CVE-2026-40487: Arbitrary File Upload > Stored XSS > Account Takeover in Postiz
After obtaining my OSWE, I started appreciating white box analysis much more. So every now and then I pick an open-source project I like, analyze the source code, try to understand how it works, and when I come across critical functions I ask myself: “what if this didn’t work as expected?”. Today I’ll tell you how I discovered this Arbitrary File Upload via MIME-type spoofing that led to a Stored XSS in Postiz, an open-source social media management tool with over 600 instances exposed on the internet. The vulnerability allows an attacker to act as the victim: steal API keys, exfiltrate tokens from all connected social integrations (Instagram, X, LinkedIn, Facebook, TikTok and 23 other providers), publish and delete posts on their behalf, and create persistent OAuth backdoors that survive password changes. ...