CVE-2021-42539

Executive Summary

CVE-2021-42539 is a missing permission validation vulnerability in Emerson WirelessHART Gateway firmware that allows an authenticated attacker with low-privilege access to restore a crafted system backup, resulting in account takeover and unauthorized settings changes. The vulnerability affects Emerson WirelessHART Gateway deployments running firmware versions prior to v4.7.105. Successful exploitation could allow an attacker to escalate privileges to administrative level, compromise gateway cryptographic material, and alter device configuration — directly impacting the integrity and availability of industrial wireless networks.

CVSS Score: 8.0 (High)

Technical Analysis

The vulnerability resides in the system backup restoration function of the Emerson WirelessHART Gateway web management interface. The CNA description indicates a "missing permission validation on system backup restore" — meaning the code path that processes backup file imports does not verify that the authenticated user possesses sufficient privileges to perform this administrative action.

Vulnerability Mechanism:

The WirelessHART Gateway exposes a management interface (web-based) where administrators can create and restore system backup files — typically containing device configuration, network settings, and user account credentials. The flaw allows any authenticated user — regardless of assigned role or privilege level — to invoke the backup restore operation. When a crafted backup file is uploaded:

1. The application fails to check whether the requesting session has administrative privileges
2. The restore routine processes the uploaded backup without additional authorization checks
3. The backup contents are applied to the gateway's active configuration

Attack Surface:

The attack surface is the HTTPS management interface of the WirelessHART Gateway. An attacker requires valid credentials for any account on the gateway — including low-privilege operator accounts, service accounts, or the default read-only user. The Network-facing management interface provides the primary vector, though a local attacker with console access could also exploit this if the web interface is bound to the same management port.

Vulnerability Class Details:

This falls under the CWE-862 (Missing Authorization) class — the application implements access controls for the backup creation function but fails to enforce the same authorization check on the restore function. This is a classic "functional-level access control" flaw where an administrative action (restore) is accessible to non-admin users.

Technical detail limitations: The public description does not specify the exact code path, function names, or endpoint identifiers. Emerson's advisory confirms the flaw exists in the restore routine but does not disclose implementation-level details. The root cause is plausible as an authentication/authorization check implemented in the UI layer but absent from the restore request handler.

Exploitation Scenarios

Attacker Profile: An unauthenticated attacker cannot exploit this directly (CVSS privilege requirement: Low). The threat actor is either:

• An insider with legitimate low-privilege gateway credentials
• An external attacker who previously compromised a low-privilege account via credential stuffing, phishing, or another vulnerability

Attack Chain:

1. Reconnaissance/Target Identification: An attacker identifies Emerson WirelessHART Gateways exposed to the internet or internal network via Shodan-style fingerprinting (device banner, HTTP title, TLS certificate attributes). Confirmation of the specific firmware version may be obtained from the login page or version disclosure endpoints.

2. Authentication: The attacker authenticates using compromised low-privilege credentials (operator, viewer, or read-only account). The gateway management interface requires authentication for all functions.

3. Crafted Backup Creation/Modification: The attacker prepares a malicious backup file. Since backup formats for industrial gateways typically contain:

   • User account database (usernames, password hashes, privilege levels)
   • Network configuration (IP addresses, routing, security keys)
   • WirelessHART network security material (network keys, join keys)

   The attacker crafts a backup that either:

   • Alters the admin account's password hash to a known value
   • Adds a new admin account to the user database
   • Changes gateway security settings (e.g., disabling authentication for subsequent access)

4. Restore Trigger: The attacker uploads the crafted backup through the management interface's restore function. Because permission validation is missing, the gateway processes the backup without checking that the requesting user has admin rights.

5. Post-Exploitation Impact: After the restore completes, the attacker:

   • Logs in as the newly-created or modified administrative account
   • Gains full administrative control of the WirelessHART Gateway
   • Can modify the WirelessHART network configuration, exfiltrate network keys, or cause a denial of service by reconfiguring the network

No public PoC available; attack chain is inferred from vulnerability class and standard backup-restore attack patterns. This assessment is consistent with the missing-permission-validation description provided by the CNA.

