An approach for hiding sensitive data in log files is provided. The approach uses a background program embedded within an operating system. The background program intercepts a write operation of an application and, in response to determining that the write operation is for a log file, gets content of the write operation. The background program checks a knowledge base that comprises information for the sensitive data and identifies the sensitive data in the content of the write operation. The background program masks the sensitive data in the content of the write operation, in response to determining that the content of the write operation has the sensitive data to be masked. In the log file, the background program writes modified content in which the sensitive data is masked.