Package com.google.json

Class JsonSanitizer

  • public final class JsonSanitizer
extends Object
    Given JSON-like content, converts it to valid JSON. This can be attached at either end of a data-pipeline to help satisfy Postel's principle:
    be conservative in what you do, be liberal in what you accept from others

    Applied to JSON-like content from others, it will produce well-formed JSON that should satisfy any parser you use.

    Applied to your output before you send, it will coerce minor mistakes in encoding and make it easier to embed your JSON in HTML and XML.

    Input

    The sanitizer takes JSON like content, and interprets it as JS eval would. Specifically, it deals with these non-standard constructs.
    • '...' Single quoted strings are converted to JSON strings.
    • \xAB Hex escapes are converted to JSON unicode escapes.
    • \012 Octal escapes are converted to JSON unicode escapes.
    • 0xAB Hex integer literals are converted to JSON decimal numbers.
    • 012 Octal integer literals are converted to JSON decimal numbers.
    • +.5 Decimal numbers are coerced to JSON's stricter format.
    • [0,,2] Elisions in arrays are filled with null.
    • [1,2,3,] Trailing commas are removed.
    • {foo:"bar"} Unquoted property names are quoted.
    • //comments JS style line and block comments are removed.
    • (...) Grouping parentheses are removed.
    The sanitizer fixes missing punctuation, end quotes, and mismatched or missing close brackets. If an input contains only white-space then the valid JSON string null is substituted.

    Output

    The output is well-formed JSON as defined by RFC 4627. The output satisfies three additional properties:
    1. The output will not contain the substring (case-insensitively) "</script" so can be embedded inside an HTML script element without further encoding.
    2. The output will not contain the substring "]]>" so can be embedded inside an XML CDATA section without further encoding.
    3. The output is a valid Javascript expression, so can be parsed by Javascript's eval builtin (after being wrapped in parentheses) or by JSON.parse. Specifically, the output will not contain any string literals with embedded JS newlines (U+2028 Paragraph separator or U+2029 Line separator).
    4. The output contains only valid Unicode scalar values (no isolated UTF-16 surrogates) that are allowed in XML unescaped.

    Security

    Since the output is well-formed JSON, passing it to eval will have no side-effects and no free variables, so is neither a code-injection vector, nor a vector for exfiltration of secrets.

    This library only ensures that the JSON string → Javascript object phase has no side effects and resolves no free variables, and cannot control how other client side code later interprets the resulting Javascript object. So if client-side code takes a part of the parsed data that is controlled by an attacker and passes it back through a powerful interpreter like eval or innerHTML then that client-side code might suffer unintended side-effects.

    Efficiency

    The sanitize method will return the input string without allocating a new buffer when the input is already valid JSON that satisfies the properties above. Thus, if used on input that is usually well formed, it has minimal memory overhead.

    The sanitize method takes O(n) time where n is the length in UTF-16 code-units.

    • Field Detail

      • DEFAULT_NESTING_DEPTH

        public static final int DEFAULT_NESTING_DEPTH
        The default for the maximumNestingDepth constructor parameter.
        See Also:
        Constant Field Values

      • MAXIMUM_NESTING_DEPTH

        public static final int MAXIMUM_NESTING_DEPTH
        The maximum value for the maximumNestingDepth constructor parameter.
        See Also:
        Constant Field Values

    • Method Detail

      • sanitize

        public static String sanitize​(String jsonish)
        Given JSON-like content, produces a string of JSON that is safe to embed, safe to pass to JavaScript's eval operator.
        Parameters:
        jsonish - JSON-like content.
        Returns:
        embeddable JSON

      • sanitize

        public static String sanitize​(String jsonish,
                              int maximumNestingDepth)
        Same as sanitize(String), but allows to set a custom maximum nesting depth.
        Parameters:
        jsonish - JSON-like content.
        maximumNestingDepth - maximum nesting depth.
        Returns:
        embeddable JSON