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The vulnerability CVE-2025-66548 exploits Right-to-Left Override (RTLO) Unicode control characters to spoof file extensions in Nextcloud Deck. When a user uploads or downloads a file, its name is rendered by the web browser. An attacker can craft a filename containing an RTLO character (U+202E). This character reverses the display order of all subsequent characters, while the actual file data and extension remain unchanged. For example, a file saved as “malwareexe.pdf” could be displayed as “fdp.exe” if the RTLO character is placed strategically before the “exe” portion. This tricks users into believing they are opening a safe PDF document when they are actually executing a malicious `.exe` file. The attack relies on user interaction, requiring them to download and open the spoofed file. The vulnerability exists in the file handling logic of Nextcloud Deck, which did not sanitize or detect these deceptive filenames before display, allowing the spoof to occur within the application’s interface.
Platform: Nextcloud Deck
Version: Prior 1.12.7
Vulnerability: RTLO Spoofing
Severity: Critical
date: 12/05/2025
Prediction: Q1 2026 Patch
What Undercode Say:
Showing bash commands and codes related to the blog
Create a filename with an RTLO character printf "malware\xe2\x80\xaexe.pdf" > spoofed_file The filename 'spoofed_file' will display as 'fdp.exe' in vulnerable interfaces
Python snippet to demonstrate RTLO in a string
rtlo_char = u'\u202E'
filename = f"malware{rtlo_char}exe.pdf"
print(f"Stored name: {repr(filename)}")
print(f"Display may show as: {filename[::-1]} or 'fdp.exe'")
How Exploit:
1. Attacker crafts file.
2. Uploads to Deck.
3. User downloads spoof.
4. User executes malware.
Protection from this CVE
Update to Nextcloud Deck version 1.12.7, 1.14.4, or 1.15.1. Implement server-side filename validation to strip or reject RTLO control characters. Use client-side security tools that detect spoofed extensions.
Impact:
Phishing Attacks
Malware Execution
Data Breach Risk
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Sources:
Reported By: nvd.nist.gov
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