a2p
accept
access
acct
addftinfo
addr2line
adjtime
afmtodit
after
aio_cancel
aio_error
aio_read
aio_return
aio_suspend
aio_waitcomplete
aio_write
alias
aliases
alloc
anvil
append
apply
apropos
ar
array
as
asa
asn1parse
at
atq
atrm
attemptckalloc
attemptckrealloc
authlib
authtest
autopoint
awk
b64decode
b64encode
basename
batch
bc
bdes
bell
bg
bgerror
biff
big5
binary
bind
bindkey
bindtags
bindtextdomain
bio
bitmap
blowfish
bn
bootparams
bootptab
bounce
brandelf
break
breaksw
brk
bsdiff
bsdtar
bsnmpd
bspatch
bthost
btsockstat
buffer
builtin
builtins
bunzip2
button
byacc
bzcat
bzegrep
bzfgrep
bzgrep
bzip2
c2ph
c89
c99
ca
cal
calendar
canvas
cap_mkdb
case
cat
catch
catman
cc
cd
cdcontrol
chdir
checkbutton
checknr
chflags
chfn
chgrp
chio
chkey
chmod
chown
chpass
chroot
chsh
ci
ciphers
ckalloc
ckdist
ckfree
ckrealloc
cksum
cleanup
clear
clipboard
clock
clock_getres
clock_gettime
clock_settime
close
cmp
co
col
colcrt
colldef
colors
colrm
column
comm
command
compile_et
complete
compress
concat
config
connect
console
continue
core
courierlogger
couriertcpd
cp
cpan
cpio
cpp
creat
crl
crontab
crunchgen
crunchide
crypt
crypto
csh
csplit
ctags
ctm
ctm_dequeue
ctm_rmail
ctm_smail
cu
cursor
cursors
cut
cvs
date
dbiprof
dbiproxy
dc
dcgettext
dcngettext
dd
dde
default
defer
deliverquota
des
destroy
devfs
df
dgettext
dgst
dh
dhparam
dialog
diff
diff3
dig
dir
dirent
dirname
dirs
discard
disktab
dngettext
do
domainname
done
dprofpp
dsa
dsaparam
dtmfdecode
du
dup
dup2
eaccess
ec
ecdsa
echo
echotc
ecparam
ed
edit
editrc
ee
egrep
elf
elfdump
elif
else
enc
enc2xs
encoding
end
endif
endsw
engine
enigma
entry
env
envsubst
eof
eqn
err
errno
error
errstr
esac
ethers
euc
eui64
eval
event
evp
ex
exec
execve
exit
expand
export
exports
expr
extattr
extattr_delete_fd
extattr_delete_file
extattr_get_fd
extattr_get_file
extattr_set_fd
extattr_set_file
f77
false
famm
famx
fblocked
fbtab
fc
fchdir
fchflags
fchmod
fchown
fcntl
fconfigure
fcopy
fdescfs
fdformat
fdread
fdwrite
fetch
fg
fgrep
fhopen
fhstat
fhstatfs
fi
file
file2c
fileevent
filename
filetest
find
find2perl
finger
flex
flock
flush
fmt
focus
fold
font
fontedit
for
foreach
fork
format
forward
fpathconf
frame
from
fs
fstab
fstat
fstatfs
fsync
ftp
ftpchroot
ftpusers
ftruncate
futimes
g711conv
gb2312
gb18030
gbk
gcc
gcore
gcov
gdb
gencat
gendsa
genrsa
gensnmptree
getconf
getdents
getdirentries
getdtablesize
getegid
geteuid
getfacl
getfh
getfsstat
getgid
getgroups
getitimer
getlogin
getopt
getopts
getpeername
getpgid
getpgrp
getpid
getppid
getpriority
getresgid
getresuid
getrlimit
