X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlsec.pod;h=dd237a08f02f3cb7a674a6686d1468456e0bf979;hb=8269e00da02a2e0f107fbb8b4a78f0c4058f3587;hp=41f96691aca4905d665c752766fa6d87f27ebaed;hpb=7b3f70378c41657f3e0c917f322e2cda58f33b5e;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlsec.pod b/pod/perlsec.pod index 41f9669..dd237a0 100644 --- a/pod/perlsec.pod +++ b/pod/perlsec.pod @@ -32,10 +32,10 @@ program more secure than the corresponding C program. You may not use data derived from outside your program to affect something else outside your program--at least, not by accident. All command line arguments, environment variables, locale information (see -L), results of certain system calls (readdir(), -readlink(), the variable of shmread(), the messages returned by -msgrcv(), the password, gcos and shell fields returned by the -getpwxxx() calls), and all file input are marked as "tainted". +L), results of certain system calls (C, +C, the variable of C, the messages returned by +C, the password, gcos and shell fields returned by the +C calls), and all file input are marked as "tainted". Tainted data may not be used directly or indirectly in any command that invokes a sub-shell, nor in any command that modifies files, directories, or processes, B: @@ -65,12 +65,14 @@ in which case they are able to run arbitrary external code. =back -The value of an expression containing tainted data will itself be -tainted, even if it is logically impossible for the tainted data to -affect the value. +For efficiency reasons, Perl takes a conservative view of +whether data is tainted. If an expression contains tainted data, +any subexpression may be considered tainted, even if the value +of the subexpression is not itself affected by the tainted data. Because taintedness is associated with each scalar value, some -elements of an array can be tainted and others not. +elements of an array or hash can be tainted and others not. +The keys of a hash are never tainted. For example: @@ -127,13 +129,29 @@ For example: If you try to do something insecure, you will get a fatal error saying something like "Insecure dependency" or "Insecure $ENV{PATH}". +The exception to the principle of "one tainted value taints the whole +expression" is with the ternary conditional operator C. Since code +with a ternary conditional + + $result = $tainted_value ? "Untainted" : "Also untainted"; + +is effectively + + if ( $tainted_value ) { + $result = "Untainted"; + } else { + $result = "Also untainted"; + } + +it doesn't make sense for C<$result> to be tainted. + =head2 Laundering and Detecting Tainted Data To test whether a variable contains tainted data, and whose use would thus trigger an "Insecure dependency" message, you can use the -tainted() function of the Scalar::Util module, available in your +C function of the Scalar::Util module, available in your nearby CPAN mirror, and included in Perl starting from the release 5.8.0. -Or you may be able to use the following I function. +Or you may be able to use the following C function. sub is_tainted { return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 }; @@ -147,7 +165,8 @@ approach is used that if any tainted value has been accessed within the same expression, the whole expression is considered tainted. But testing for taintedness gets you only so far. Sometimes you have just -to clear your data's taintedness. The only way to bypass the tainting +to clear your data's taintedness. Values may be untainted by using them +as keys in a hash; otherwise the only way to bypass the tainting mechanism is by referencing subpatterns from a regular expression match. Perl presumes that if you reference a substring using $1, $2, etc., that you knew what you were doing when you wrote the pattern. That means using @@ -176,7 +195,7 @@ Laundering data using regular expression is the I mechanism for untainting dirty data, unless you use the strategy detailed below to fork a child of lesser privilege. -The example does not untaint $data if C is in effect, +The example does not untaint C<$data> if C is in effect, because the characters matched by C<\w> are determined by the locale. Perl considers that locale definitions are untrustworthy because they contain data from outside the program. If you are writing a @@ -195,18 +214,43 @@ line, so you may need to use something like C<-wU> instead of C<-w -U> under such systems. (This issue should arise only in Unix or Unix-like environments that support #! and setuid or setgid scripts.) +=head2 Taint mode and @INC + +When the taint mode (C<-T>) is in effect, the "." directory is removed +from C<@INC>, and the environment variables C and C +are ignored by Perl. You can still adjust C<@INC> from outside the +program by using the C<-I> command line option as explained in +L. The two environment variables are ignored because +they are obscured, and a user running a program could be unaware that +they are set, whereas the C<-I> option is clearly visible and +therefore permitted. + +Another way to modify C<@INC> without modifying the program, is to use +the C pragma, e.g.: + + perl -Mlib=/foo