PERLSEC(1)

📦
version 5.42.3 · 22 versions
, in fedora 43, fedora 44

✨ NAME ✨

perlsec — Perl security 🛡️

(that's straight outta the Perl Programmers Reference Guide, bestie, section 1 of the manual 💅)

💖 DESCRIPTION 💖

okay so first things first: Perl was literally designed so you can write secure code without losing your mind, even when your program is running with extra privileges — like setuid or setgid programs. 👑

Most command line shells do this thing where they do multiple substitution passes on every single line of your script, and that's where the sneaky bugs live. Perl doesn't do that. Perl uses a more conventional evaluation scheme with way fewer hidden snags. Less chaos, more slay. ✨

And because Perl just has more builtin functionality, it can rely less on external (and possibly untrustworthy 👀) programs to get stuff done. Fewer outside helpers = fewer ways to get betrayed.

🚨 SECURITY VULNERABILITY CONTACT INFORMATION 🚨

If you think you found a security vulnerability in the Perl interpreter, or in modules maintained in the core Perl codebase, email the deets to perl-security@perl.org <mailto:perl-security@perl.org>. 📧

Heads up: that address is a closed membership mailing list monitored by the Perl security team. So don't just vibe in there, okay? 🫶

See perlsecpolicy for additional information. 📄

🔐 SECURITY MECHANISMS AND CONCERNS 🔐

🌊 Taint mode 🌊

By default, Perl automatically switches on a whole set of special security checks called taint mode when it notices your program is running with differing real and effective user or group IDs. 😳

So like — the setuid bit in Unix permissions is mode 04000, and the setgid bit is mode 02000. Either one, or both, can be set. 👯

You can also turn taint mode on explicitly with the -T command line flag. This flag is strongly suggested for server programs and literally any program you run on behalf of someone else — think CGI scripts. 🖥️ And once taint mode is on? It's on for the rest of your script. No take-backs. Periodt.

While you're in this mode, Perl takes special precautions called taint checks to prevent both the obvious traps and the subtle ones. Some of these checks are pretty simple — like verifying that path directories aren't writable by others. Careful programmers have always done checks like that. But other checks are best supported by the language itself, and it's especially these checks that make a set-id Perl program more secure than the equivalent C program. 💪

ngl, support for taint checks adds overhead to all Perl programs, whether or not you're using the taint features. Perl 5.18 introduced C preprocessor symbols that can be used to disable the taint features when building perl. 🔧

When taint is enabled, you may not use data derived from outside your program to affect something else outside your program — at least, not by accident. 🚫

Here's the full list of stuff that gets marked as "tainted":

  • all command line arguments
  • environment variables
  • locale information (see perllocale)
  • 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
  • all file input

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. 😤 With the following exceptions:

  • Arguments to print and syswrite are not checked for taintedness. 🖨️
  • Symbolic methods
$obj->$method(@args);

and symbolic sub references

&{$foo}(@args);
$foo->(@args);

are not checked for taintedness. ⚠️ This requires extra carefulness unless you want external data to affect your control flow. Unless you carefully limit what these symbolic values are, people are able to call functions outside your Perl code, such as POSIX::system, in which case they are able to run arbitrary external code. 💀 (yeah. that's the scary one.)

  • Hash keys are never tainted. 🔑

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. 🤷♀️

And 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. 🎯

For example:

$arg = shift;               # $arg is tainted
$hid = $arg . 'bar';        # $hid is also tainted
$line = <>;                 # Tainted
$line = <STDIN>;            # Also tainted
open FOO, "/home/me/bar" or die $!;
$line = <FOO>;              # Still tainted
$path = $ENV{'PATH'};       # Tainted, but see below
$data = 'abc';              # Not tainted

system "echo $arg";         # Insecure
system "/bin/echo", $arg;   # Considered insecure
                            # (Perl doesn't know about /bin/echo)
system "echo $hid";         # Insecure
system "echo $data";        # Insecure until PATH set

$path = $ENV{'PATH'};       # $path now tainted

$ENV{'PATH'} = '/bin:/usr/bin';
delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'};

$path = $ENV{'PATH'};       # $path now NOT tainted
system "echo $data";        # Is secure now!

open(FOO, "< $arg");        # OK - read-only file
open(FOO, "> $arg");        # Not OK - trying to write

open(FOO,"echo $arg|");     # Not OK
open(FOO,"-|")
    or exec 'echo', $arg;   # Also not OK

$shout = `echo $arg`;       # Insecure, $shout now tainted

unlink $data, $arg;         # Insecure
umask $arg;                 # Insecure

exec "echo $arg";           # Insecure
exec "echo", $arg;          # Insecure
exec "sh", '-c', $arg;      # Very insecure!