getrusage
gets
getsid
getsockname
getsockopt
gettext
gettextize
gettimeofday
gettytab
getuid
glob
global
gmake
goto
gperf
gprof
grab
grep
grid
grn
grodvi
groff
groff_font
groff_out
groff_tmac
grog
grolbp
grolj4
grops
grotty
group
groups
gunzip
gzcat
gzexe
gzip
h2ph
h2xs
hash
hashstat
hd
head
help2man
hesinfo
hexdump
history
host
hostname
hosts
hosts_access
hosts_options
hpftodit
http
hup
i386_get_ioperm
i386_get_ldt
i386_set_ioperm
i386_set_ldt
i386_vm86
iconv
id
ident
idprio
if
ifnames253
ifnames259
image
imapd
incr
indent
indxbib
info
infokey
inode
install
instmodsh
interp
intro
introduction
ioctl
ipcrm
ipcs
ipf
ipftest
ipnat
ippool
ipresend
issetugid
jail
jail_attach
jobid
jobs
join
jot
kbdcontrol
kbdmap
kcon
kdestroy
kdump
kenv
kevent
keycap
keylogin
keylogout
keymap
keysyms
kgdb
kill
killall
killpg
kinit
kldfind
kldfirstmod
kldload
kldnext
kldstat
kldsym
kldunload
klist
kpasswd
kqueue
kse
kse_create
kse_exit
kse_release
kse_switchin
kse_thr_interrupt
kse_wakeup
ktrace
label
labelframe
lam
lappend
last
lastcomm
lastlog
lchflags
lchmod
lchown
ld
ldap
ldapadd
ldapcompare
ldapdelete
ldapmodify
ldapmodrdn
ldappasswd
ldapsearch
ldapwhoami
ldd
leave
less
lesskey
lex
lgetfh
lhash
libnetcfg
library
limit
limits
lindex
link
linprocfs
linsert
lint
lio_listio
list
listbox
listen
lj4_font
lkbib
llength
lmtp
ln
load
loadfont
local
locale
locate
lock
lockf
log
logger
login
logins
logname
logout
look
lookbib
lorder
lower
lp
lpq
lpr
lprm
lptest
lrange
lreplace
ls
lsearch
lseek
lset
lsort
lstat
lsvfs
lutimes
lynx
m4
madvise
magic
mail
maildiracl
maildirkw
maildirmake
mailq
mailx
make
makeinfo
makewhatis
man
manpath
master
mc
mcedit
mcview
md2
md4
md5
mdc2
memory
menu
menubar
menubutton
merge
mesg
message
mincore
minherit
minigzip
mkdep
mkdir
mkfifo
mkimapdcert
mklocale
mknod
mkpop3dcert
mkstr
mktemp
mlock
mlockall
mmap
mmroff
modfind
modfnext
modnext
modstat
moduli
more
motd
mount
mprotect
mptable
msdos
msdosfs
msgattrib
msgcat
msgcmp
msgcomm
msgconv
msgen
msgexec
msgfilter
msgfmt
msggrep
msginit
msgmerge
msgs
msgunfmt
msguniq
mskanji
msql2mysql
msync
mt
munlock
munlockall
munmap
mv
myisamchk
myisamlog
myisampack
mysql
mysqlaccess
mysqladmin
mysqlbinlog
mysqlcheck
mysqld
mysqldump
mysqld_multi
mysqld_safe
mysqlhotcopy
mysqlimport
mysqlshow
mysql_config
mysql_fix_privilege_tables
mysql_zap
namespace
nanosleep
nawk
nc
ncal
ncplist
ncplogin
ncplogout
neqn
netconfig
netgroup
netid
netstat
networks
newaliases
newgrp
nex
nfsstat
nfssvc
ngettext
nice
nl
nm
nmount
nohup
nologin
notify
nroff
nseq
nslookup
ntp_adjtime
ntp_gettime
