CVE-2021-42538

Executive Summary

CVE-2021-42538 is a parameter injection vulnerability affecting the Emerson WirelessHART Gateway, a critical component used in industrial wireless mesh networks for process automation. The flaw resides in the gateway's handling of the passphrase parameter, allowing an authenticated attacker to supply uncontrolled input that can modify command behavior or inject unintended parameters. With a CVSS score of 8.0, successful exploitation could result in unauthorized configuration changes, denial of service, or potential command execution within the gateway's context. The patch is claimed by the CNA but not independently confirmed (confidence 2/4), and Emerson recommends upgrading to firmware v4.7.105.

Technical Analysis

The vulnerability is classified as a parameter injection flaw, specifically triggered via the passphrase input field on the Emerson WirelessHART Gateway. Parameter injection occurs when an application constructs internal commands, filesystem paths, or configuration strings by concatenating user-supplied input without adequate sanitization or validation. In this case, an attacker who can reach the gateway's management interface can supply specially crafted passphrase values that break out of the intended parameter context and inject additional parameters or commands that the application processes with elevated trust.

The attack surface is the gateway's administrative interface, which is typically exposed on the network for configuration and monitoring purposes. WirelessHART gateways are deployed at the edge of industrial wireless mesh networks, bridging HART field devices to host control systems. The passphrase parameter is commonly used to authenticate access to the gateway's wireless network configuration, making it a natural target for injection because it is frequently passed to underlying system utilities (e.g., shell commands, key generation tools, or cryptographic functions) during device provisioning or reconfiguration.

Specific technical implementation details are not disclosed in the NVD description, and no CWE classification was assigned. Based on the vulnerability class, the core mechanism involves the gateway's failure to properly quote or escape the passphrase value before embedding it into a command string or configuration parameter. A malicious input such as a semicolon, newline, or whitespace-delimited string could terminate the current argument and append attacker-controlled content that the underlying parser interprets as new parameters or operations.

The vulnerability is remotely exploitable over the network (AV:N), requires low attack complexity (AC:L), and requires only low-privilege authentication (PR:L) — likely standard access to the gateway's web-based management interface. Exploitation yields high impact on confidentiality, integrity, and availability (C:H/I:H/A:H), and the scope is unchanged (S:U), meaning the compromise is confined to the gateway device itself. Note that the CVSS vector was not provided in the source data; the vector presented here is an inferred representation consistent with the CVSS score of 8.0.

The EPSS score of 0.00917 (56.9th percentile) indicates low-to-moderate exploitation likelihood in the wild, and the vulnerability is not listed on CISA's Known Exploited Vulnerabilities catalog. No public PoC, Nuclei template, or exploit code is currently available.

Exploitation Scenarios

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

Step 1 — Reconnaissance / Target Identification:
An attacker scans the network for Emerson WirelessHART Gateways using Shodan or similar services, searching for exposed web management interfaces on common industrial gateway ports. The gateway may respond with distinctive HTTP banners, product names, or TLS certificates identifying it as an Emerson WirelessHART device. The attacker also determines whether the gateway is running firmware versions prior to v4.7.105, which are suspected to be vulnerable.

Step 2 — Triggering the Injection:
The attacker authenticates to the gateway's management interface using default credentials (if not changed), acquired credentials, or a low-privilege account. They navigate to the wireless network configuration section where the passphrase parameter is set. The attacker supplies a crafted passphrase value containing injection delimiters (e.g., ;, \n, &, or spaces) followed by additional parameters or command fragments. Because the application fails to validate or sanitize this input, the underlying command or configuration parser interprets the injected content as new parameters rather than as part of the passphrase string.

Step 3 — Payload / Execution Method:
At the vulnerability-class level, the injected parameters may be used to:

• Override configuration file paths to point to attacker-controlled locations.
• Add new command-line arguments that trigger unintended functionality (e.g., verbose logging, debug mode, or data exfiltration).
• Reference untrusted files or executable utilities accessible within the gateway's environment.
• Modify the gateway’s wireless network settings to disable security or redirect traffic.

The exact payload format depends on the underlying system that processes the passphrase. Without further technical detail, it is not possible to specify the precise injection syntax or the exact commands that can be executed.

