What is CVE-2026-76816?

CVE-2026-76816 is Netty: MQTT Topic Name and Client ID Validation Bypass. The supplied enrichment data classifies it as Input Validation mapped to CWE-20 Improper Input Validation; CWE-626 CWE-626.

Which versions are affected?

Affected product and version evidence is listed in the product table above. Current public advisories should be checked for exact version scope before patch targeting is finalized.

Is there active exploitation?

Active exploitation is not confirmed from current sources for CVE-2026-76816. If exploitation is not confirmed from current sources, teams should document that status and continue monitoring KEV, vendor, and telemetry updates.

No. CVE-2026-76816 is not listed in CISA KEV in the supplied enrichment data. KEV status should be monitored because it can change after initial disclosure or after exploitation becomes publicly confirmed.

Is there a public exploit?

A public PoC is not confirmed from current sources for CVE-2026-76816. Public exploit or PoC availability should raise triage priority, especially for exposed or business-critical systems.

How can defenders detect it?

Start with Not reported where available. Look for vulnerability-class behavior, product errors, suspicious authentication events, exploit probes, and abnormal process activity based only on evidence that applies to the affected product.

How should teams prioritize it?

Prioritize CVE-2026-76816 using CVSS 3.5, EPSS 0%, KEV status, public PoC status, and asset exposure. Internet-facing, privileged, or business-critical deployments should be handled first.

Are workarounds available?

The supplied mitigation guidance says to apply vendor patches and restrict external exposure until fixed. Fixed-version evidence is listed in the mitigation section above.

Which threat actors are linked publicly?

No public threat actor linkage is confirmed from current sources. Do not attribute exploitation to a threat actor unless the source evidence explicitly supports that linkage.

What should be done in the next 24 hours?

Identify exposed assets running affected vendor/product/version combinations. Prioritize based on EPSS, PoC availability, and external exposure. Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class. Open remediation tickets with affected assets, owners, evidence status, and detection coverage so decisions are traceable.

lowInput Validation

CVE-2026-76816

CVE-2026-76816 — Netty: MQTT Topic Name and Client ID Validation Bypass

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.

Published Aug 24, 2026Updated Aug 24, 2026Sources: NVD, netty (CNA), GitHub, SOCRadar CTI

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the scoring sources call it.

Exploitation

Unreported

no source claims exploitation

EPSS

ahead of 0% of scored CVEs

Affects

netty

2 products listed

CVSS base

3.5

low

Remediation

The vendor's own words where we have them.

Upgrade netty to a fixed release. Apply vendor patches per advisory and restrict external exposure of the affected component until patched.

First 24 hours

Ordered from the record's own fields — exposure first, because you cannot patch what you have not found.

Identify exposed assets running affected vendor/product/version combinations.
Prioritize based on EPSS, PoC availability, and external exposure.
Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class.

Affected scope

Vendor, product and version as the advisories word them.

Vendor	Product	Versions	Status
netty	netty	>= 4.2.0.Final, < 4.1.137.Final	Vulnerable
netty	netty	< 4.1.137.Final	Vulnerable

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Availability

None

Confidentiality

None

Integrity

Low

Scope

Unchanged

Attack Complexity

Low

Attack Vector

Adjacent

Privileges Req

Low

User Interaction

None

Every base score collected

Sources score independently and disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Source
3.5	CVSS 3.1	low	2.1	1.4	GitHub (CNA), netty (CNA)

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

What this weakness leads to

MITRE's own consequences and mitigations for the weakness class — the authority's wording, not guidance derived from the CVSS vector.

MITRE

CWE-626 · Null Byte Interaction Error (Poison Null Byte)

Unexpected State::

Mitigation: Remove null bytes from all incoming strings.::

CWE-20 · Improper Input Validation

DoS: Crash, Exit, or Restart
DoS: Resource Consumption (CPU)
DoS: Resource Consumption (Memory)
Read Memory

Mitigation: Consider using language-theoretic security (LangSec) techniques that characterize inputs using a formal language and build recognizers for that language. This effectively requires parsing to be a distinct layer that effectively enforces a boundary between raw input and internal data representations, instead of allowing parser code to be scattered throughout the program, where it could be subject to errors or inconsistencies that create weaknesses. [REF-1109] [REF-1110] [REF-1111]

Weakness & attack patterns

CWE-20Improper Input Validation
CWE-626

Attack patterns reported against this CVE. The ATT&CK techniques below are inferred from its weakness class.

CAPEC-10 · Buffer Overflow via Environment VariablesCAPEC-101 · Server Side Include (SSI) InjectionCAPEC-104 · Cross Zone ScriptingCAPEC-108 · Command Line Execution through SQL InjectionCAPEC-109 · Object Relational Mapping InjectionCAPEC-110 · SQL Injection through SOAP Parameter TamperingCAPEC-120 · Double EncodingCAPEC-13 · Subverting Environment Variable ValuesCAPEC-135 · Format String InjectionCAPEC-136 · LDAP InjectionCAPEC-14 · Client-side Injection-induced Buffer OverflowCAPEC-153 · Input Data Manipulation

T1562.003Impair Defenses: Impair Command History Logging
T1574.006Hijack Execution Flow: Dynamic Linker Hijacking
T1574.007Hijack Execution Flow: Path Interception by PATH Environment Variable

Timeline

What happened to this CVE, newest first — with the readings a source repeats on a schedule counted underneath rather than listed.

2026
Added · CVSS 3.1 3.5 (AV:A)
Aug 24, 2026 · NVD
Initial · CVE published
Aug 24, 2026 · NVD

References

3 on the record

github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7
Exploit, Third Party Advisory, Mitre
github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961
Exploit, Third Party Advisory, Mitre
github.com/netty/netty/security/advisories/GHSA-43fm-7cxg-hf3j
Exploit, Third Party Advisory, Mitre

Elsewhere on this site

nettyevery CVE for this vendor
Input Validationsame class
CWE-20other pages naming this weakness

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record1

What should defenders know first?

CVE-2026-76816 is Netty: MQTT Topic Name and Client ID Validation Bypass, a low vulnerability affecting netty from netty. The current evidence does not list it in CISA KEV, and the exploit status is: Active exploitation is not confirmed from current sources for CVE-2026-76816. Public exploit evidence is: A public PoC is not confirmed from current sources for CVE-2026-76816. The affected-version evidence is listed in the key facts and affected products tables. Defenders should first verify whether exposed or business-critical assets run those versions, then apply vendor patches or mitigations, restrict reachable attack surface, and preserve logs for detection review. CVSS 3.5 describes technical severity, while EPSS 0% helps estimate near-term exploit likelihood; neither replaces asset context. Unknown fields should remain explicit in tickets, and threat actor, IOC, victimology, or payload claims should not be added unless a cited source supports them. Monitor CISA KEV, vendor advisories, NVD changes, public PoC repositories, and internal telemetry for update triggers.