CVE-2026-76816: Input Validation Flaw (CVSS 3.5)

Description

Netty is an asynchronous, event-driven network application framework. Prior to versions 4.1.137.Final and 4.2.17.Final, MqttEncoder does not validate client identifiers, will topics, usernames, and PUBLISH topic names before encoding, allowing prohibited null bytes in MQTT UTF-8 string fields and potentially causing routing, access-control, or identity mismatches in downstream brokers. The vulnerability is exploitable when an application uses Netty's MQTT encoder to construct messages from user-controlled input. This issue is fixed in versions 4.1.137.Final and 4.2.17.Final.

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CVSS v3.1 Score

3.5

LOW

CVSS:3.1/AV:A/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N

EPSS — Exploit Prediction

0.0017

Probability of exploitation

0.07%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-20 Improper Input Validation

CWE-626 CWE-626

References

Other References

https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7 https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961 https://github.com/netty/netty/security/advisories/GHSA-43fm-7cxg-hf3j

Frequently Asked Questions

What is CVE-2026-76816? +

Netty is an asynchronous, event-driven network application framework. Prior to versions 4.1.137.Final and 4.2.17.Final, MqttEncoder does not validate client identifiers, will topics, usernames, and PUBLISH topic names before encoding, allowing prohibited null bytes in MQTT UTF-8 string fields and potentially causing routing, access-control, or identity mismatches in downstream brokers. The vulnerability is exploitable when an application uses Netty's MQTT encoder to construct messages from user-controlled input. This issue is fixed in versions 4.1.137.Final and 4.2.17.Final. It has a CVSS v3.1 base score of 3.5 (LOW).

How severe is CVE-2026-76816? +

CVE-2026-76816 has a CVSS v3.1 score of 3.5 out of 10, rated LOW. This is a low-severity vulnerability with limited impact. The EPSS score is 0.0017, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2026-76816? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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