@files = <*.c>;             # insecure (uses readdir() or similar)
@files = glob('*.c');       # insecure (uses readdir() or similar)

# In either case, the results of glob are tainted, since the list of
# filenames comes from outside of the program.

$bad = ($arg, 23);          # $bad will be tainted
$arg, `true`;               # Insecure (although it isn't really)

If you try to do something insecure, you will get a fatal error saying something like "Insecure dependency" or "Insecure $ENV{PATH}". 💥

Now — the one exception to the principle of "one tainted value taints the whole expression" is with the ternary conditional operator ?:. Since code with a ternary conditional like

$result = $tainted_value ? "Untainted" : "Also untainted";

is effectively

if ( $tainted_value ) {
    $result = "Untainted";
} else {
    $result = "Also untainted";
}

it doesn't make sense for $result to be tainted. 🧠✨

🧼 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. It's 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 is_tainted() function.

sub is_tainted {
    local $@;   # Don't pollute caller's value.
    return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 };
}

This function makes use of the fact that the presence of tainted data anywhere within an expression renders the entire expression tainted. It would be inefficient for every operator to test every argument for taintedness. Instead, the slightly more efficient and conservative approach is used: 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. 🧽

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. in a non-tainting pattern, that you knew what you were doing when you wrote that pattern. 👀 That means using a bit of thought — don't just blindly untaint anything, or you defeat the entire mechanism.

Bestie, listen: it's better to verify that the variable has only good characters (for certain values of "good") rather than checking whether it has any bad characters. That's because it's far too easy to miss bad characters that you never thought of. 🕵️♀️

Here's a test to make sure that the data contains nothing but "word" characters (alphabetics, numerics, and underscores), a hyphen, an at sign, or a dot.

if ($data =~ /^([-\@\w.]+)$/) {
    $data = $1;                     # $data now untainted
} else {
    die "Bad data in '$data'";      # log this somewhere
}

This is fairly secure because "/\w+/" doesn't normally match shell metacharacters, nor are dot, dash, or at going to mean something special to the shell. 🐚 Use of "/.+/" would have been insecure in theory because it lets everything through, but Perl doesn't check for that. The lesson is that when untainting, you must be exceedingly careful with your patterns. 😬

Laundering data using regular expression is the only 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 use locale is in effect, because the characters matched by "\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 locale-aware program, and want to launder data with a regular expression containing "\w", put no locale ahead of the expression in the same block. See "SECURITY" in perllocale for further discussion and examples. 🌍

🪄 Switches On the "#!" Line 🪄

When you make a script executable, in order to make it usable as a command, the system will pass switches to perl from the script's #! line. Perl checks that any command line switches given to a setuid (or setgid) script actually match the ones set on the #! line. ✅

Some Unix and Unix-like environments impose a one-switch limit on the #! line, so you may need to use something like "-wU" instead of "-w -U" under such systems. (This issue should arise only in Unix or Unix-like environments that support #! and setuid or setgid scripts.) 🐧

📚 Taint mode and @INC 📚

When taint mode ("-T") is in effect, the environment variables "PERL5LIB", "PERLLIB", and "PERL_USE_UNSAFE_INC" are ignored by Perl. 🚫

You can still adjust @INC from outside the program by using the "-I" command line option as explained in perlrun. 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 "-I" option is clearly visible and therefore permitted. 👁️

Another way to modify @INC without modifying the program is to use the "lib" pragma, e.g.:

perl -Mlib=/foo program

The benefit of using "-Mlib=/foo" over "-I/foo" is that the former will automagically remove any duplicated directories, while the latter will not. 🪄✨

So real talk, bestie: if a tainted string sneaks its way into @INC, here's the vibe you're gonna get:

Insecure dependency in require while running with -T switch

And okay, history lesson ✨: on Perl versions before 5.26, flipping on taint mode would also yeet the current directory (".") out of @INC's default value. Since 5.26 though, the current directory just isn't in @INC by default at all. Periodt.

