The server hosting dozens of free-to-use online file conversion websites has been hacked several times in the past year using a well-known, easy-to-use exploit.
The security researcher, who asked not to be named for fear of legal repercussions, told ZDNet that the attacker obtained "full root access" to the server and its contents.
The researcher said the level of access would allow an attacker to quietly exfiltrate any file uploaded to the sites, but said it was "impossible to tell" what the shells were for, or if they were in actively used.
The Paris-based server hosted sites -- including combinepdf.com, imagetopdf.com, jpg2pdf.com, pdftoimage.com, pdfcompressor.com, and wordtojpeg.com, among others -- that let users convert files and documents to other formats.
These are hardly the most popular sites in the world, but thousands of people use the sites each day, based on various traffic metrics and statistics sites. Key search terms like "pdf convert" and "image convert" bring up several of the affected sites in the first page of Google search results, giving them an edge over other conversion sites.
