Critical severity · published Sep 22, 2026
CVE-2026-87121
Out-of-bounds write in lwIP TCP/IP Stack MQTT Client Application
9.8
CVSS
0.8%
EPSS
No
On KEV
Description
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
High
Integrity
High
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.8% · higher than 56% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
TS
TCP/IP Stack MQTT · lwIP
2.0.1 through 2.2.1 · source: vendor
In the news
VulnerabilityCritical
lwIP TCP/IP Stack MQTT Client Application
View CSAF
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
Expand All +
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.
View CVE Details
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
Remediations
Mitigation Users of lwIP are encouraged to update their version of lwIP using the repository found at https://savannah.nongnu.org/projects/lwip . The commit identifier that contains the fix is f89407ea711879c04d91c92b35d67be78bbaf0f1.
Relevant CWE: CWE-787 Out-of-bounds Write
Metrics
CVSS Versi...
Sep 22, 2026·CISA Advisories
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-87121
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-87121
github.com
https://github.com/cisagov/CSAF/blob/develop/csaf_files/OT/white/2026/icsa-26-265-01.json
savannah.nongnu.org
https://savannah.nongnu.org/projects/lwip
www.cisa.gov
https://www.cisa.gov/news-events/ics-advisories/icsa-26-265-01
