lwIP up to 2.2.1 MQTT out-of-bounds write

CVSS Meta Temp Score
CVSS is a standardized scoring system to determine possibilities of attacks. The Temp Score considers temporal factors like disclosure, exploit and countermeasures. The unique Meta Score calculates the average score of different sources to provide a normalized scoring system.
Current Exploit Price (≈)
Our analysts are monitoring exploit markets and are in contact with vulnerability brokers. The range indicates the observed or calculated exploit price to be seen on exploit markets. A good indicator to understand the monetary effort required for and the popularity of an attack.
CTI Interest Score
Our Cyber Threat Intelligence team is monitoring different web sites, mailing lists, exploit markets and social media networks. The CTI Interest Score identifies the interest of attackers and the security community for this specific vulnerability in real-time. A high score indicates an elevated risk to be targeted for this vulnerability.
9.9$0-$5k1.44

Summaryinfo

A vulnerability was found in lwIP up to 2.2.1 and classified as very critical. This issue affects some unknown processing of the component MQTT. The manipulation results in out-of-bounds write. This vulnerability is known as CVE-2026-87121. It is possible to launch the attack remotely. No exploit is available.

Detailsinfo

A vulnerability, which was classified as very critical, was found in lwIP up to 2.2.1. Affected is an unknown code of the component MQTT. The manipulation with an unknown input leads to a out-of-bounds write vulnerability. CWE is classifying the issue as CWE-787. The product writes data past the end, or before the beginning, of the intended buffer. This is going to have an impact on confidentiality, integrity, and availability. CVE summarizes:

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.

The advisory is shared for download at savannah.nongnu.org. This vulnerability is traded as CVE-2026-87121 since 09/09/2026. The exploitability is told to be easy. It is possible to launch the attack remotely. The exploitation doesn't require any form of authentication. There are neither technical details nor an exploit publicly available. The current price for an exploit might be approx. USD $0-$5k (estimation calculated on 09/22/2026).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

VulDB is the best source for vulnerability data and more expert information about this specific topic.

Productinfo

Name

Version

CPE 2.3info

CPE 2.2info

CVSSv4info

VulDB Vector: 🔒
VulDB Reliability: 🔍

CNA CVSS-B Score: 🔒
CNA CVSS-BT Score: 🔒
CNA Vector: 🔒

CVSSv3info

VulDB Meta Base Score: 9.9
VulDB Meta Temp Score: 9.9

VulDB Base Score: 10.0
VulDB Temp Score: 10.0
VulDB Vector: 🔒
VulDB Reliability: 🔍

CNA Base Score: 9.8
CNA Vector (icscert): 🔒

CVSSv2info

AVACAuCIA
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VectorComplexityAuthenticationConfidentialityIntegrityAvailability
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VulDB Base Score: 🔒
VulDB Temp Score: 🔒
VulDB Reliability: 🔍

Exploitinginfo

Class: Out-of-bounds write
CWE: CWE-787 / CWE-119
CAPEC: 🔒
ATT&CK: 🔒

Physical: No
Local: No
Remote: Yes

Availability: 🔒
Status: Not defined
Price Prediction: 🔍
Current Price Estimation: 🔒

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Threat Intelligenceinfo

Interest: 🔍
Active Actors: 🔍
Active APT Groups: 🔍

Countermeasuresinfo

Recommended: no mitigation known
Status: 🔍

0-Day Time: 🔒

Timelineinfo

09/09/2026 CVE reserved
09/22/2026 +13 days Advisory disclosed
09/22/2026 +0 days VulDB entry created
09/22/2026 +0 days VulDB entry last update

Sourcesinfo

Advisory: savannah.nongnu.org
Status: Not defined

CVE: CVE-2026-87121 (🔒)
GCVE (CVE): GCVE-0-2026-87121
GCVE (VulDB): GCVE-100-408720

Entryinfo

Created: 09/22/2026 22:13
Changes: 09/22/2026 22:13 (72)
Complete: 🔍
Cache ID: 216::103

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