CleanShot X deserialization flaw enables RCE
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A CleanShot X for macOS flaw lets malicious .cleanshot files achieve code execution when opened, patched in versions up to 4.8.10.
CERT Polska coordinated disclosure of CVE-2026-16477, an insecure deserialization vulnerability (CWE-502) in MTW's CleanShot X screenshot annotation tool for macOS, affecting versions 4.0.0 through 4.5.2. The flaw resides in how the application parses .cleanshot project files, which store editable screenshot annotations; untrusted data embedded in these files is deserialized without proper validation, allowing an attacker-crafted file to execute arbitrary code in the context of the user who opens it.
Exploitation requires social engineering: an attacker must create a malicious .cleanshot file and convince a target to open it in CleanShot X. There is no indication of in-the-wild exploitation or a public proof-of-concept; this was reported directly to CERT Polska and resolved through coordinated disclosure. Users who have never opened .cleanshot files from untrusted sources are not affected.
The vendor has released fixes in versions 4.5.2, 4.6.3, 4.7.8, and 4.8.10. Defenders should ensure CleanShot X installations are updated to one of these patched versions or later, and should treat unsolicited .cleanshot files from external or unknown sources with the same caution as other untrusted document types capable of triggering code execution upon opening.
Mentioned in this report
Detection guidance
CleanShot X Spawning Shell or Scripting Interpreter
Detects the CleanShot X macOS app spawning a shell, scripting interpreter or download utility, consistent with code execution from a malicious .cleanshot file (CVE-2026-16477 deserialization). Auto-generated starting point — validate and tune in your environment before deploying. IOC matches can false-positive on shared infrastructure and decay as adversary infrastructure rotates.
title: CleanShot X Spawning Shell or Scripting Interpreter
description: Detects the CleanShot X macOS application spawning a shell, scripting
interpreter or download utility. The screenshot tool does not normally launch these,
so a child like this suggests code execution through insecure deserialization of
a crafted .cleanshot project file (CVE-2026-16477).
tags:
- attack.execution
- attack.t1203
- attack.t1204.002
logsource:
category: process_creation
product: macos
detection:
selection_parent:
ParentImage|contains: /CleanShot X.app/Contents/
selection_child:
Image|endswith:
- /sh
- /bash
- /zsh
- /osascript
- /curl
- /python
- /python3
- /perl
- /ruby
- /nscurl
condition: selection_parent and selection_child
falsepositives:
- Custom CleanShot X post-capture automation or user-configured scripts that call
a shell
- Application updater or helper routines that invoke curl or sh on some versions
level: high
id: 30cb48ae-91a2-53b5-9ca7-6dd8a92bc977
status: experimental
author: Vorant
references:
- https://cert.pl/en/posts/2026/10/CVE-2026-16477
Behavioural rules are generated from public reporting — validate in your environment before deploying.
Source reporting: https://cert.pl/en/posts/2026/10/CVE-2026-16477
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