# CVE-2026-16477: Deserialization of Untrusted Data (CWE-502) in MTW CleanShot X (macOS) leading to code execution via malicious .cleanshot file

> CERT Polska reports a CWE-502 deserialization vulnerability in MTW CleanShot X for macOS (4.0.0 through 4.5.2). Opening a crafted .cleanshot project file executes an attacker payload during parsing, running code as the current user. The vendor shipped security updates 4.5.2, 4.6.3, 4.7.8 and 4.8.10 on 2026-07-21.

- **Published:** 2026-10-22T15:07:45Z
- **Last reviewed:** 2026-10-22T15:07:45Z
- **Canonical:** https://intel.threadlinqs.com/threat/TL-2026-4153
- **ID:** TL-2026-4153
- **Severity:** MEDIUM
- **Category:** VULNERABILITY
- **Status:** PATCHED
- **Detections:** 9 · **IOCs:** 9 (full data via the Threadlinqs MCP server — Purple tier)
- **CVEs:** CVE-2026-16477

## Description

CVE-2026-16477 is a deserialization of untrusted data weakness (CWE-502) in CleanShot X for macOS, developed by MTW. The editable .cleanshot project format was introduced in CleanShot X 4.0 (2021-12-13) and lets users save annotated screenshots as projects that can be reopened and edited. According to the CERT Polska advisory (published 2026-10-22), a specially crafted .cleanshot file carries a payload that is executed automatically during parsing, resulting in arbitrary code execution in the context of the current user.

Exploitation requires user interaction: the victim must open an attacker-supplied .cleanshot file obtained through some untrusted delivery channel (the advisory refers to 'untrusted file delivery' without naming a specific channel). No network-reachable attack surface is described. Code runs with the privileges of the logged-in user, not root.

The vendor changelog (cleanshot.com/changelog) lists versions 4.5.2, 4.6.3, 4.7.8 and 4.8.10, all dated 2026-07-21 and each labelled 'Important security update recommended for all users'. These are parallel fixes across the 4.5, 4.6, 4.7 and 4.8 release branches. This explains why the advisory lists 4.5.2 as both the upper end of the affected range and a fixed version: the affected range most likely means releases before the fix on each branch. The advisory text does not state this, so it is an analyst inference. Users should update to the latest release on whichever branch they run.

The advisory does not provide a CVSS score. The CVE.org page and the NVD API returned no usable record at research time (NVD returned zero results). The advisory reports no CISA KEV listing, no public proof-of-concept, no in-the-wild exploitation, no IOCs and no attribution. The discoverer is credited as Dharan24. Severity MEDIUM is analyst-assigned, reflecting the required user interaction and current-user privilege. BeaconBeagle was not queried because the sources contain no network IOCs.

## MITRE ATT&CK

- T1566 Phishing
- T1203 Exploitation for Client Execution
- T1204.002 User Execution: Malicious File
- T1059 Command and Scripting Interpreter
- T1005 Data from Local System

## Sources

- [CERT Polska: Vulnerability in MTW CleanShot X software (CVE-2026-16477)](https://cert.pl/en/posts/2026/10/CVE-2026-16477/)
- [CVE record CVE-2026-16477](https://www.cve.org/CVERecord?id=CVE-2026-16477)
- [NVD API query for CVE-2026-16477](https://services.nvd.nist.gov/rest/json/cves/2.0?cveId=CVE-2026-16477)
- [CleanShot X Changelog (security updates 4.5.2, 4.6.3, 4.7.8, 4.8.10)](https://cleanshot.com/changelog)
- [CERT Polska Coordinated Vulnerability Disclosure](https://cert.pl/en/cvd/)
- [9to5Mac: CleanShot X launches own file format with new update (.cleanshot introduction)](https://9to5mac.com/2021/12/18/mad-screen-capturing-tool-cleanshot-x-launches-own-file-format-with-new-update/)
- [CWE-502: Deserialization of Untrusted Data](https://cwe.mitre.org/data/definitions/502.html)

## Full data

Detection queries (Splunk SPL / Microsoft KQL / Sigma) and IOC values require the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp

Canonical: https://intel.threadlinqs.com/threat/TL-2026-4153