From: Alex Holst Sent: 18 April 2002 03:49 To: dev@subversion.tigris.org Subject: Subversion and assurance. Hi. I've been bribed with bananas again. This time the guilty party is gstein who requested that I post a note with my thoughts about security and assurance, and what steps can be taken to reduce the possible number of security flaws in subversion 1.0. First, a brief introduction: When people ask you, as a developer, about security in Subversion, you might say Subversion is secure. Subversion has access control, it supports SSL, committers need no system accounts, and other nice things. These are _security_ features, not nessesarily _secure_ features. You may have access control, but what if the code implementing this access control was written poorly, and contains a buffer overflow? 2 hours ago you worried about who could read or write to a document in your repository. Now you discover that an attacker can execute arbitary code as the userid your service is running as. This is not ideal. Hence, we distinquish between "security features" and assurance. Brian Snow, a technical director at the NSA, defines assurance as follows: "Confidence-building activities that demonstrate that a system possesses the desired properties and only these properties and that functions are implemented correctly. Assurance can be provided through a structured design process, documentation, and testing." Assurance is what protects the user in the case of misuse or when faced with malice. Today, cars come with safety functions such as seatbelts, ABS breaks, airbags, etc, all of which means that you have a very good chance of walking away from accidents. This was not so 50 years ago. I strongly recommend listening to Brian Snow's full talk on assurance, which is available as a RealPlayer stream from Blackhat.com: The two most important steps that Subversion can take are: Establish secure coding guidelines that are communicated to all developers and enforced by the project leads. Improve the documentation: A diagram much like qmail's Big Picture which shows how code and data flows within the program. It allows for fast identification of security boundaries. These steps will enable greatly improved looks into the Subversion code for someone who has not spent the last few months getting familiar with the Subversion code. Additional steps include: Establish a QA section on the website containing documentation about the tests that are run against Subversion. Document how new tests for both server and client can be written and encourage users who are in need of assurance to participate in the QA process. The tests against the server should specifically include things like attempting to break ACLs, attempt to issue legal commands in an inproper order, use very long strings for filenames and arguments, etc. The more you document, the more likely it is that someone with the knowledge to spot problems will take a look at what you have done. Websites that help: "Secure Programming for Linux and UNIX" by David Wheeler http://www.dwheeler.com/secure-programs/ Software Quality Assurance: Documentation and Review http://hissa.ncsl.nist.gov/publications/nistir4909/ Books that help: "Safer C" by Les Hatton "Solid Software" by Hatton, Howell & Pfleeger "Building Secure Software" by Viega & McGraw "Writing Secure Code" by Howard & LeBlanc "Writing Solid Software" by Maguire I'll be delighted to answer any questions. Thanks for your time. -- I prefer the dark of the night, after midnight and before four-thirty, when it's more bare, more hollow. http://a.area51.dk/