nvi
nview
objcopy
objdump
objformat
ocsp
od
onintr
open
openssl
opieaccess
opieinfo
opiekey
opiekeys
opiepasswd
option
options
oqmgr
pack
package
packagens
pagesize
palette
pam_auth
panedwindow
parray
passwd
paste
patch
pathchk
pathconf
pawd
pax
pbm
pcre
pcreapi
pcrebuild
pcrecallout
pcrecompat
pcrecpp
pcregrep
pcrematching
pcrepartial
pcrepattern
pcreperform
pcreposix
pcreprecompile
pcresample
pcretest
perl
perl56delta
perl58delta
perl561delta
perl570delta
perl571delta
perl572delta
perl573delta
perl581delta
perl582delta
perl583delta
perl584delta
perl585delta
perl586delta
perl587delta
perl588delta
perl5004delta
perl5005delta
perlaix
perlamiga
perlapi
perlapio
perlapollo
perlartistic
perlbeos
perlbook
perlboot
perlbot
perlbs2000
perlbug
perlcall
perlcc
perlce
perlcheat
perlclib
perlcn
perlcompile
perlcygwin
perldata
perldbmfilter
perldebguts
perldebtut
perldebug
perldelta
perldgux
perldiag
perldoc
perldos
perldsc
perlebcdic
perlembed
perlepoc
perlfaq
perlfaq1
perlfaq2
perlfaq3
perlfaq4
perlfaq5
perlfaq6
perlfaq7
perlfaq8
perlfaq9
perlfilter
perlfork
perlform
perlfreebsd
perlfunc
perlglossary
perlgpl
perlguts
perlhack
perlhist
perlhpux
perlhurd
perlintern
perlintro
perliol
perlipc
perlirix
perlivp
perljp
perlko
perllexwarn
perllinux
perllocale
perllol
perlmachten
perlmacos
perlmacosx
perlmint
perlmod
perlmodinstall
perlmodlib
perlmodstyle
perlmpeix
perlnetware
perlnewmod
perlnumber
perlobj
perlop
perlopenbsd
perlopentut
perlos2
perlos390
perlos400
perlothrtut
perlpacktut
perlplan9
perlpod
perlpodspec
perlport
perlqnx
perlre
perlref
perlreftut
perlrequick
perlreref
perlretut
perlrun
perlsec
perlsolaris
perlstyle
perlsub
perlsyn
perlthrtut
perltie
perltoc
perltodo
perltooc
perltoot
perltrap
perltru64
perltw
perlunicode
perluniintro
perlutil
perluts
perlvar
perlvmesa
perlvms
perlvos
perlwin32
perlxs
perlxstut
perror
pfbtops
pftp
pgrep
phones
photo
pic
pickup
piconv
pid
pipe
pkcs7
pkcs8
pkcs12
pkg_add
pkg_check
pkg_create
pkg_delete
pkg_info
pkg_sign
pkg_version
pkill
pl2pm
place
pod2html
pod2latex
pod2man
pod2text
pod2usage
podchecker
podselect
poll
popd
popup
posix_madvise
postalias
postcat
postconf
postdrop
postfix
postkick
postlock
postlog
postmap
postqueue
postsuper
pr
pread
preadv
printcap
printenv
printf
proc
procfs
profil
protocols
prove
proxymap
ps
psed
psroff
pstruct
ptrace
publickey
pushd
puts
pwd
pwrite
pwritev
qmgr
qmqpd
quota
quotactl
radiobutton
raise
rand
ranlib
rcp
rcs
rcsclean
rcsdiff
rcsfile
rcsfreeze
rcsintro
rcsmerge
read
readelf
readlink
readonly
readv
realpath
reboot
recv
recvfrom
