lwIP TCP/IP Stack MQTT Client Application
CISA warns lwIP MQTT client versions 2.0.1 through 2.2.1 have critical CVE-2026-87121.
CISA published ICSA-26-265-01 for an out-of-bounds write in the lwIP MQTT client, CVE-2026-87121, affecting versions 2.0.1 through 2.2.1. CVSS v3.1 is 9.8 and CVSS v4.0 is 9.3; successful exploitation could allow full code execution. The stack is deployed across multiple critical infrastructure sectors worldwide. CISA says no known public exploitation has been reported.
- CVE-2026-87121 is an out-of-bounds write in lwIP's MQTT client.
- Affected versions are 2.0.1 through 2.2.1; CVSS v3.1 score is 9.8.
- Exploitation could yield full code execution without authentication.
- CISA reports no known public exploitation at this time.
Vulnerabilities mentionedAll →
- CVE-2026-871219.3—lwIP TCP/IP Stack MQTT is vulnerable to an out-of-bounds write, which may allow an attacker to gain full code execution on the devicepublished
| CVE | Vulnerability | CVSS | EPSS | Flags | Affected | Exposure | Published |
|---|---|---|---|---|---|---|---|
| CVE-2026-87121 | lwIP TCP/IP Stack MQTT is vulnerable to an out-of-bounds write, which may allow an attacker to gain full code execution on the device lwIP TCP/IP Stack MQTT is vulnerable to an out-of-bounds write, which may allow an attacker to gain full code execution on the device. NVD description · AI analysis pending | 9.3 | — | — |
Full article490 words · extracted from cisa.gov · click to collapse
Summary
Successful exploitation of this vulnerability could allow an attacker to gain full code execution on the device.
The following versions of lwIP TCP/IP Stack MQTT Client Application are affected:
- MQTT Client Application >=2.0.1|<=2.2.1 (CVE-2026-87121)
| CVSS | Vendor | Equipment | Vulnerabilities |
|---|---|---|---|
| v3 9.8 | lwIP | lwIP TCP/IP Stack MQTT Client Application | Out-of-bounds Write |
Background
- Critical Infrastructure Sectors: Chemical, Communications, Critical Manufacturing, Energy, Financial Services, Healthcare and Public Health, Transportation Systems, Water and Wastewater Systems
- Countries/Areas Deployed: Worldwide
- Company Headquarters Location: Sweden
Vulnerabilities
CVE-2026-87121
The affected product is vulnerable to an out-of-bounds write, which may allow an attacker to gain full code execution on the device.
Affected Products
lwIP TCP/IP Stack MQTT Client Application
Vendor:
lwIP
Product Version:
lwIP MQTT Client Application: >=2.0.1|<=2.2.1
Product Status:
known_affected
Relevant CWE: CWE-787 Out-of-bounds Write
Metrics
| CVSS Version | Base Score | Base Severity | Vector String |
|---|---|---|---|
| 3.1 | 9.8 | CRITICAL | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H |
| 4.0 | 9.3 | CRITICAL | CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N |
Acknowledgments
- Shahriyar Jalayeri of ByteRay Ltd. reported this vulnerability to CISA.
Legal Notice and Terms of Use
This product is provided subject to this Notification (https://www.cisa.gov/notification) and this Privacy & Use policy (https://www.cisa.gov/privacy-policy).
Recommended Practices
CISA recommends users take defensive measures to minimize the risk of exploitation of this vulnerability.
- Minimize network exposure for all control system devices and/or systems, ensuring they are not accessible from the internet.
- Locate control system networks and remote devices behind firewalls and isolating them from business networks.
- When remote access is required, use more secure methods, such as Virtual Private Networks (VPNs), recognizing VPNs may have vulnerabilities and should be updated to the most current version available. Also recognize VPN is only as secure as the connected devices.
CISA reminds organizations to perform proper impact analysis and risk assessment prior to deploying defensive measures.
CISA also provides a section for control systems security recommended practices on the ICS webpage on cisa.gov/ics. Several CISA products detailing cyber defense best practices are available for reading and download, including Improving Industrial Control Systems Cybersecurity with Defense-in-Depth Strategies.
CISA encourages organizations to implement recommended cybersecurity strategies for proactive defense of ICS assets.
Additional mitigation guidance and recommended practices are publicly available on the ICS webpage at cisa.gov/ics in the technical information paper, ICS-TIP-12-146-01B--Targeted Cyber Intrusion Detection and Mitigation Strategies.
Organizations observing suspected malicious activity should follow established internal procedures and report findings to CISA for tracking and correlation against other incidents.
CISA also recommends users take the following measures to protect themselves from social engineering attacks:
- Do not click web links or open attachments in unsolicited email messages.
- Refer to Recognizing and Avoiding Email Scams for more information on avoiding email scams.
- Refer to Avoiding Social Engineering and Phishing Attacks for more information on social engineering attacks.
No known public exploitation specifically targeting this vulnerability has been reported to CISA at this time.
Revision History
- Initial Release Date: 2026-09-22
| Date | Revision | Summary |
|---|---|---|
| 2026-09-22 | 1 | Initial Publication |