Broker crash via oversized MQTT v5 varint
Published Mar 11, 2026 · Updated Mar 11, 2026
Out-of-bounds read in NanoMQ 0.24.6 and earlier allows remote attackers to crash the broker via a malformed MQTT v5 CONNECT packet. get_var_integer() accepts a fifth variable-byte integer byte and reads past the packet buffer when the first four bytes retain continuation bits. No authentication or user interaction is required; the demonstrated result is a one-byte out-of-bounds read and process termination under AddressSanitizer.
Summary
What happened
Out-of-bounds read in NanoMQ 0.24.6 and earlier allows remote attackers to crash the broker via a malformed MQTT v5 CONNECT packet. get_var_integer() accepts a fifth variable-byte integer byte and reads past the packet buffer when the first four bytes retain continuation bits. No authentication or user interaction is required; the demonstrated result is a one-byte out-of-bounds read and process termination under AddressSanitizer.
The record
- CVE
- CVE-2026-21888
- Published
- Mar 11, 2026
- Updated
- Mar 11, 2026
- Vendor
- EMQ Technologies
- Product
- NanoMQ
- Classifications
- CWE-125, T1499.004
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Mar 11, 2026CVE publishedPublication date reported by the CVE source.
- Mar 11, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=<= 0.24.6
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Public exploit references
- Malformed MQTT v5 CONNECT packet proof of conceptproof of concept · demonstrated
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
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Your stack
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