recvmsg
red
ree
refer
regexp
registry
regsub
rehash
remote
rename
repeat
replace
req
reset
resolver
resource
return
rev
revoke
rfcomm_sppd
rfork
rhosts
ripemd
ripemd160
rlog
rlogin
rm
rmd160
rmdir
rpc
rpcgen
rs
rsa
rsautl
rsh
rtld
rtprio
rup
ruptime
rusers
rwall
rwho
s2p
safe
sasl
sasldblistusers2
saslpasswd2
sbrk
scache
scale
scan
sched
sched_getparam
sched_getscheduler
sched_get_priority_max
sched_get_priority_min
sched_rr_get_interval
sched_setparam
sched_setscheduler
sched_yield
scon
scp
script
scrollbar
sdiff
sed
seek
select
selection
semctl
semget
semop
send
sendbug
sendfile
sendmail
sendmsg
sendto
services
sess_id
set
setegid
setenv
seteuid
setfacl
setgid
setgroups
setitimer
setlogin
setpgid
setpgrp
setpriority
setregid
setresgid
setresuid
setreuid
setrlimit
setsid
setsockopt
settc
settimeofday
setty
setuid
setvar
sftp
sh
sha
sha1
sha256
shar
shells
shift
shmat
shmctl
shmdt
shmget
showq
shutdown
sigaction
sigaltstack
sigblock
sigmask
sigpause
sigpending
sigprocmask
sigreturn
sigsetmask
sigstack
sigsuspend
sigvec
sigwait
size
slapadd
slapcat
slapd
slapdn
slapindex
slappasswd
slaptest
sleep
slogin
slurpd
smbutil
smime
smtp
smtpd
socket
socketpair
sockstat
soelim
sort
source
spawn
speed
spinbox
spkac
splain
split
squid
squid_ldap_auth
squid_ldap_group
squid_unix_group
sscop
ssh
sshd_config
ssh_config
stab
startslip
stat
statfs
stop
string
strings
strip
stty
su
subst
sum
suspend
swapoff
swapon
switch
symlink
sync
sysarch
syscall
sysconftool
sysconftoolcheck
systat
s_client
s_server
s_time
tabs
tail
talk
tar
tbl
tclsh
tcltest
tclvars
tcopy
tcpdump
tcpslice
tcsh
tee
tell
telltc
telnet
term
termcap
terminfo
test
texindex
texinfo
text
textdomain
tfmtodit
tftp
then
threads
time
tip
tk
tkerror
tkvars
tkwait
tlsmgr
tmac
top
toplevel
touch
tput
tr
trace
trafshow
trap
troff
true
truncate
truss
tset
tsort
tty
ttys
type
tzfile
ui
ul
ulimit
umask
unalias
uname
uncomplete
uncompress
undelete
unexpand
unhash
unifdef
unifdefall
uniq
units
unknown
unlimit
unlink
unmount
unset
unsetenv
until
unvis
update
uplevel
uptime
upvar
usbhidaction
usbhidctl
users
utf8
utimes
utmp
utrace
uudecode
uuencode
uuidgen
vacation
variable
verify
version
vfork
vgrind
vgrindefs
vi
vidcontrol
vidfont
view
virtual
vis
vt220keys
vwait
w
wait
wait3
wait4
waitpid
wall
wc
wget
what
whatis
where
whereis
which
while
who
whoami
whois
window
winfo
wish
wm
write
writev
wtmp
x509
xargs
xgettext
xmlwf
xstr
xsubpp
yacc
yes
ypcat
ypchfn
ypchpass
ypchsh
ypmatch
yppasswd
ypwhich
yyfix
zcat
zcmp
zdiff
zegrep
zfgrep
zforce
zgrep
zmore
znew
_exit
__syscall
 
