CVE-2026-30867

A vulnerability exists in the packet parsing logic of CocoaMQTT that allows an attacker (or a compromised/malicious MQTT broker) to remotely crash the host iOS/macOS/tvOS application.

The vulnerability is located in Source/FramePublish.swift during the extraction of the Topic string from the incoming byte array.

When parsing the Variable Header of a PUBLISH frame, the library reads the first two bytes to determine the topicLength. It then adds this length to the current position (pos) and attempts to slice the byte array to extract the string:

if let data = NSString(bytes: [UInt8](bytes[2...(pos-1)]), length: Int(len), encoding: String.Encoding.utf8.rawValue) {
    topic = data as String
}

If a packet is received where the Topic Length evaluates to 0 (e.g., 0x00 0x00), the len variable becomes 0, and pos evaluates to 2.

The slicing logic dynamically calculates bytes[2...(2-1)], which becomes bytes[2...1]. Swift's ClosedRange operator (...) requires the lower bound to be less than or equal to the upper bound. Because 2 is not less than 1, Swift detects an out-of-bounds access attempt and immediately triggers a runtime trap (Fatal error: Range requires lowerBound <= upperBound), crashing the host application.

If an attacker publishes this 4-byte malformed payload to a shared topic with the RETAIN flag set to true, the MQTT broker will persist the payload. Any time a vulnerable client connects and subscribes to that topic, the broker will automatically push the malformed packet. The app will instantly crash in the background before the user can even interact with it. This effectively "bricks" the mobile application (a persistent DoS) until the retained message is manually wiped from the broker database.

CVE-2026-30867 has a CVSS score of 5.7 (Medium). The vector is network-reachable, low privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.2.2); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-30867? CVE-2026-30867 is a medium-severity security vulnerability in CocoaMQTT (swift), affecting versions < 2.2.2. It is fixed in 2.2.2.
2. How severe is CVE-2026-30867? CVE-2026-30867 has a CVSS score of 5.7 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of CocoaMQTT are affected by CVE-2026-30867? CocoaMQTT (swift) versions < 2.2.2 is affected.
4. Is there a fix for CVE-2026-30867? Yes. CVE-2026-30867 is fixed in 2.2.2. Upgrade to this version or later.
5. Is CVE-2026-30867 exploitable, and should I be worried? Whether CVE-2026-30867 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-30867 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-30867? Upgrade CocoaMQTT to 2.2.2 or later.