Step 4 — Post-Exploitation Impact:
Successful exploitation could allow the attacker to:

• Modify the gateway's wireless configuration, potentially altering the HART mesh network topology, disabling encryption, or adding rogue devices.
• Cause a denial of service by supplying malformed parameters that crash the configuration process or gateway firmware.
• Read sensitive configuration data including network keys, device credentials, or operational parameters.
• Potentially achieve code execution if the injected parameters enable loading of arbitrary libraries or execution of system commands, leading to full gateway compromise and lateral movement within the industrial control network.

Impact Assessment

Business Impact: The Emerson WirelessHART Gateway is a foundational component of WirelessHART networks used in process industries (oil and gas, chemicals, pharmaceuticals, water/wastewater). Compromise of this gateway can disrupt real-time process monitoring and control, leading to operational downtime, safety incidents, regulatory non-compliance, and significant financial losses. The High/High/High CVSS subscores for confidentiality, integrity, and availability underscore that all three security objectives are at risk.

Affected Systems: Emerson WirelessHART Gateways running firmware versions prior to v4.7.105. The exact version range is not specified in the NVD data beyond the fixed version. Organizations with WirelessHART deployments — particularly those exposing gateway management interfaces beyond the OT network perimeter — are at elevated risk.

Blast Radius: While the CVSS scope metric (S:U) indicates the compromise is confined to the gateway itself, the operational blast radius in an industrial context can be substantial. A gateway compromise can interrupt communication to potentially hundreds of wireless field devices, blind the host control system, or serve as an entry point into the broader OT network if segmentation is insufficient.

CVSS Assessment: The 8.0 (High) score reflects the combination of network exploitability, low authentication barrier, and severe impact across all security dimensions. The low EPSS score suggests that public exploitation activity is not widespread, but the high-impact potential makes proactive patching critical for asset owners.

Detection Guidance

The following detection content is intended to identify exploitation attempts of CVE-2021-42538 at the host, network, and log levels. These rules are based on the vulnerability class and should be validated in your environment.

Sigma Rule

title: Emerson WirelessHART Gateway - Passphrase Parameter Injection Attempt
id: 3a8c7f2e-9d41-4b6a-a8e1-2c5f9d7b4e21
status: experimental
description: Detects suspicious passphrase values containing command injection or parameter injection delimiters in Emerson WirelessHART Gateway management logs.
logsource:
    category: application
    product: emerson
    service: wirelesshart_gateway
detection:
    selection:
        event_data.message|contains:
            - 'passphrase'
            - 'Passphrase'
    injection_chars:
        event_data.message|contains|all:
            - ';'
            - '|'
            - '&'
            - '\n'
            - '\r'
            - '`'
            - '$('
    condition: selection and injection_chars
level: high
falsepositives:
    - Legitimate passphrase rotatation scripts using special characters
    - Misconfigured automation that sends complex passphrases
tags:
    - attack.initial-access
    - attack.t1190
    - cve.2021-42538

YARA Rule

rule CVE_2021_42538_Emerson_WirelessHART_Passphrase_Injection {
    meta:
        author = "ValterSIT Threat Intelligence"
        description = "Detects suspicious passphrase request patterns targeting Emerson WirelessHART Gateway"
        date = "2024-01-15"
        cve = "CVE-2021-42538"
        severity = "high"
    strings:
        $passphrase = "passphrase" nocase
        $inject1 = ";" ascii
        $inject2 = "|" ascii
        $inject3 = "&" ascii
        $inject4 = "`" ascii
        $inject5 = "$(" ascii
        $inject6 = "\n" ascii
        $inject7 = "&&" ascii
        $inject8 = "||" ascii
        $inject9 = "> " ascii
        $inject10 = ">> " ascii
    condition:
        $passphrase in (0..filesize) and
        (any of ($inject*) in (0..filesize))
}

Suricata IDS Rule

alert http any any -> $HOME_NET any (
    msg:"CVE-2021-42538 - Emerson WirelessHART Gateway Passphrase Injection Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"passphrase"; http_client_body; nocase;
    pcre:"/(;|\||&|`|\$\(|\\n|\\r|&&|\|\||>>)/Ui";
    classtype:attempted-admin;
    sid:1000051;
    rev:1;
    metadata:created_at 2024_01_15, updated_at 2024_01_15, cve CVE_2021_42538;
)

Elastic Detection

sequence by source.ip with maxspan=5m
  [ http where event.action == "post" and
    http.request.body.content contains "passphrase" and
    (http.request.body.content contains ";" or
     http.request.body.content contains "\\n" or
     http.request.body.content contains "&&" or
     http.request.body.content contains "|" or
     http.request.body.content contains "`" or
     http.request.body.content contains "$(") ]
  [ file where process.name : "gateway*" and file.path : "*passphrase*" ]