🧼 Cleaning Up Your Path 🧼

Okay so if you're getting hit with Insecure $ENV{PATH} messages, here's the tea ☕: you gotta set $ENV{'PATH'} to a known value, and every single directory in that path has to be absolute and not writable by anyone who isn't the owner or the group. Careful though — on Unix-ish environments, an empty component of the PATH might get read as if it were "." (the local directory), and that'll trigger this message too. 🙈

And like, don't be shocked if you get this message even when the pathname to your executable is fully qualified. It's not mad because you didn't give a full path to the program — it's mad because you never set your PATH environment variable, or you set it to something that wasn't safe. The reason? Perl literally cannot guarantee that the executable in question won't turn around and run some other program that depends on your PATH. So it makes sure you set the PATH. It's giving protective bestie energy, fr fr. 🫶

And not me about to tell you — PATH is not the only environment variable that can cause problems. Because some shells use the variables IFS, CDPATH, ENV, and BASH_ENV, Perl checks that those are either empty or untainted when it starts subprocesses. So you might wanna add something like this to your setid and taint-checking scripts:

delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};   # Make %ENV safer

It's also possible to get into trouble with other operations that don't care whether they use tainted values. So be judicious with your file tests when you're dealing with user-supplied filenames. When you can, do opens and such after properly dropping any special user (or group!) privileges. Perl won't stop you from opening tainted filenames for reading, so watch what you print out. The whole tainting mechanism is here to prevent stupid mistakes — it is not here to remove the need for thought. It's not your mom, it's your safety net. 💀

Also, Perl does not call the shell to expand wild cards when you pass system and exec explicit parameter lists instead of strings with possible shell wildcards in them. Lowkey iconic.</> Unfortunately, the open, glob, and backtick functions don't offer that alternate calling convention, so you're gonna need more subterfuge. 🙃

But wait, there's a slay way to do this 👑: 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. Here's the steps:

  1. First, fork a child using the special open syntax that connects the parent and child by a pipe.
  2. Now the child resets its ID set and any other per-process attributes — environment variables, umasks, current working directories — back to the originals or known safe values.
  3. Then the child process, which no longer has any special permissions, does the open or other system call.
  4. Finally, the child passes the data it managed to access back to the parent.

And the reason this eats? Because the file or pipe was opened in the child while running under less privilege than the parent, so 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 exec 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. Periodt. 💅

use English;
die "Can't fork: $!" unless defined($pid = open(KID, "-|"));
if ($pid) {           # parent
    while (<KID>) {
        # do something
    }
    close KID;
} else {
    my @temp     = ($EUID, $EGID);
    my $orig_uid = $UID;
    my $orig_gid = $GID;
    $EUID = $UID;
    $EGID = $GID;
    # Drop privileges
    $UID  = $orig_uid;
    $GID  = $orig_gid;
    # Make sure privs are really gone
    ($EUID, $EGID) = @temp;
    die "Can't drop privileges"
        unless $UID == $EUID  && $GID eq $EGID;
    $ENV{PATH} = "/bin:/usr/bin"; # Minimal PATH.
    # Consider sanitizing the environment even more.
    exec 'myprog', 'arg1', 'arg2'
        or die "can't exec myprog: $!";
}

A similar strategy would work for wildcard expansion via glob, although you can use readdir instead. ✨

Taint checking is most useful when, like, you trust yourself not to have written a program that gives away the farm — but you don't necessarily trust the people who end up using it not to try to trick it into doing something bad. This is the kind of security checking that's useful for set-id programs and programs launched on someone else's behalf, like CGI programs. 🌸

Okay but this is quite different from not even trusting the writer of the code not to do something evil. That's the kind of trust you need when someone hands you a program you've never seen before and says, "Here, run this." 🫠 For that kind of safety, you might wanna check out the Safe module, which is included standard in the Perl distribution. This module lets the programmer set up special compartments where all system operations are trapped and namespace access is carefully controlled.

But heads up bestie: Safe should not be considered bullet-proof. It will not prevent foreign code from setting up infinite loops, allocating gigabytes of memory, or even abusing perl bugs to make the host interpreter crash or behave in unpredictable ways. And honestly? It's better avoided completely if you're really concerned about security. 😭

🐚 Shebang Race Condition 🐚

Beyond the obvious problems that come from giving special privileges to systems as flexible as scripts, on many versions of Unix, set-id scripts are inherently insecure right from the start. The problem is a race condition in the kernel. Between the time the kernel opens the file to see which interpreter to run and when the (now-set-id) interpreter turns around and reopens the file to interpret it, the file in question may have changed — especially if you have symbolic links on your system. 💀

Some Unixes, especially the more recent ones, are free of this inherent security bug. On those systems, when the kernel passes the name of the set-id script to open to the interpreter, rather than using a pathname that's subject to meddling, it instead passes /dev/fd/3. That's a special file that's already opened on the script, so there can be no race condition for evil scripts to exploit. 👑