Impact Assessment

CVSS v3.x Rationale:

• Attack Vector: Network (AV:N) — The management interface is network-accessible
• Attack Complexity: Low (AC:L) — No special conditions; the flaw is straightforwardly exploitable
• Privileges Required: Low (PR:L) — Requires only any authenticated user account
• User Interaction: None (UI:N) — No victim interaction required
• Scope: Unchanged (S:U) — The compromise stays within the gateway
• Confidentiality: High (C:H) — Attacker gains access to all gateway configuration, including potentially network security keys
• Integrity: High (I:H) — Full configuration and user-database modification possible
• Availability: High (A:H) — Attacker can render the gateway non-functional or misconfigure the wireless network

Business Impact:

1. Operational disruption: Compromised gateway allows attacker to reconfigure or disable the WirelessHART network, halting process monitoring in industrial facilities
2. Data integrity loss: Sensor data integrity on the network can be undermined if the attacker reconfigures gateway routing/data handling
3. Security key compromise: WirelessHART network keys can be extracted, enabling passive network decryption or rogue device injection — a critical concern for safety-instrumented functions

Affected Systems:

• All Emerson WirelessHART Gateway models running firmware prior to v4.7.105
• Typical deployments: Oil & gas refineries, chemical plants, water treatment facilities, pharmaceutical manufacturing — any environment using WirelessHART for industrial process monitoring

Blast Radius: Containment is limited to the gateway itself (Scope: Unchanged), but the functional blast radius extends to all WirelessHART field devices connected to the compromised gateway and the industrial processes depending on that telemetry.

Detection Guidance

The following detection rules target the attack patterns associated with this CVE — namely, unusual backup file uploads to Emerson WirelessHART Gateway management interfaces, and post-exploitation administrative actions. Because the vulnerability involves file upload and subsequent configuration changes, network-level detection focuses on HTTP uploads to management endpoints.

Sigma Rule

title: Emerson WirelessHART Gateway Backup Restore Attempt
id: 7e61f19c-4b24-4ff8-aa91-6c9db883f6ef
status: experimental
description: Detects HTTP requests to Emerson WirelessHART Gateway backup restore endpoints
author: Valtersit Threat Intel
date: 2024/04/15
logsource:
  category: webserver
  product: iis
  service: http
detection:
  selection_restore:
    cs-method: 'POST'
    cs-uri-path|contains:
      - 'restore'
      - 'backup'
      - 'restoration'
  selection_gateway:
    cs-user-agent|contains:
      - 'WirelessHART'
    OR
    cs-host|contains:
      - 'hart'
      - 'gateway'
  condition: selection_restore
fields:
  - client_ip
  - cs-username
  - cs-uri-path
  - sc-status
falsepositives:
  - Legitimate administrator backup restorations
level: medium

YARA Rule

rule Emerson_WirelessHART_Backup_Restore_Traffic
{
    meta:
        author = "Valtersit Threat Intel"
        description = "Detect HTTP traffic referencing backup restore on Emerson WirelessHART Gateway"
        date = "2024-04-15"
        reference = "CVE-2021-42539"
        score = 60

    strings:
        $s1 = "restore" ascii wide nocase
        $s2 = "backup" ascii wide nocase
        $s3 = "WirelessHART" ascii wide nocase
        $s4 = "gateway" ascii wide nocase
        $s5 = "POST" ascii wide nocase
        $s6 = "multipart/form-data" ascii wide nocase

    condition:
        uint16(0) == 0x0D0A and  // CRLF at start (HTTP header start)
        any of ($s*) and
        3 of ($s1, $s2, $s3, $s4, $s6) and
        filesize < 1MB
}

Suricata IDS Rule

alert http any any -> $HOME_NET any (msg:"CVE-2021-42539 - Emerson WirelessHART Gateway Backup Restore Attempt"; flow:established,to_server; content:"POST"; http_method; content:"restore"; http_uri; content:"backup"; http_uri; content:"multipart/form-data"; http_client_body; pcre:"/(restore|restoration|backuprestore)/i"; reference:cve,2021-42539; classtype:attempted-admin; sid:2021425391; rev:1;)

