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Add assertion to test XPath filters against an allow-list for axes and functions
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3 files changed

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‎src/Assert/Assert.php‎

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<?php
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declare(strict_types=1);
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namespace SimpleSAML\XML\Assert;
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use BadMethodCallException; // Requires ext-spl
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use DateTime; // requires ext-date
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use DateTimeImmutable; // requires ext-date
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use InvalidArgumentException; // Requires ext-spl
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use SimpleSAML\Assert\Assert as BaseAssert;
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use SimpleSAML\Assert\AssertionFailedException;
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use Throwable;
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use function array_pop;
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use function array_unshift;
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use function call_user_func_array;
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use function end;
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use function enum_exists;
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use function function_exists;
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use function get_class;
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use function is_object;
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use function is_resource;
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use function is_string;
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use function is_subclass_of;
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use function lcfirst;
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use function method_exists;
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use function preg_match; // Requires ext-pcre
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use function strval;
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/**
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* SimpleSAML\XML\Assert\Assert wrapper class
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*
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* @package simplesamlphp/xml-common
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*
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* @method static void allowedXPathFilter(mixed $value, array $allowed_axes, $array $allowed_functions, string $message = '', string $exception = '')
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* @method static void nullOrAllowedXPathFilter(mixed $value, array $allowed_axes, $array $allowed_functions, string $message = '', string $exception = '')
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* @method static void allAllowedXPathFilter(mixed $value, array $allowed_axes, $array $allowed_functions, string $message = '', string $exception = '')
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*/
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final class Assert
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{
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use CustomAssertionTrait;
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/**
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* @param string $name
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* @param array<mixed> $arguments
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*/
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public static function __callStatic(string $name, array $arguments): void
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{
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// Handle Exception-parameter
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$exception = AssertionFailedException::class;
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$last = end($arguments);
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if (is_string($last) && class_exists($last) && is_subclass_of($last, Throwable::class)) {
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$exception = $last;
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array_pop($arguments);
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}
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try {
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if (method_exists(static::class, $name)) {
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call_user_func_array([static::class, $name], $arguments);
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return;
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} elseif (preg_match('/^nullOr(.*)$/i', $name, $matches)) {
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$method = lcfirst($matches[1]);
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if (method_exists(static::class, $method)) {
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call_user_func_array([static::class, 'nullOr'], [[static::class, $method], $arguments]);
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} elseif (method_exists(BaseAssert::class, $method)) {
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call_user_func_array([static::class, 'nullOr'], [[BaseAssert::class, $method], $arguments]);
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} else {
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throw new BadMethodCallException(sprintf("Assertion named `%s` does not exists.", $method));
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}
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} elseif (preg_match('/^all(.*)$/i', $name, $matches)) {
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$method = lcfirst($matches[1]);
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if (method_exists(static::class, $method)) {
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call_user_func_array([static::class, 'all'], [[static::class, $method], $arguments]);
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} elseif (method_exists(BaseAssert::class, $method)) {
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call_user_func_array([static::class, 'all'], [[BaseAssert::class, $method], $arguments]);
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} else {
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throw new BadMethodCallException(sprintf("Assertion named `%s` does not exists.", $method));
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}
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} else {
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throw new BadMethodCallException(sprintf("Assertion named `%s` does not exists.", $name));
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}
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} catch (InvalidArgumentException $e) {
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throw new $exception($e->getMessage());
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}
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}
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/**
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* Handle nullOr* for either Webmozart or for our custom assertions
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*
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* @param callable $method
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* @param array<mixed> $arguments
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* @return void
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*/
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private static function nullOr(callable $method, array $arguments): void
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{
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$value = reset($arguments);
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($value === null) || call_user_func_array($method, $arguments);
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}
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/**
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* all* for our custom assertions
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*
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* @param callable $method
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* @param array<mixed> $arguments
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* @return void
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*/
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private static function all(callable $method, array $arguments): void
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{
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$values = array_pop($arguments);
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foreach ($values as $value) {
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$tmp = $arguments;
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array_unshift($tmp, $value);
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call_user_func_array($method, $tmp);
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}
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}
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/**
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* @param mixed $value
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*
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* @return string
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*/
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protected static function valueToString(mixed $value): string
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{
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if (is_resource($value)) {
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return 'resource';
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}
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if (null === $value) {
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return 'null';
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}
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if (true === $value) {
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return 'true';
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}
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if (false === $value) {
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return 'false';
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}
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if (is_array($value)) {
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return 'array';
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}
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if (is_object($value)) {
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if (method_exists($value, '__toString')) {
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return $value::class . ': ' . self::valueToString($value->__toString());
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}
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if ($value instanceof DateTime || $value instanceof DateTimeImmutable) {
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return $value::class . ': ' . self::valueToString($value->format('c'));
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}
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if (function_exists('enum_exists') && enum_exists(get_class($value))) {
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return get_class($value) . '::' . $value->name;
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}
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return $value::class;
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}
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if (is_string($value)) {
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return '"' . $value . '"';
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}
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return strval($value);
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}
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}