Splunk SPL Query

index=emerson_wirelesshart sourcetype="gateway:config"
| where request_body LIKE "%passphrase%" AND 
    (request_body LIKE "*;*" OR 
     request_body LIKE "*|*" OR 
     request_body LIKE "*&*" OR 
     request_body LIKE "*`*" OR 
     request_body LIKE "*$(*" OR 
     request_body LIKE "*\\n*" OR 
     request_body LIKE "*&&*" OR 
     request_body LIKE "*||*")
| stats count by src_ip, dest_ip, command, uri, _time
| where count > 3
| sort - _time

Wazuh OSSEC Rule

<group name="emerson_wirelesshart,">
  <rule id="100251" level="12">
    <decoded_as>json</decoded_as>
    <field name="event_data.message" type="pcre2">passphrase</field>
    <field name="event_data.message" type="pcre2">(;|\||&|`|\$\(|\\n|\\r|&&|\|\||>>)</field>
    <description>CVE-2021-42538: Emerson WirelessHART Gateway passphrase parameter injection attempt detected from $(srcip)</description>
    <group>attack.initial_access,attack.t1190,cve.2021-42538</group>
    <mitre><id>T1190</id></mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence: 2/4 — The patch is claimed by a single source (CNA) and has not been independently confirmed.

1. Immediate Mitigations:

   • Restrict access to the WirelessHART Gateway management interface to only trusted administrative hosts via network segmentation and firewall rules.
   • Disable remote access to the gateway management interface from the internet or unsecured networks.
   • Enforce strong, unique passphrases and rotate them periodically to reduce the risk of credential-based access.
   • Monitor gateway logs for anomalous passphrase change attempts, failed authentication, or unexpected configuration changes.
   • If possible, place the gateway behind a jump host or bastion with multi-factor authentication.

2. Patch/Upgrade Steps:

   • Emerson recommends upgrading to firmware v4.7.105 to address this vulnerability.
   • Visit the Emerson Gate Firmware site for download instructions.
   • If you do not have a free Guardian account, follow the updated Emerson Gateway Firmware download process by accessing the link above and viewing the download guide.
   • Before upgrading, verify the firmware version is compatible with your current hardware and network configuration. Perform the upgrade during a planned maintenance window to minimize operational disruption.
   • After the upgrade, verify system functionality and re-confirm that the vulnerability is resolved by testing passphrase handling.

3. Long-Term Hardening:

   • Establish a routine firmware update cycle aligned with Emerson's security advisories.
   • Implement OT network segmentation — place WirelessHART gateways on isolated VLANs with strict firewall policies.
   • Deploy an intrusion detection/prevention system with coverage for ICS/SCADA protocols to detect anomalous traffic targeting the gateway.
   • Conduct regular vulnerability assessments of OT infrastructure using credentialed scans.
   • Subscribe to Emerson security advisories and CISA ICS advisories for future disclosures affecting this product line.
   • Document and enforce an incident response plan specifically for OT device compromises, including procedures for isolating affected segments.

References

1. NVD Entry — CVE-2021-42538: https://nvd.nist.gov/vuln/detail/CVE-2021-42538 (National Vulnerability Database)
2. CISA ICS Advisories — CISA Known Exploited Vulnerabilities catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog (CVE not listed — confirmed absence as of the report date)
3. Emerson Security Advisory — Emerson WirelessHART Gateway firmware update guidance, referencing upgrade to v4.7.105. Accessible via the Emerson Gateway Firmware site (registration with Guardian account required).
4. ATT&CK Framework — T1190: Exploit Public-Facing Application: https://attack.mitre.org/techniques/T1190/
5. EPSS Data Source — FIRST EPSS Score Database: https://www.first.org/epss/data-stats (EPSS: 0.00917, 56.9% percentile)

Threat Actor Activity: No known threat actor campaigns specifically targeting CVE-2021-42538 have been publicly documented as of the report date. The vulnerability is not listed on CISA's KEV catalog, and no exploit-in-the-wild evidence has been reported. However, given the high CVSS score and the strategic value of industrial gateways to state-sponsored and financially motivated threat actors, monitoring for opportunistic exploitation is recommended.