The use of Linux in systems requiring high levels of security has been a frequent topic of controversy. Supporters have claimed that Linux's open source approach improves security due to exposure to an enormous, worldwide community of developers and users.

Detractors have maintained that the complexity and architecture of Linux make it unsuitable for high criticality applications. With a lifetime of over 15 years, there are now plenty of public statistics with which to analyze Linux security.

Linux in Government Security Systems
Some powerful organizations have sided with the supporters. Linux is the trusted operating system in HP's NetTop [1], the IBM Trusted Thin Client [2], and the General Dynamics Trusted Virtual Environment (TVE) - a product of NSA's High Assurance Platform (HAP) program [3].

All of these products are designed to consolidate computers used by government personnel to access classified and unclassified networks. The specialized computer provides multiple "virtual" desktops and is trusted to protect sensitive information.

In order to prepare it better it for the task of becoming the "touching point" between physically distinct networks, Linux was enhanced by the NSA's National Information Assurance Research Laboratory with additional security controls, known as Security-Enhanced Linux (SELinux)[4]. The SELinux extensions have been adopted by the Linux community and these systems.

Along with their investment in Linux, these military suppliers have made forceful claims about the trustworthiness of these products. According to General Dynamics, the TVE provides "high robustness" and a "quantum leap in the way military and government security levels are accessed." [5]

It is interesting to note, however, that the NSA's developers were careful not to claim suitability for high criticality systems, stating that SELinux is "very unlikely by itself to meet any interesting definition of secure system."[6] Furthermore, the SELinux effort has included "no work focused upon increasing the assurance of Linux itself." [7]

Vulnerabilities in Linux
While many discussions about the security of Linux have been clouded by hyperbole and commercial agendas, a number of independent resources, many published by the Linux community, are painting a more complete, unbiased picture about Linux security.

In the first release of Linux 2.6 (2.6.0), the Linux kernel consisted of more than 5 million lines of code.10 In 2.6.30, that number has grown to over 11 million lines.[8]

In addition, Linux development follows general commercial practices, not compliant with any stringent safety or security standard. While Linux's open source exposure has enabled it to achieve a low defect rate relative to most commercial software, the size of the kernel assures a large dose of flaws. In 2004, an automated static analysis tool discovered almost 1,000 bugs in the Linux kernel.[9]

NIST and the DHS National Security Cyber Division publish a catalog, the National Vulnerability Database (NVD), of security defects in commercial software products. As of August 16, 2009, a search on Linux yields 1288 entries, 457 of which are considered "High Severity."[10]

134 high severity vulnerabilities are associated with the Linux kernel. The NVD reports 91, 77, and 87 Linux kernel vulnerabilities for each of the years 2006, 2007, and 2008, respectively[11].

It is statistically assured that a similar number will be found in future years, implying that numerous vulnerabilities exist in today's shipping version. These numbers of course do not account for unreported defects.

Some critical software components gain assurance over time. This occurs when the software is relatively simple, changes very little (except perhaps for bug fixes), and is deployed for a long period of time in a variety of environments.

The Linux kernel, however, undergoes continuous modification, including in the field (e.g. over-the-air patching). The latest major version of Linux, 2.6, has changed more rapidly than previous versions and regularly undergoes major modifications to the "stable branch" of the kernel.[12]

As an example, Linux developer Greg Kroah-Hartman reported that the 2.6.24 kernel saw approximately 5,000 lines of code added per day during a three month period, prompting his lament, "It's fricken scarily amazing that things are still working at all"[13]

The rate of change has been accelerating. Kroah-Hartman reported that over 12,000 lines of code were added per day on average during the 2.6.30 development cycle[14]. Since 2005, the Linux kernel has been modified by over 5000 different people [15], at a rate which now exceeds 6 changes per hour.[16]

Another good example was provided by Jim Ready, founder of Linux vendor Montavista, who discussed NVD defect CVE-2006-1528 which was patched in Linux version 2.6.13. In order to get the bug fix in a supported release, a user running 2.6.10 would have been forced to take in 846,233 new lines of code (representing the changes between 2.6.10 and 2.6.13) [17].

On August 10, 2009, a memory leak in the SELinux security extensions was published in the NVD.[18] A few days later, five more vulnerabilities were published.

One of these, CVE-2009-2692, reports a severe kernel defect that can be trivially exploited by a user to take complete control of the system.[19] This vulnerability exists in every Linux operating system deployed over the past eight years.