On these systems, Perl should be compiled with -DSETUID_SCRIPTS_ARE_SECURE_NOW. The Configure 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. ✨

If you don't have the safe version of set-id scripts, all is not lost! Sometimes this kernel "feature" can be disabled, so that the kernel either doesn't run set-id scripts with the set-id, or doesn't run them at all. Either way avoids the exploitability of the race condition, but doesn't help in actually running scripts set-id. 🙃

If the kernel set-id script feature isn't disabled, then any set-id script provides an exploitable vulnerability. Perl can't avoid being exploitable, but it will point out vulnerable scripts where it can. If Perl detects that it's being applied to a set-id script, then it will complain loudly that your set-id script is insecure, and won't run it. When Perl complains, you need to remove the set-id bit from the script to eliminate the vulnerability. Refusing to run the script doesn't in itself close the vulnerability — it's just Perl's way of encouraging you to do this. 🫶

To actually run a script set-id, if you don't have the safe version of set-id scripts, you'll need to put a C wrapper around the script. A C wrapper is just a compiled program that does nothing except call your Perl program. Compiled programs are not subject to the kernel bug that plagues set-id scripts. Here's a simple wrapper, written in C:

#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>

#define REAL_PATH "/path/to/script"

int main(int argc, char **argv)
{
    execv(REAL_PATH, argv);
    fprintf(stderr, "%s: %s: %s\n",
                    argv[0], REAL_PATH, strerror(errno));
    return 127;
}

Compile this wrapper into a binary executable and then make it rather than your script setuid or setgid. Note though: this wrapper isn't doing anything to sanitise the execution environment other than ensuring that a safe path to the script is used. It only avoids the shebang race condition. It relies on Perl's own features, and on the script itself being careful, to make it safe enough to run the script set-id. No cap. 💅

🔒 Protecting Your Programs 🔒

There are a number of ways to hide the source to your Perl programs, with varying levels of "security."

First of all, though — you can't take away read permission, because the source code has to be readable in order to be compiled and interpreted. (That doesn't mean that a CGI script's source is readable by people on the web, though.) So you have to leave the permissions at the socially friendly 0755 level. This lets people on your local system only see your source. 🎀

Some people mistakenly regard this as a security problem. But bestie, if your program does insecure things and relies on people not knowing how to exploit those insecurities, it is not secure. It's often possible for someone to determine the insecure things and exploit them without viewing the source. "Security through obscurity" — which is the name for hiding your bugs instead of fixing them — is little security indeed. 🙈

You can try using encryption via source filters (Filter::* from CPAN, or Filter::Util::Call and Filter::Simple since Perl 5.8). But crackers might be able to decrypt it. You can try using the byte code compiler and interpreter described below, but crackers might be able to de-compile it. You can try using the native-code compiler described below, but crackers might be able to disassemble it. These pose varying degrees of difficulty to people wanting to get at your code, but none can definitively conceal it (this is true of every language, not just Perl). It's giving "the math is mathing." ✨

okay so if your whole personality is "people are gonna make BANK off my code and i cannot" 😤 then bestie, the brutal tea is this: the only thing that gives you actual legal security is a restrictive license. that's the bottom line. so license your software, and then go absolutely feral with threatening little 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.

and fr fr, go see a lawyer so you know your license wording will actually stand up in court. 💅

🦋 Unicode 🦋

Unicode is a whole new and complex technology, and it is SO easy to overlook certain security pitfalls in it. go read perluniintro for the overview and perlunicode for the details, and specifically "Security Implications of Unicode" in perlunicode if you want the security implications in particular. 💜

💀 Algorithmic Complexity Attacks 💀

Certain internal algorithms used in Perl's implementation can be attacked by choosing your input very carefully so that it eats up large amounts of time, or space, or both. and that, bestie, is how you end up with the so-called Denial of Service (DoS) attacks. not the vibe. 🙈

  • Hash Algorithm — hash algorithms like the one Perl uses are well known to be vulnerable to collision attacks on their hash function. the attack goes like this: construct a set of keys that all collide into the same bucket → inefficient behavior → suffering. these attacks usually depend on discovering the seed of the hash function that maps keys to buckets; once you've got that seed you can brute-force a key set and mount a denial of service attack. Perl 5.8.1 introduced changes to harden Perl against exactly this, and then later, in Perl 5.18.0, those features got enhanced and extra protections were added. ✨

At the time of this writing, Perl 5.18.0 is considered to be well-hardened against algorithmic complexity attacks on its hash implementation. this is largely thanks to the following measures, which mitigate attacks:

  • Hash Seed Randomization — so that it's impossible to know which seed to build an attack key set for, this seed is randomly initialized at process start. you can override it with the PERL_HASH_SEED environment variable, see "PERL_HASH_SEED" in perlrun. this environment variable controls how items are actually stored, not how they're presented via "keys", "values" and "each".
  • Hash Traversal Randomization — independent of which seed is used in the hash function, "keys", "values", and "each" return items in a per-hash randomized order. modify a hash by insertion and the iteration order of that hash changes. you can override this behavior with hash_traversal_mask() from Hash::Util, or with the PERL_PERTURB_KEYS environment variable, see "PERL_PERTURB_KEYS" in perlrun. note that this feature controls the visible order of the keys, not the actual order they're stored in. 👑
  • Bucket Order Perturbance — when items collide into a given hash bucket, the order they're stored in the chain is no longer predictable as of Perl 5.18. the whole intention here is to make it harder to observe a collision. override it with the PERL_PERTURB_KEYS environment variable, see "PERL_PERTURB_KEYS" in perlrun.
  • New Default Hash Function — the default hash function has been modified with the intention of making it harder to infer the hash seed.
  • Alternative Hash Functions — the source code includes multiple hash algorithms to choose from. while we believe the default perl hash is robust to attack, the hash function Siphash is included as a fall-back option. at the time of release of Perl 5.18.0, Siphash is believed to be of cryptographic strength. it is not the default because it's much slower than the default hash.

Without compiling a special Perl, there is no way to get the exact same behavior of any versions prior to Perl 5.18.0. the closest one can get is by setting PERL_PERTURB_KEYS to 0 and setting the PERL_HASH_SEED to a known value. we do not advise those settings for production use, because of all the security considerations above. 🙈

Perl has never guaranteed any ordering of the hash keys, and the ordering has already changed several times during the lifetime of Perl 5. also, the ordering of hash keys has always been — and continues to be — affected by the insertion order and the history of changes made to the hash over its lifetime.

Also note: even though the order of the hash elements might be randomized, this "pseudo-ordering" should not be used for applications like shuffling a list randomly (use List::Util::shuffle() for that, see List::Util, a standard core module since Perl 5.8.0; or the CPAN module "Algorithm::Numerical::Shuffle"), or for generating permutations (use e.g. the CPAN modules "Algorithm::Permute" or "Algorithm::FastPermute"), or for any cryptographic applications. periodt. 💅

Tied hashes may have their own ordering and algorithmic complexity attacks. 🌀

  • Regular expressions — Perl's regular expression engine is a so-called NFA (Non-deterministic Finite Automaton), which among other things means it can rather easily consume large amounts of both time and space if the regular expression may match in several ways. careful crafting of your regular expressions can help, but quite often there really isn't much one can do (the book "Mastering Regular Expressions" is required reading, see perlfaq2). running out of space manifests itself by Perl running out of memory. 😭
  • Sorting — the quicksort algorithm used in Perls before 5.8.0 to implement the sort() function was very easy to trick into misbehaving, 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. ate. 🔥

See <https://www.usenix.org/legacy/events/sec03/tech/full_papers/crosby/crosby.pdf> for more information, plus literally any computer science textbook on algorithmic complexity.

🫶 Using Sudo 🫶

The popular tool "sudo" gives users a controlled way to run programs as other users. it sanitises the execution environment to some extent, and it will avoid the shebang race condition. if you don't have the safe version of set-id scripts, then "sudo" may be a more convenient way of executing a script as another user than writing a C wrapper would be.

However — and this is the part where you sit up, bestie — "sudo" sets the real user or group ID to that of the target identity, not just the effective ID the way set-id bits do. as a result, Perl can't detect that it's running under "sudo", and so it won't automatically take its own security precautions, such as turning on taint mode. where "sudo" configuration dictates exactly which command can be run, the approved command may include a -T option to perl to enable taint mode.

in general, it is necessary to evaluate the suitability of a script to run under "sudo" specifically with that kind of execution environment in mind. it is neither necessary nor sufficient for the same script to be suitable to run in a traditional set-id arrangement, though many of the issues overlap. 🧚

💖 SEE ALSO 💖

  • "ENVIRONMENT" in perlrun for its description of cleaning up environment variables. 🌈

and that's a wrap on this whole page, bestie 💖 written against perl v5.42.3, dated 2026-08-03, and yeah, it's PERLSEC(1) — go forth and be secure, no cap. 🎀🦋✨

📦 where this page comes from