program + +The benefit of using C<-Mlib=/foo> over C<-I/foo>, is that the former +will automagically remove any duplicated directories, while the later +will not. + +Note that if a tainted string is added to C<@INC>, the following +problem will be reported: + + Insecure dependency in require while running with -T switch + =head2 Cleaning Up Your Path -For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to a -known value, and each directory in the path must be non-writable by others -than its owner and group. You may be surprised to get this message even -if the pathname to your executable is fully qualified. This is I -generated because you didn't supply a full path to the program; instead, -it's generated because you never set your PATH environment variable, or -you didn't set it to something that was safe. Because Perl can't -guarantee that the executable in question isn't itself going to turn -around and execute some other program that is dependent on your PATH, it -makes sure you set the PATH. +For "Insecure C<$ENV{PATH}>" messages, you need to set C<$ENV{'PATH'}> to +a known value, and each directory in the path must be absolute and +non-writable by others than its owner and group. You may be surprised to +get this message even if the pathname to your executable is fully +qualified. This is I generated because you didn't supply a full path +to the program; instead, it's generated because you never set your PATH +environment variable, or you didn't set it to something that was safe. +Because Perl can't guarantee that the executable in question isn't itself +going to turn around and execute some other program that is dependent on +your PATH, it makes sure you set the PATH. The PATH isn't the only environment variable which can cause problems. Because some shells may use the variables IFS, CDPATH, ENV, and @@ -224,26 +268,26 @@ privileges. Perl doesn't prevent you from opening tainted filenames for reading, so be careful what you print out. The tainting mechanism is intended to prevent stupid mistakes, not to remove the need for thought. -Perl does not call the shell to expand wild cards when you pass B -and B explicit parameter lists instead of strings with possible shell -wildcards in them. Unfortunately, the B, B, and +Perl does not call the shell to expand wild cards when you pass C +and C explicit parameter lists instead of strings with possible shell +wildcards in them. Unfortunately, the C, C, and backtick functions provide no such alternate calling convention, so more subterfuge will be required. Perl provides a reasonably safe way to open a file or pipe from a setuid or setgid program: just create a child process with reduced privilege who does the dirty work for you. First, fork a child using the special -B syntax that connects the parent and child by a pipe. Now the +C syntax that connects the parent and child by a pipe. Now the child resets its ID set and any other per-process attributes, like environment variables, umasks, current working directories, back to the originals or known safe values. Then the child process, which no longer -has any special permissions, does the B or other system call. +has any special permissions, does the C or other system call. Finally, the child passes the data it managed to access back to the parent. Because the file or pipe was opened in the child while running under less privilege than the parent, it's not apt to be tricked into doing something it shouldn't. -Here's a way to do backticks reasonably safely. Notice how the B is +Here's a way to do backticks reasonably safely. Notice how the C is not called with a string that the shell could expand. This is by far the best way to call something that might be subjected to shell escapes: just never call the shell at all. @@ -307,7 +351,7 @@ outlaw scripts with any set-id bit set, which doesn't help much. Alternately, it can simply ignore the set-id bits on scripts. If the latter is true, Perl can emulate the setuid and setgid mechanism when it notices the otherwise useless setuid/gid bits on Perl scripts. It does -this via a special executable called B that is automatically +this via a special executable called F that is automatically invoked for you if it's needed. However, if the kernel set-id script feature isn't disabled, Perl will @@ -334,12 +378,12 @@ of the set-id script to open to the interpreter, rather than using a pathname subject to meddling, it instead passes I. This is a special file already opened on the script, so that there can be no race condition for evil scripts to exploit. On these systems, Perl should be -compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The B +compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>. The F program that builds Perl tries to figure this out for itself, so you should never have to specify this yourself. Most modern releases of SysVr4 and BSD 4.4 use this approach to avoid the kernel race condition. -Prior to release 5.6.1 of Perl, bugs in the code of B could +Prior to release 5.6.1 of Perl, bugs in the code of F could introduce a security hole. =head2 Protecting Your Programs