Elastic Detection

sequence by source.ip with maxspan=5m
  [network where event.category == "web" and network.protocol == "http"]
  [http where request.method == "POST" and 
     (url.path contains "restore" or url.path contains "backup") and 
     user_agent.name == "Unknown"]

Splunk SPL Query

index=web sourcetype=access_combined
| where method="POST" AND (uri_path LIKE "%restore%" OR uri_path LIKE "%backup%")
| eval threat_score=case(
    match(uri_path,"(?i)restore"),     "80 (High)",
    match(uri_path,"(?i)backup"),      "60 (Medium)",
    1=1,                              "30 (Low)")
| table _time, clientip, user, uri_path, status, threat_score
| sort - _time

Wazuh OSSEC Rule

<group name="emerson_wirelesshart,">
  <rule id="100211" level="12">
    <decoded_as>json</decoded_as>
    <field name="method">POST</field>
    <field name="url" type="pcre2">(?i).*(restore|restoration).*</field>
    <description>Emerson WirelessHART Gateway backup restore detected - potential CVE-2021-42539 exploitation</description>
    <mitre>
      <id>T1548</id>
    </mitre>
    <group>attack_initial_access,attack_privilege_escalation,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence: 2/4 (CLAIMED — single source claims patch, not independently confirmed)

1. Immediate Mitigations (if patch cannot be applied)

• Restrict network access: Place management interface behind VPN or only on trusted management VLANs. Block direct internet exposure of the gateway's HTTPS port.
• Enforce least privilege: Audit existing gateway user accounts; disable default accounts; ensure only authorized personnel have credentials.
• Monitor backup restore activity: Deploy SIEM monitoring for restore operations; establish a baseline of legitimate backup restore events and alert on anomalies.
• Use network segmentation: Separate the WirelessHART Gateway management network from corporate OT networks and the broader IT infrastructure.

2. Patch/Upgrade Steps

• Vendor fix: Emerson recommends upgrading gateway firmware to v4.7.105 to address this vulnerability.
• Download: Visit the Emerson Gate Firmware site (requires a free Guardian account). Follow the updated Emerson Gateway Firmware download process as documented in the vendor's download guide.
• Verification: After upgrading, verify the firmware version is v4.7.105 or later. Restrict network access to the gateway during and immediately after the upgrade window.

3. Long-Term Hardening

• Patch management program: Establish a recurring firmware update cadence for all Emerson WirelessHART Gateways — not just for CVE-2021-42539, but for future disclosed vulnerabilities.
• Credential hygiene: Enforce strong password policies for gateway accounts; rotate passwords periodically; disallow shared service accounts.
• Encrypt backup files: If the backup format supports encryption, apply it. Crafted backups are only exploitable if an attacker can produce a valid file — encryption makes this significantly harder.
• Inventory & exposure scanning: Maintain an inventory of all WirelessHART Gateways and periodically scan for unauthorized exposure.
• Reference monitoring: Since this vulnerability allows account takeover, review gateway user accounts after any suspicious activity for new or modified admin accounts.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2021-42539

   • NVD provides the official CVE scoring (8.0 CVSS) and contains the description used in this analysis.

2. Emerson Vendor Advisory: (Raw CNA advisory) — described in the CVE intelligence data, references upgrade to v4.7.105

3. MITRE ATT&CK: https://attack.mitre.org/techniques/T1548/

   • Relevant technique: "Abuse Elevation Control Mechanism"

4. CWE-862: https://cwe.mitre.org/data/definitions/862.html

   • Missing Authorization class — the underlying weakness

5. Emerson Guardian Account Portal: Required for downloading the patched firmware. Refer to Emerson's official website for Guardian account registration and Firmware download instructions.

Campaign/Exploit Insights:

• No known exploit-in-the-wild usage has been confirmed.
• EPSS score is 0.00740 (51.3% percentile) — has not yet been observed exploited in the wild per CISA KEV (not listed).
• The vulnerability is likely to be most attractive to nation-state APT actors targeting critical infrastructure and OT networks, though no specific campaign attribution is available at this time.