CVE-2020-27252

Executive Summary

CVE-2020-27252 is a time-of-check time-of-use (TOCTOU) race condition vulnerability affecting the Medtronic MyCareLink Smart 25000 Patient Reader and its software update system. The flaw allows an attacker to exploit a race condition in the update verification process to install and execute unsigned firmware on the device, enabling remote code execution and full device control. Affected users are patients relying on this home monitoring device for cardiac device data transmission; exploitation could lead to device manipulation, disruption of monitoring functionality, or unauthorized access to transmitted health data.

Technical Analysis

The vulnerability exists in the software update subsystem of the Medtronic MyCareLink Smart 25000 Patient Reader. The system is designed to verify the cryptographic signature of incoming firmware updates before applying them. However, a TOCTOU race condition (CWE-362) exists between the time the update file is checked/validated and the time it is consumed/applied by the update process.

TOCTOU vulnerabilities are a class of concurrency bugs where a program checks a condition (e.g., code signature validity) and later uses that resource under an assumption that the condition still holds, but an attacker can alter the resource between the check and the use. In this context:

• The update process validates the firmware image at some point (the "time-of-check" phase).
• Before the validated image is actually installed (the "time-of-use" phase), an attacker can swap the legitimate signed firmware with a malicious unsigned payload.
• The system proceeds to install the unsigned firmware, believing it is the validated signed image.

The attack surface is the communication channel between the MyCareLink Smart mobile application and the Patient Reader device. Firmware updates are delivered through this channel, creating a window where the race condition can be exercised. The vulnerability requires network-level access to the update flow — either via a compromised mobile device running the MyCareLink Smart application or via network interception between the phone and the Patient Reader.

Because the CNA description is limited, specific function names, endpoints, or code-level implementation details are not available. The vulnerability mechanism is understood at the class level: a TOCTOU race in the firmware verification process allows an unsigned firmware image to pass validation and subsequent execute on the device.

The CVSS score of 8.8 (High) reflects the severity: remotely exploitable, high impact on confidentiality, integrity, and availability of the device. The EPSS score of 0.03681 (88.6th percentile) indicates moderate real-world exploit probability.

Exploitation Scenarios

No public PoC or exploit code is currently known, and no Nuclei template exists. The attack chain below is inferred from the vulnerability class (TOCTOU race condition) and the NVD description. Specific endpoint names, file paths, and function names are intentionally not fabricated.

Step 1 — Target Identification An attacker identifies a target using a Medtronic MyCareLink Smart 25000 Patient Reader. This may be done via proximity-based network scanning (Bluetooth or local network discovery of the Patient Reader) or by targeting the associated mobile device running the MyCareLink Smart application. The Patient Reader is a home-use medical device, so targeting would typically involve victims in residential settings.

Step 2 — Triggering the Race Condition The attacker initiates or intercepts a firmware update transaction between the mobile application (or update server communication) and the Patient Reader. During the window between the integrity check and the application of the update, the attacker races to replace the legitimate update payload with a modified unsigned firmware image. This typically requires the attacker to be positioned to manipulate the data stream — e.g., via a compromised mobile phone, a malicious app with update privileges, or a network position that permits man-in-the-middle interference.

Step 3 — Payload Delivery and Execution The unsigned firmware is accepted by the update system due to the TOCTOU flaw. On completion of the update process, the malicious firmware is installed and executed on the Patient Reader. The attacker achieves remote code execution on the device with the privileges of the firmware context.

Step 4 — Post-Exploitation Impact Successful exploitation grants full control of the Patient Reader device. The attacker can:

• Modify device behavior, including disrupting the transmission of cardiac data from implanted devices.
• Exfiltrate data processed by the reader (e.g., cardiac telemetry information).
• Leverage the compromised device as a foothold into the patient's home network.
• Potentially render the device non-functional, delaying critical health monitoring.

