X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlsec.pod;h=59980d6a86edc36d58fc7f63d7ebf4f24ccb9a97;hb=9f2f055aa1e8c86d97b5ea42473ab1747f518f3a;hp=5a09e32d8eceb42f4f0f3cf861bc4e4ff4f9ad47;hpb=595bde10f833ec6ce0053cdb47ce14644ea67e2d;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlsec.pod b/pod/perlsec.pod index 5a09e32..59980d6 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: @@ -63,6 +63,10 @@ you carefully limit what these symbolic values are, people are able to call functions B your Perl code, such as POSIX::system, in which case they are able to run arbitrary external code. +=item * + +Hash keys are B tainted. + =back For efficiency reasons, Perl takes a conservative view of @@ -72,7 +76,7 @@ of the subexpression is not itself affected by the tainted data. Because taintedness is associated with each scalar value, some elements of an array or hash can be tainted and others not. -The keys of a hash are never tainted. +The keys of a hash are B tainted. For example: @@ -129,11 +133,27 @@ 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 C function. @@ -179,7 +199,7 @@ 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 @@ -218,18 +238,23 @@ 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 @@ -247,26 +272,26 @@ 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. @@ -330,7 +355,7 @@ 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 @@ -357,12 +382,12 @@ 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 @@ -395,11 +420,11 @@ code, but none can definitively conceal it (this is true of every language, not just Perl). If you're concerned about people profiting from your code, then the -bottom line is that nothing but a restrictive licence will give you +bottom line is that nothing but a restrictive license will give you legal security. License your software and pepper it with threatening statements like "This is unpublished proprietary software of XYZ Corp. Your access to it does not give you permission to use it blah blah -blah." You should see a lawyer to be sure your licence's wording will +blah." You should see a lawyer to be sure your license's wording will stand up in court. =head2 Unicode @@ -432,13 +457,16 @@ the hash function is randomly perturbed by a pseudorandom seed which makes generating such naughty hash keys harder. See L for more information. -The random perturbation is done by default but if one wants for some -reason emulate the old behaviour one can set the environment variable -PERL_HASH_SEED to zero (or any other integer). One possible reason -for wanting to emulate the old behaviour is that in the new behaviour -consecutive runs of Perl will order hash keys differently, which may -confuse some applications (like Data::Dumper: the outputs of two -different runs are no more identical). +In Perl 5.8.1 the random perturbation was done by default, but as of +5.8.2 it is only used on individual hashes if the internals detect the +insertion of pathological data. If one wants for some reason emulate the +old behaviour (and expose oneself to DoS attacks) one can set the +environment variable PERL_HASH_SEED to zero to disable the protection +(or any other integer to force a known perturbation, rather than random). +One possible reason for wanting to emulate the old behaviour is that in the +new behaviour consecutive runs of Perl will order hash keys differently, +which may confuse some applications (like Data::Dumper: the outputs of two +different runs are no longer identical). B, and the ordering has already changed several times during the lifetime of @@ -455,9 +483,9 @@ Algorithm::FastPermute), or for any cryptographic applications. =item * -Regular expressions - Perl's regular expression engine is so called -NFA (Non-Finite Automaton), which among other things means that it can -rather easily consume large amounts of both time and space if the +Regular expressions - Perl's regular expression engine is so called NFA +(Non-deterministic Finite Automaton), which among other things means that +it can rather easily consume large amounts of both time and space if the regular expression may match in several ways. Careful crafting of the regular expressions can help but quite often there really isn't much one can do (the book "Mastering Regular Expressions" is required @@ -468,15 +496,14 @@ Perl running out of memory. Sorting - the quicksort algorithm used in Perls before 5.8.0 to implement the sort() function is very easy to trick into misbehaving -so that it consumes a lot of time. Nothing more is required than -resorting a list already sorted. Starting from Perl 5.8.0 a different -sorting algorithm, mergesort, is used. Mergesort is insensitive to -its input data, so it cannot be similarly fooled. +so that it consumes a lot of time. Starting from Perl 5.8.0 a different +sorting algorithm, mergesort, is used by default. Mergesort cannot +misbehave on any input. =back See L for more information, -and any computer science text book on the algorithmic complexity. +and any computer science textbook on algorithmic complexity. =head1 SEE ALSO