How an Insecure 'File Upload' Took Down 4chan and Key Insights!
Last week, a hacker breached 4chan (the infamous internet forum) and posted: "U GOT HACKED!"
Hours later, they leaked backend access, source code and moderator identities. Total chaos.
How did 4chan, a site built on anonymity for years, fall apart?
It all started with a simple 'File Upload' bug!
๐๐๐๐ฎ๐ฐ๐ธ ๐๐น๐ผ๐ (as per hacker's own post):
Attacker finds a functionality in 4chan site that allows user to upload PDF files to message forums.
Attacker identifies that the app does not validate whether itโs a PDF file or not > Creates a malicious PostScript file.
Attacker uploads it as a normal file > 4chanโs backend (running Ghostscript) attempts to process the file.
The malicious file triggers remote code execution (RCE) on the backend server > Attacker gets access to server!
Attacker now uses a misconfigured SUID binary > Escalates privileges to root (more on this later) > Game over!
Attacker accesses and exfiltrates sensitive internal data > Stolen data is leaked publicly > Anonymous identities of moderators leaked > Site is down for more than a week.
Key Insights:
If you think about it, multiple things lead to this catastrophe - an insecure file upload, an outdated interpreter software, a misconfigured SUID. In isolation, each one of these may not seem deadly enough but when combined they can be devastating.
Half of problems in application security comes down to one core principle: Never trust anything coming the client side. Not text. Not inputs. Not files.
Files like PDF or PostScript contain not only text but also instructions for rendering. Those instructions are processed by the interpreter. If you have a vulnerable interpreter, it can be tricked into running commands.
A file is not what it claims to be. A file is what it actually contains. Validate deeply to confirm. Check MIME types, inspect content, scan for malware.
Good security assumes things will eventually break and attempts to minimize the damage. Had 4chan isolated their file processor, the blast radius of the attack could have been limited and not let to its entire collapse.
Outdated software don't age out quietly. Its like a ticking time bomb. It sits quietly in the background. And one day, it blows everything apart.
SUID (Set User ID) binaries run with the permissions of their owner, not the user executing them. When misconfigured, they become privilege escalation highways. The attacker found a SUID program owned by root but accessible to all users. This allowed them to execute commands with administrative privileges.
Its been over a week since the attack and 4chan is still down. Lesson: You don't prepare for war during war, you prepare during peace.