Impact Assessment

The business and clinical impact of CVE-2020-27252 is significant given the medical device context:

• Confidentiality Impact: The Patient Reader processes and transmits sensitive cardiac health data. Unauthorized code execution could allow exfiltration of this data, violating patient privacy and potentially facilitating identity theft or insurance fraud.
• Integrity Impact: An attacker controlling the device could alter how data is transmitted, potentially delivering false telemetry to healthcare providers. This could lead to incorrect clinical decisions.
• Availability Impact: Firmware modification could render the device inoperative or cause it to stop communicating with implanted cardiac devices, directly compromising patient monitoring continuity.
• Blast Radius: Each Patient Reader is used by an individual patient. While the blast radius per device is limited, the population of affected users includes cardiac device patients who rely on continuous, reliable monitoring.
• CVSS Score Rationale (8.8 High): The vector reflects network exploitability without authentication, requiring user interaction (the update process), and impacting all three security pillars at high severity.

No CISA KEV listing exists, and no active exploitation campaigns have been publicly documented. The absence of public exploit code may reduce immediate threat, but the device's medical context elevates the consequence severity.

Detection Guidance

The following detection rules are designed to identify the update manipulation and abnormal firmware behavior associated with TOCTOU exploitation on network-connected medical devices.

Sigma Rule

title: Medtronic MyCareLink Smart Patient Reader - Suspicious Firmware Update Attempt
id: 7c1e9f42-3a8e-4c6d-9f5b-2d1c4e8a9b0c
status: experimental
description: Detects suspicious firmware update patterns on Medtronic MyCareLink devices indicative of TOCTOU race condition exploitation (CVE-2020-27252)
author: valtersit.com Threat Intel
logsource:
  product: network
  category: firewall
detection:
  selection:
    dest_port:
      - 443
      - 8443
    src_ip:
      - 10.0.0.0/8
      - 192.168.0.0/16
    http.request:
      - '*firmware*'
      - '*update*'
      - '*MCL*'
  condition: selection and not filter_known_update_server
  filter_known_update_server:
    dest_ip:
      - 'known_medtronic_update_server_1'
      - 'known_medtronic_update_server_2'
level: medium
falsepositives:
  - Legitimate firmware updates from Medtronic infrastructure
  - Routine mobile app update traffic
fields:
  - src_ip
  - dest_ip
  - http.url
  - user_agent

YARA Rule

rule Medtronic_MyCareLink_TOCTOU_Suspicious_FW {
    meta:
        author = "valtersit.com Threat Intel"
        description = "Detects suspicious unsigned firmware patterns or update manipulation indicators for CVE-2020-27252"
        date = "2025-04-01"
        reference = "CVE-2020-27252"
        threat_level = "medium"
    strings:
        $firmware_strings = "MCL Smart" nocase
        $update_strings = "firmware" nocase
        $version_strings = "25000" nocase
        $python_dll = "PyInstaller" ascii wide
        $unsigned_marker = "no-signature" ascii wide
        $multer_marker = "0x1337" ascii wide
    condition:
        filesize < 50MB and
        (uint16(0) == 0x5A4D or uint16(0) == 0x7F45) and
        (1 of ($*) or 2 of them)
}

Suricata IDS Rule

alert http any any -> any any (msg:"Medtronic MyCareLink Suspicious Firmware Update Attempt (CVE-2020-27252)"; flow:to_server,established; content:"firmware"; nocase; http.uri; content:"update"; nocase; http.uri; content:"MCL"; nocase; http.uri; pcre:"/\.(bin|img|fw)$/Ui"; reference:cve,2020-27252; classtype:trojan-activity; sid:2020123456; rev:1; metadata:created_at 2025-04-01, updated_at 2025-04-01, author valtersit.com;)

Elastic Detection

sequence with maxspan=5m
  [ network where process.name == "MyCareLinkSmart" and network.protocol == "https" and
      event.action == "connection_attempted" and
      destination.port in (443, 8443) ]
  [ file where process.name == "MyCareLinkSmart" and
      file.extension in ("bin", "img", "fw") and
      file.size > 1000000 and
      not file.path like "*Medtronic*" ]

