rev:1;)

Elastic Detection

sequence by host.id with maxspan=5m
  [ network where process.name == "curl" or process.name == "wget" and destination.port == 80 or destination.port == 443 ]
  [ file where event.action == "created" and file.extension == "zip" or file.extension == "tar" and file.path : "*restore*" ]
  [ process where event.action == "executed" and process.name == "unzip" or process.name == "tar" and process.args : "*../../*" ]

Splunk SPL Query

index=emerson_gateway OR index=web_access
  (sourcetype=access_combined OR sourcetype=gateway_web)
  method=POST
  AND (uri=*/restore* OR uri=*/config* OR uri=*/import*)
  AND (
    // Look for path traversal in uploaded file structure
    form_data=*../../*
    OR form_data=*../../../etc/*
    OR form_data=*C:\*
    // Or unusually large POST body (could be malicious payload)
    OR bytes > 10000000
  )
  | stats count by src_ip, uri, status, host
  | sort -count

Wazuh OSSEC Rule

<group name="emerson,attacks,">
    <rule id="877064" level="12">
        <decoded_as>web</decoded_as>
        <match>restore</match>
        <match>POST</match>
        <regex>(\S+\s+())</regex>
        <description>CVE-2021-38485 - Unauthorized configuration restore upload to Emerson WirelessHART Gateway</description>
        <group>emerson,gateway,cve_2021_38485,attack.web</group>
        <mitre>
            <id>T1499</id>
        </mitre>
    </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence: 2/4 — CLAIMED — a single source (the CNA) claims a patch exists (v4.7.105). This is not independently confirmed; treat the patch as validated only after internal testing.

1) Immediate Mitigations

• Disable or restrict remote restore functionality: If the restore-import interface is not needed for normal operations, disable it or restrict access by source IP (via firewall or gateway-level access control lists).
• Network segmentation: Place WirelessHART Gateways in a dedicated OT VLAN with strict egress/ingress ACLs. Do not allow direct internet access to the management interface.
• Enable strong authentication: Ensure the gateway does not use default credentials. Enforce complex, unique passwords for all gateway accounts. Implement multi-factor authentication if the device supports it.
• Restrict physical access: Limit physical console access to the gateway; a malicious local actor could also attempt to trigger the restore function.
• Monitor logs: Enable and regularly review gateway audit logs, focusing on configuration changes, restore operations, and file modification timestamps.

2) Patch/Upgrade Steps

• Verify the current firmware version of the affected gateway by logging into the management interface.
• If running a version older than 4.7.105, download the updated firmware from the Emerson Gate Firmware site (requires a free Guardian account). Link available in the vendor advisory.
• Follow Emerson's firmware upgrade procedure: download the firmware bundle, upload to the device, and allow sufficient time for the upgrade to complete without interrupting the device's power or network connection.
• After upgrade, verify the running version reflects 4.7.105 or later.
• Test the restore-import function post-upgrade to confirm the fix blocks malicious archive paths.

3) Long-Term Hardening

• Implement IEC 62443 / ISA-99 zones and conduits to isolate OT devices from IT and external networks.
• Deploy an industrial intrusion detection system (IIDS) capable of parsing OT-specific protocols to monitor traffic to/from the gateway.
• Enforce a formal change-management process for all configuration changes on the gateway.
• Segment WirelessHART field device traffic from management traffic; use a VLAN dedicated to management (e.g., VLAN 5).
• Replace default HTTP(S) management certificates with enterprise-issued ones to prevent MITM attacks during configuration exchange.
• Maintain an up-to-date inventory of all Emerson WirelessHART Gateways, including firmware versions, and subscribe to Emerson security advisories.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2021-38485
2. CISA ICS-CERT Advisory (if applicable): — see ICS-CERT database for CVE-2021-38485 details.
3. Emerson Vendor Advisory: — as referenced in the CNA patch metadata; consult the Emerson Guardian portal for firmware v4.7.105 release notes.
4. CISA Known Exploited Vulnerabilities Catalog: — CVE-2021-38485 is NOT listed in CISA KEV (as of the data provided).
5. Exploit/Threat-Actor Evidence: — No known threat actor campaigns or in-the-wild exploitation have been publicly documented for this CVE. The EPSS score of 0.00870 (55.5% percentile) suggests a low-to-moderate likelihood of exploitation in the wild, but no confirmed incidents exist.

Report Prepared By: Valtersit Threat Intelligence Date: 2024-01-15 Classification: TLP:WHITE — Distribution unlimited