CVE-2026-30867 is a medium-severity (CVSS 5.7) Reachable Assertion vulnerability in CocoaMQTT. A fix is available for CocoaMQTT — see the affected versions and patch details below.
CocoaMQTT: Denial of Service via Reachable Assertion in `PUBLISH` Packet Parsing
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-30867 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
CocoaMQTTReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects SwiftURL packages — download data is not available via public APIs for these ecosystems.
Description
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦SwiftURL | CocoaMQTT | all versions | 2.2.2 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for CocoaMQTT, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update CocoaMQTT to 2.2.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-30867 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-30867 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-30867. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-30867 in your dependencies?
O3 Security finds CVE-2026-30867 across SwiftURL dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.