Mishandling private information, such as customer passwords or social security numbers, can compromise user privacy, and is often illegal.
Privacy violations occur when:
- Private user information enters the program.
- The data is written to an external location, such as the console, file system, or network.
Private data can enter a program in a variety of ways:
- Directly from the user in the form of a password or personal information
- Accessed from a database or other data store by the application
- Indirectly from a partner or other third party
Sometimes data that is not labeled as private can have a privacy implication in a different context. For example, student identification numbers are usually not considered private because there is no explicit and publicly-available mapping to an individual student's personal information. However, if a school generates identification numbers based on student social security numbers, then the identification numbers should be considered private.
Security and privacy concerns often seem to compete with each other. From a security perspective, you should record all important operations so that any anomalous activity can later be identified. However, when private data is involved, this practice can in fact create risk.
Although there are many ways in which private data can be handled unsafely, a common risk stems from misplaced trust. Programmers often trust the operating environment in which a program runs, and therefore believe that it is acceptable to store private information on the file system, in the registry, or in other locally-controlled resources. However, even if access to certain resources is restricted, this does not guarantee that the individuals who do have access can be trusted. For example, in 2004, an unscrupulous employee at AOL sold approximately 92 million private customer e-mail addresses to a spammer marketing an offshore gambling web site .
In response to such high-profile exploits, the collection and management of private data is becoming increasingly regulated. Depending on its location, the type of business it conducts, and the nature of any private data it handles, an organization may be required to comply with one or more of the following federal and state regulations:
- Safe Harbor Privacy Framework 
- Gramm-Leach Bliley Act (GLBA) 
- Health Insurance Portability and Accountability Act (HIPAA) 
- California SB-1386 
Despite these regulations, privacy violations continue to occur with alarming frequency.
The following code contains a logging statement that tracks the contents of records added to a database by storing them in a log file. Among other values that are stored, the getPassword() function returns the user-supplied plaintext password associated with the account.
pass = getPassword(); ... dbmsLog.println(id+":"+pass+":"+type+":"+tstamp);
The code in the example above logs a plaintext password to the filesystem. Although many developers trust the filesystem as a safe storage location for data, it should not be trusted implicitly, particularly when privacy is a concern.
-  J. Oates. AOL man pleads guilty to selling 92m email addresses. The Register, 2005. http://www.theregister.co.uk/2005/02/07/aol_email_theft/
-  Safe Harbor Privacy Framework. U.S. Department of Commerce. http://www.export.gov/safeharbor/.
-  Financial Privacy: The Gramm-Leach Bliley Act (GLBA). Federal Trade Commission. http://www.ftc.gov/privacy/privacyinitiatives/glbact.html.
-  Health Insurance Portability and Accountability Act (HIPAA). U.S. Department of Human Services. http://www.hhs.gov/ocr/hipaa/.
-  California SB-1386. Government of the State of California, 2002. http://info.sen.ca.gov/pub/01-02/bill/sen/sb_1351-1400/sb_1386_bill_20020926_chaptered.html.