Manual Reference Pages - SPROG (7)
- secure programming practices
Security issues have crept into many systems over the years.
This document is a guide for programming practices that prevent these problems.
Writing secure applications takes a very scrutinous and pessimistic outlook.
Applications should be run with the principle of
so that no process is ever running with more than the bare minimum access it
needs to accomplish its function.
Previously tested code should be reused whenever possible.
Generally, anything beyond the control of a program should never be trusted.
This includes all forms of user input, system resources, interprocess
communication, and the timing of events.
One of the most common types of security problems is the buffer overflow.
In short, if a program is not careful with the data it receives, it may be
possible for this data to be written across memory, overwriting the return
address for a function call, and the program will be forced to run code that
does unfriendly things.
A good number of functions in the standard C library make it difficult or
even impossible to prevent buffer overflows when used.
Many other functions that deal with strings can also open up a potential
buffer overflow when not used carefully.
does not go out of its way to provide
Of course, the proper length must always be specified.
ensure that strings are null terminated and of the specified length.
Functions that receive a string format must also be used carefully.
It is possible for a string to contain additional format specifiers, which
open up another possibility for a buffer overflow.
Never pass a string with untrusted data without using
Always use the proper secure idiom:
function( %s, string);
There are mechanisms that provide a backstop for these problems at the
library and compiler levels, however, there is no substitute for simply
writing good code.
In many cases, it may be necessary for a program to operate with an increased
set of permissions.
Reasons for this include binding to protected sockets, reading and writing
certain files and directories, and access to various resources.
Using a setuid program is frequently the solution.
However, it is important that programs give up these privileges as soon as
For example, if a program is binding to a protected socket, it should give
up its privileges as soon as it has finished binding to that socket.
This is accomplished with the
family of system calls.
The traditional method of restricting a process is with the
This system call changes the root directory from which all other paths are
referenced for a process and any child processes.
Of course, the process must have access to this path to begin with.
The new environment does not actually take effect until
is called to place the process into the new environment.
Unfortunately, a process can break out of this environment if root access is
can be used to create a more complete and enclosed environment than
A jail limits all processes inside that environment, including processes with
Fine grained privileges, as described by
extensions, are currently a work in progress, and the focus of the
More information can be found at
Programs should not make assumptions about the environment in which they are
This includes user input, signals, environment variables, system resources,
interprocess communications, and shared memory, amongst other things that are
beyond the control of the program.
They should not assume that all forms of invalid data can be detected either.
Instead, they should use positive filtering, and only allow a specific subset
of inputs that are known to be safe.
This is the same logic that an administrator should apply to a firewall, that
is, deny by default and specify what is to be accepted.
A race condition is anomalous behavior caused by the relative timing of
Programs should not assume that a particular event will occur before another.
The most common causes of race conditions are signals, access checks, and
Signals are asynchronous by nature, so special care must be taken
while dealing with them.
Attempting to check access with sequential non-atomic operations is a very
bad idea, as files can be moved and changed at any given time.
Instead of using a sequence of
and then call
.An Eric Melville Aq eric@FreeBSD.org
originally wrote this document based on a chapter of the
.An Murray Stokely Aq murray@FreeBSD.org .
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