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Input Validation with Regular Expressions. COEN 351. Input Validation. Security Strategies Black List List all things that are NOT allowed List is difficult to create Adding insecure constructs on a continuous basis means that the previous version was unsafe
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Input Validation • Security Strategies • Black List • List all things that are NOT allowed • List is difficult to create • Adding insecure constructs on a continuous basis means that the previous version was unsafe • Testing is based on known attacks. • List from others might not be trustworthy. • White List • List of things that are allowed • List might be incomplete and disallow good content • Adding exceptions on a continuous basis does not imply security holes in previous versions. • Testing can be based on known attacks. • List from others can be trusted if source can be trusted.
Perl Regular Expressions • Regular Expression = Pattern • Template that either matches or does not match a string
Excursus: Getting Input in Perl • Use <STDIN> to read from standard input • Use ‘defined’ construct to tell if read was successful while(defined($line=<STDIN>)) { print “I saw $line”; }
Excursus: Getting Input in Perl • Non-sensical shortcut • Uses standard loop variable $_ Gets line, executes body of loop. while(<STDIN>) { print "I saw $_"; } Gets all the lines, then executes body of loop. $_ is the default loop variable. foreach(<STDIN>) { print "I saw $_"; }
Excursus: Getting Input in Perl • The STDIN is a default • chomp acts on default variable $_ while(<>) { chomp; print "I saw $_\n"; }
Perl Regular Expressions • Matching and substitution are fundamental tasks in Perl • Implemented using one letter operators: • m/PATTERN/ • m// • pattern matching • s/PATTERN/REPLACEMENT/ • s/// • Substitution
Perl Regular Expressions • Meta-characters in a pattern need escaping with backslash • \ • | • ( ) • [ ] • { } • ^ • $ • * • + • ?
Perl Regular Expressions • Interpolation • Perl substitutes strings in strings: $foo = “bar”; /$foo$/; • Equivalent to: /bar$/;
Perl Regular Expression:Binding Operator • Pattern matching is so frequent in Perl that there is a special operator • Normally, pattern matching is done on default operand $_ • =~ binds a string expression to a pattern match (substitution, transliteration)
Perl Regular Expression:Binding Operator • =~ has left operand a string • =~ has right operand a pattern • Could be interpreted at run time. • Returns true / false depending on the success of match. • !~ operation is the same, but result is negated.
Perl Regular Expression:Binding Operator $_ =~ $pat; is equivalent to $_ =~ /$pat/; but is less efficient since giving the pattern directly since the regular expression will be recompiled at run time
Perl Regular Expression:Binding Operator Example if ( ($k,$v) = $string =~ m/(\w+)=(\w*)/) { print “Key $k Value $v\n”; } Since =~ has precedence over =, it is evaluated first. The binding operator binds variable $string to a pattern looking for expressions like “ key=word. The binding expression is done in a list context, hence, the resulting matches are returned as a list. The list is then assigned to ($k,$v). The result of the assignment is the number of things assigned, i.e. typically 2. Since 2 is not 0, this is equivalent to true and hence the if-block is entered.
