The Ten Immutable Laws of Security (taken from here. Via here.)
1. If a bad guy can persuade you to run his program on your computer, it's not your computer anymore.
2. If a bad guy can alter the operating system on your computer, it's not your computer anymore.
3. If a bad guy has unrestricted physical access to your computer, it's not your computer anymore.
4. If you allow a bad guy to upload programs to your web site, it's not your web site any more.
5. Weak passwords trump strong security.
6. A machine is only as secure as the administrator is trustworthy.
7. Encrypted data is only as secure as the decryption key.
8. An out of date virus scanner is only marginally better than no virus scanner at all.
9. Absolute anonymity isn't practical, in real life or on the web.
10. Technology is not a panacea.