It is a fact that software has bugs. These bugs can lead to failures. Especially dependability and security failures are a great threat to software users. This thesis introduces four novel approaches that can be used to automatically harden software at the user's site. Automatic hardening removes bugs from already deployed software. All four approaches are automated, i.e., they require little support from the end-user. However, some support from the software developer is needed for two of these approaches. The presented approaches can be grouped into error toleration and bug removal. The two error toleration approaches are focused primarily on fast detection of security errors. When an error is detected it can be tolerated with well-known existing approaches. The other two approaches are bug removal approaches. They remove dependability bugs from already deployed software. We tested all approaches with existing benchmarks and applications, like the Apache web-server.