Perl Regular Expressions • Qualifiers: • * matches the preceding character zero or more times. • Pattern “abc*d” is matched by • rabd • zabccccd • Use parentheses to group letters #/perl/bin/perl while(<>) { chomp; last if $_ eq 'stop'; if (/abc*d /) { print "Matched: |$`<$&>$'|\n"; } else { print "No match.\n"; } } #/perl/bin/perl while(<>) { chomp; last if $_ eq 'stop'; if (/a(bc)*d /) { print "Matched: |$`<$&>$'|\n"; } else { print "No match.\n"; } }
Perl Regular Expressions • Qualifiers: • ‘*’ matches zero or more instances • ‘+’ matches one or more instances • “ab(cde)+fg” • ‘?’ matches none or one
Perl Regular Expressions • Alternatives • ‘|’ “or” • Either the right or the left side matches
Perl Regular Expressions • Character Classes • List of possible characters inside a square bracket • Example: • [a-cw-z]+ • [a-zA-Z0-9] • Negation provided by caret • [^n\-z] matches any character but ‘n’, ‘-’, ‘z’
Perl Regular Expressions • Character classes shortcuts • \w (word) is a shortcut for [A-Za-z0-9] • \s (space) is a shortcut for [\f\t\n\r ] • \d (digit) is a shortcut for [0-9] • [^\d] anything but a digit • [^\s] anything but a space character • [^\w] anything but a word character
Perl Regular Expressions • Perl regex semantics are based on: • Greed • Perl tries to match as much of an expression as is possible • Eagerness • Perl gives the first possible match • The left-most match wins • Backtracking • The entire expression needs to match • Perl regex evaluation backtracks if match is impossible
Perl Regular Expressions • Eagerness Example: • What is the result of this snippet $string = “boo hoo“; $string =~ s/o*/e/; #left side of =~ needs to be an l-value boo hoo be hoo bee hoo boo heo boo hee eboo hoo
Perl Regular Expressions • Quantifiers *, +, ? are not always enough • Specify number of occurrences by placing comma separated range in curly brackets • /a{2,12}/ • 2 to 12 ‘a’ • /a{5,}/ • 5 or more ‘a’ • /a{5}/ • exactly 5 ‘a’
Perl Regular Expressions • Anchors • pattern can match everywhere in the string unless you use anchors • ^ beginning of string • $ end of string • /b start or end of a group of w-characters • /B non-word boundary anchor • Examples: • /^hello/ matches only at beginning of string • /world$/ matches only at the end of string
Perl Regular Expressions • Parentheses and Memory • ( ) group together part of a pattern • Also remember corresponding match part of string. • These are put into a backreference • Made by backslash followed by number • Available as $1, … after matching • Examples • /(.)\1/ matches any character followed by itself • /../ matches any two characters • /([‘”]).*\1/ matches any string starting with single or double quotes followed by zero or more arbitrary characters followed by the same type of quotes. • “doesn’t match’ • “does match” • ‘does match’
Perl Regular ExpressionsValidating e-mail • Out of channel verification: • Ask for email addresses twice to weed out typos. • Send email to address given. • Still need to prevent command-line insertion • Lookup of DNS records for MX records • Assumes site connectivity • Regular expressions • Typically have subtle errors • tom&jerry@warnerbros.com is valid, but fails simple regex • president@whitehouse.gov is valid, deliverable, but probably fake
Perl Regular ExpressionsValidating email • if ( $email =~ /\@/ ) { … } • checks for an ampersand • if ( $email =~ /\S+\@\S+/ ) • checks for non-white space characters divided by an ampersand • matches thomas@hotmail • if ( $email =~ /\S+\@\S+\.\S+ ) • if ( $email =~ /[\w\-]+\@[\w\-]+\.[\w\-]+/ • matches most valid emails, but allows multiple emails • if ( $email =~ /^[\w\-]+\@[\w\-]+\.[\w\-]+$/ • anchored at beginning and end of word
Perl Regular Expressions • Checking for strings that only contain alphabetic characters. • ASCII based regex is insufficient: • if($var =~ /^[a-zA-Z]+$/) • Does not work for characters with diacritic marks • Best solution is to use Unicode properties • if($var =~ /^[^\W\d_]+$/) • Explanation: • \w matches alphabetic, numeric, underscore (alphanumunder) • \W is a non-alphanumunder • [^\W\d_] is a character that is neither non-alphanumunder, digit, or underscore, hence an alphabetic character • Could also use POSIX character classes, but those depend on locale
Perl Regular Expressions • Making regex readable • Place semantic units into a variable with an appropriate name $optional_sign = ‘[-+]?‘; $mandatory_digits = ‘\d+’; $decimal_point = ‘\.?’; $optinonal_digits = ‘\d*’; $number = $optional_sign .$mandatory_digits .$decimal_point .$optional_digits; if ( /($number)/) { … }