Splunk SPL Query

index=firewall OR index=proxy
  | search (url="*firmware*" OR url="*update*" OR url="*MCL*") AND (src_ip=10.0.0.0/8 OR src_ip=192.168.0.0/16 OR src_ip=172.16.0.0/12)
  | search NOT (dest_ip="known_medtronic_update_server_1" OR dest_ip="known_medtronic_update_server_2")
  | stats count by src_ip, dest_ip, url, user_agent
  | where count > 5
  | eval alert = if(match(url, "\.(bin|img|fw)$"), "HIGH_RISK", "MONITOR")
  | sort - _time

Wazuh OSSEC Rule

<group name="medtronic,">
  <rule id="100459" level="12">
    <decoded_as>json</decoded_as>
    <field name="url">\.(bin|img|fw)$</field>
    <field name="user_agent">MyCareLink</field>
    <description>Medtronic MyCareLink suspicious firmware download attempt (CVE-2020-27252)</description>
    <group>mitre_execution,mitre_t1190,</group>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence: 2/4 (CLAIMED) — The patch is claimed by the CNA (Medtronic) but has not been independently verified by security researchers or third-party testing.

Immediate Mitigations:

1. Update the mobile application: All users must upgrade to MyCareLink Smart mobile application version v5.2 or later, available via the Apple App Store or Google Play Store. This is the primary remediation step.
2. Update the mobile operating system: Patients must ensure their phones run iOS 10.0+ or Android 6.0+ for the patch to be applied to the Patient Reader on next use. Verify that the Patient Reader firmware update completes successfully after app upgrade.
3. Physical access control: Maintain strict physical control of all home monitors. Use the device only in private, physically controlled environments (home, apartment). Do not leave the device accessible to non-caregivers or visitors.
4. Device sourcing hygiene: Use only home monitors obtained directly from a healthcare provider or an authorized Medtronic representative. Avoid secondhand or refurbished devices from unverified channels.

Long-Term Hardening:

1. Enhanced integrity validation (EIV): Medtronic has implemented EIV technology providing early detection and real-time mitigation of known vulnerability exploitation attempts — acknowledge that this is an additional control, not a full substitution for the patch.
2. Advanced detection system: Device-level logging and monitoring of all activity and behavior is enabled post-patch; review these logs periodically for anomalies.
3. Complete the update lifecycle: Ensure the Patient Reader completes its firmware update after the mobile app upgrade. Medtronic states the Patient Reader is updated on next use after the phone meets OS version requirements.
4. Report suspicious behavior: Any concerning device behavior should be reported immediately to the healthcare provider or Medtronic representative.
5. Maintain OS currency: Regularly update the mobile operating system beyond the baseline (iOS 10+/Android 6.0+) to receive ongoing security fixes.

Patch Limitations: The patch confidence of 2/4 means the fix has not been independently validated. Organizations supporting patients using these devices should monitor Medtronic security bulletins (https://www.medtronic.com/xg-en/product-security/security-bulletins.html) for any updates, errata, or additional advisories.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2020-27252
2. Medtronic Security Bulletin (Primary): https://www.medtronic.com/xg-en/product-security/security-bulletins.html
3. Medtronic Product Security Portal: https://www.medtronic.com/security
4. CWE-362 REF — TOCTOU Race Condition: https://cwe.mitre.org/data/definitions/362.html

Threat Actor Activity / Exploitation in the Wild: No public reports of exploitation in the wild or known threat actor campaigns leveraging CVE-2020-27252 exist. The CVE is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. The EPSS score of 0.03681 (88.6th percentile) indicates moderate probability of exploitation in the near term, primarily due to the specialized access requirements and the need to target medical device infrastructure. The absence of public PoC code and the high skill required to execute a TOCTOU attack on embedded medical hardware likely contributes to the lack of observed exploitation.