FreeBSD/Linux/UNIX General Commands Manual
Hypertext Man Pages
perlsec
 
PERLSEC(1)	       Perl Programmers Reference Guide 	    PERLSEC(1)



NAME
       perlsec - Perl security

DESCRIPTION
       Perl is designed to make it easy to program securely even when running
       with extra privileges, like setuid or setgid programs.  Unlike most
       command line shells, which are based on multiple substitution passes on
       each line of the script, Perl uses a more conventional evaluation
       scheme with fewer hidden snags.	Additionally, because the language has
       more builtin functionality, it can rely less upon external (and possi-
       bly untrustworthy) programs to accomplish its purposes.

       Perl automatically enables a set of special security checks, called
       taint mode, when it detects its program running with differing real and
       effective user or group IDs.  The setuid bit in Unix permissions is
       mode 04000, the setgid bit mode 02000; either or both may be set.  You
       can also enable taint mode explicitly by using the -T command line
       flag. This flag is strongly suggested for server programs and any pro-
       gram run on behalf of someone else, such as a CGI script. Once taint
       mode is on, it's on for the remainder of your script.

       While in this mode, Perl takes special precautions called taint checks
       to prevent both obvious and subtle traps.  Some of these checks are
       reasonably simple, such as verifying that path directories aren't
       writable by others; careful programmers have always used checks like
       these.  Other checks, however, are best supported by the language
       itself, and it is these checks especially that contribute to making a
       set-id Perl program more secure than the corresponding C program.

       You may not use data derived from outside your program to affect some-
       thing else outside your program--at least, not by accident.  All com-
       mand line arguments, environment variables, locale information (see
       perllocale), results of certain system calls ("readdir()", "read-
       link()", the variable of "shmread()", the messages returned by
       "msgrcv()", the password, gcos and shell fields returned by the "getp-
       wxxx()" 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, with the following exceptions:

       o   Arguments to "print" and "syswrite" are not checked for tainted-
	   ness.

       o   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.

       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 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 = ;	       # Also tainted
	   open FOO, "/home/me/bar" or die $!;
	   $line = ;	       # 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 Perl releases older than 5.6.0 the <*.c> and glob('*.c') would
	   # have used an external program to do the filename expansion; but in
	   # either case the result is 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}".

       The exception to the principle of "one tainted value taints the whole
       expression" is with the ternary conditional operator "?:".  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 $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, 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()" func-
       tion.

	   sub is_tainted {
	       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 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.  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 expres-
       sion 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 a bit of thought--don't just blindly untaint anything,
       or you defeat the entire mechanism.  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 spe-
       cial 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 con-
       taining "\w", put "no locale" ahead of the expression in the same
       block.  See "SECURITY" in perllocale for further discussion and exam-
       ples.

       Switches On the "#!" Line

       When you make a script executable, in order to make it usable as a com-
       mand, the system will pass switches to perl from the script's #!  line.
       Perl checks that any command line switches given to a setuid (or set-
       gid) 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 environ-
       ments that support #! and setuid or setgid scripts.)

       Taint mode and @INC

       When the taint mode ("-T") is in effect, the "." directory is removed
       from @INC, and the environment variables "PERL5LIB" and "PERLLIB" 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 later
       will not.

       Note that if a tainted string is added to @INC, the following problem
       will be reported:

	 Insecure dependency in require while running with -T switch

       Cleaning Up Your Path

       For "Insecure $ENV{PATH}" messages, you need to set $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 qual-
       ified.  This is not 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
       BASH_ENV, Perl checks that those are either empty or untainted when
       starting subprocesses. You may wish to 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.  Make judicious use of the file
       tests in dealing with any user-supplied filenames.  When possible, do
       opens and such after properly dropping any special user (or group!)
       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 "sys-
       tem" and "exec" explicit parameter lists instead of strings with possi-
       ble shell wildcards in them.  Unfortunately, the "open", "glob", 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 "open" 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 "open" 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 "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.

	       use English '-no_match_vars';
	       die "Can't fork: $!" unless defined($pid = open(KID, "-|"));
	       if ($pid) {	     # parent
		   while () {
		       # do something
		   }
		   close KID;
	       } else {
		   my @temp	= ($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 although you trust yourself not to
       have written a program to give away the farm, you don't necessarily
       trust those who end up using it not to try to trick it into doing some-
       thing 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.

