Inside popular software

Javascript

• Javascript can connect to other sites
• Javascript can be used to do port scans

PDF

• PDF files can now contain Javascript

C Compiler Hack

• Ken Thompson wrote a self-producing code fragment
• Modify the C compiler to detect that it's compiling login, enable backdoor.

sendmail

• Eric Allman, authoer of sendmail, installed a password-protected back door because he didn't have development access to a production system.