‎src/Assert/CustomAssertionTrait.php‎

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<?php
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declare(strict_types=1);
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namespace SimpleSAML\XML\Assert;
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use InvalidArgumentException;
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use SimpleSAML\Assert\Assert as BaseAssert;
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use SimpleSAML\XML\Constants as C;
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use function in_array;
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use function preg_match_all;
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use function preg_replace;
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use function sprintf;
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use function strlen;
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/**
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* @package simplesamlphp/xml-common
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*/
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trait CustomAssertionTrait
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{
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/**
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* Remove the content from all single or double-quoted strings in $input, leaving only quotes.
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* Use possessive quantifiers (i.e. *+ and ++ instead of * and + respectively) to prevent backtracking.
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*
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* '/(["\'])(?:(?!\1).)*+\1/'
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* (["\']) # Match a single or double quote and capture it in group 1
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* (?: # Start a non-capturing group
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* (?! # Negative lookahead
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* \1 # Match the same quote as in group 1
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* ) # End of negative lookahead
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* . # Match any character (that is not a quote, because of the negative lookahead)
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* )*+ # Repeat the non-capturing group zero or more times, possessively
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* \1 # Match the same quote as in group 1
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*/
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private static string $regex_xpfilter_remove_strings = '/(["\'])(?:(?!\1).)*+\1/';
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/**
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* Function names are lower-case alpha (i.e. [a-z]) and can contain one or more hyphens,
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* but cannot start or end with a hyphen. To match this, we start with matching one or more
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* lower-case alpha characters, followed by zero or more atomic groups that start with a hyphen
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* and then match one or more lower-case alpha characters. This ensures that the function name
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* cannot start or end with a hyphen, but can contain one or more hyphens.
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* More than one consecutive hyphen does not match.
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*
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* '/([a-z]++(?>-[a-z]++)*+)\s*+\(/'
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* ( # Start a capturing group
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* [a-z]++ # Match one or more lower-case alpha characters
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* (?> # Start an atomic group (no capturing)
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* - # Match a hyphen
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* [a-z]++ # Match one or more lower-case alpha characters, possessively
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* )*+ # Repeat the atomic group zero or more times,
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* ) # End of the capturing group
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* \s*+ # Match zero or more whitespace characters, possessively
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* \( # Match an opening parenthesis
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*/
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private static string $regex_xpfilter_functions = '/([a-z]++(?>-[a-z]++)*+)\\s*+\\(/';
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/**
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* We use the same rules for matching Axis names as we do for function names.
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* The only difference is that we match the '::' instead of the '('
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* so everything that was said about the regular expression for function names
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* applies here as well.
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*
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* '/([a-z]++(?>-[a-z]++)*+)\s*+::'
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* ( # Start a capturing group
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* [a-z]++ # Match one or more lower-case alpha characters
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* (?> # Start an atomic group (no capturing)
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* - # Match a hyphen
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* [a-z]++ # Match one or more lower-case alpha characters, possessively
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* )*+ # Repeat the atomic group zero or more times,
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* ) # End of the capturing group
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* \s*+ # Match zero or more whitespace characters, possessively
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* \( # Match an opening parenthesis
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*/
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private static string $regex_xpfilter_axes = '/([a-z]++(?>-[a-z]++)*+)\\s*+::/';
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/***********************************************************************************
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* NOTE: Custom assertions may be added below this line. *
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* They SHOULD be marked as `private` to ensure the call is forced *
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* through __callStatic(). *
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* Assertions marked `public` are called directly and will *
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* not handle any custom exception passed to it. *
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***********************************************************************************/
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/**
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* Check an XPath expression for allowed axes and functions
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* The goal is preventing DoS attacks by limiting the complexity of the XPath expression by only allowing