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

       Security Bugs

       Beyond the obvious problems that stem 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) inter-
       preter 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.

       Fortunately, sometimes this kernel "feature" can be disabled.  Unfortu-
       nately, there are two ways to disable it.  The system can simply 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 suidperl that is automatically invoked
       for you if it's needed.

       However, if the kernel set-id script feature isn't disabled, Perl will
       complain loudly that your set-id script is insecure.  You'll need to
       either disable the kernel set-id script feature, or 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 sub-
       ject to the kernel bug that plagues set-id scripts.  Here's a simple
       wrapper, written in C:

	   #define REAL_PATH "/path/to/script"
	   main(ac, av)
	       char **av;
	   {
	       execv(REAL_PATH, av);
	   }

       Compile this wrapper into a binary executable and then make it rather
       than your script setuid or setgid.

       In recent years, vendors have begun to supply systems free of this
       inherent security bug.  On such systems, when the kernel passes the
       name of the set-id script to open to the interpreter, rather than using
       a pathname subject to meddling, it instead passes /dev/fd/3.  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 "-DSETUID_SCRIPTS_ARE_SECURE_NOW".  The Configure pro-
       gram 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 condi-
       tion.

       Prior to release 5.6.1 of Perl, bugs in the code of suidperl could
       introduce a security hole.

       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, however, 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.  If your pro-
       gram does insecure things, and relies on people not knowing how to
       exploit those insecurities, it is not secure.  It is often possible for
       someone to determine the insecure things and exploit them without view-
       ing the source.	Security through obscurity, 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-com-
       pile 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).

       If you're concerned about people profiting from your code, then the
       bottom line is that nothing but a restrictive licence 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
       stand up in court.

       Unicode

       Unicode is a new and complex technology and one may easily overlook
       certain security pitfalls.  See perluniintro for an overview and perlu-
       nicode for details, and "Security Implications of Unicode" in perluni-
       code for security implications in particular.

       Algorithmic Complexity Attacks

       Certain internal algorithms used in the implementation of Perl can be
       attacked by choosing the input carefully to consume large amounts of
       either time or space or both.  This can lead into the so-called Denial
       of Service (DoS) attacks.

       o   Hash Function - the algorithm used to "order" hash elements has
	   been changed several times during the development of Perl, mainly
	   to be reasonably fast.  In Perl 5.8.1 also the security aspect was
	   taken into account.

	   In Perls before 5.8.1 one could rather easily generate data that as
	   hash keys would cause Perl to consume large amounts of time because
	   internal structure of hashes would badly degenerate.  In Perl 5.8.1
	   the hash function is randomly perturbed by a pseudorandom seed
	   which makes generating such naughty hash keys harder.  See
	   "PERL_HASH_SEED" in perlrun 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 possi-
	   ble reason for wanting to emulate the old behaviour is that in the
	   new behaviour consecutive runs of Perl will order hash keys differ-
	   ently, which may confuse some applications (like Data::Dumper: the
	   outputs of two different runs are no more identical).

	   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 con-
	   tinues to be, affected by the insertion order.

	   Also note that while the order of the hash elements might be ran-
	   domised, this "pseudoordering" 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 per-
	   mutations (use e.g. the CPAN modules Algorithm::Permute or Algo-
	   rithm::FastPermute), or for any cryptographic applications.

       o   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 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 reading, see perlfaq2).  Running out of space manifests
	   itself by Perl running out of memory.

       o   Sorting - the quicksort algorithm used in Perls before 5.8.0 to
	   implement the sort() function is very easy to trick into misbehav-
	   ing 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.

       See  for more information, and
       any computer science text book on the algorithmic complexity.

SEE ALSO
       perlrun for its description of cleaning up environment variables.



perl v5.8.8			  2006-01-07			    PERLSEC(1)
=30123
+79
(5)