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* a select subset of functions and axes.
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* The check uses a list of allowed functions and axes, and throws an exception when an unknown function
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* or axis is found in the $xpath_expression.
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*
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* Limitations:
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* - The implementation is based on regular expressions, and does not employ an XPath 1.0 parser. It may not
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* evaluate all possible valid XPath expressions correctly and cause either false positives for valid
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* expressions or false negatives for invalid expressions.
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* - The check may still allow expressions that are not safe, I.e. expressions that consist of only
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* functions and axes that are deemed "save", but that are still slow to evaluate. The time it takes to
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* evaluate an XPath expression depends on the complexity of both the XPath expression and the XML document.
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* This check, however, does not take the XML document into account, nor is it aware of the internals of the
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* XPath processor that will evaluate the expression.
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* - The check was written with the XPath 1.0 syntax in mind, but should work equally well for XPath 2.0 and 3.0.
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*
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* @param string $value
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* @param array $allowed_axes
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* @param array $allowed_functions
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* @param string $message
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*/
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private static function allowedXPathFilter(
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string $value,
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array $allowed_axes = C::DEFAULT_ALLOWED_AXES,
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array $allowed_functions = C::DEFAULT_ALLOWED_FUNCTIONS,
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string $message = '',
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): void {
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BaseAssert::maxLength(
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$value,
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C::XPATH_FILTER_MAX_LENGTH,
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sprintf('XPath Filter exceeds the limit of 100 characters.'),
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);
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$strippedValue = preg_replace(
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self::$regex_xpfilter_remove_strings,
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// Replace the content with two of the quotes that were matched
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"\\1\\1",
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$value,
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);
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if ($strippedValue === null) {
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throw new Exception("Error in preg_replace.");
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}
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/**
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* Check if the $xpath_expression uses an XPath function that is not in the list of allowed functions
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*
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* Look for the function specifier '(' and look for a function name before it.
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* Ignoring whitespace before the '(' and the function name.
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* All functions must match a string on a list of allowed function names
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*/
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$matches = [];
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$res = preg_match_all(self::$regex_xpfilter_functions, $strippedValue, $matches);
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if ($res === null) {
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throw new Exception("Error in preg_match_all.");
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}
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// Check that all the function names we found are in the list of allowed function names
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foreach ($matches[1] as $match) {
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if (!in_array($match, $allowed_functions)) {
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throw new InvalidArgumentException(sprintf(
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$message ?: '\'%s\' is not an allowed XPath function.',
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$match,
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));
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}
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}
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/**
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* Check if the $xpath_expression uses an XPath axis that is not in the list of allowed axes
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*
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* Look for the axis specifier '::' and look for a function name before it.
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* Ignoring whitespace before the '::' and the axis name.
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* All axes must match a string on a list of allowed axis names
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*/
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$matches = [];
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$res = preg_match_all(self::$regex_xpfilter_axes, $strippedValue, $matches);
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if ($res === null) {
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throw new Exception("Error in preg_match_all.");
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}
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// Check that all the axes names we found are in the list of allowed axes names
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foreach ($matches[1] as $match) {
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if (!in_array($match, $allowed_axes)) {
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throw new InvalidArgumentException(sprintf(
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$message ?: '\'%s\' is not an allowed XPath axis.',
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$match,
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));
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}
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}
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}
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}

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