问题描述

来自Python标准json模块的json.loads是否易受任意代码执行或任何其他安全问题的攻击？

我的应用程序可以接收来自不可信源的JSON消息。

---

解决方案

请注意，以下答案与Windows1064位的默认Python3.4安装相关。另请注意，此答案仅查看py扫描程序，而不是c扫描程序。

有关源文件，请参阅https://hg.python.org/cpython/file/tip/Lib/json或在本地python安装中找到它们。

研究

_{请参阅本文底部本研究旁边的参考实现}

json.loads(s)调用的解析函数在Libjsonscanner.py中定义：

parse_object = context.parse_object
parse_array = context.parse_array
parse_string = context.parse_string
parse_float = context.parse_float
parse_int = context.parse_int
parse_constant = context.parse_constant

context是JSONDecoder中定义的JSONDecoder类的实例，使用以下解析器：

self.parse_float = parse_float or float
self.parse_int = parse_int or int
self.parse_constant = parse_constant or _CONSTANTS.__getitem__
self.parse_string = scanstring
self.parse_object = JSONObject
self.parse_array = JSONArray

从这里我们可以查看每个单独的解析器，以确定它是否容易受到任意代码执行的影响：

---

PARSE_FLOAT

这使用默认的float函数，因此是安全的。

---

parse_int

这使用默认的int函数，因此是安全的。

---

parse_constant

_CONSTANTS在同一个文件中定义为：

_CONSTANTS = {
    '-Infinity': NegInf,
    'Infinity': PosInf,
    'NaN': NaN,
}

所以执行了一个简单的查找，所以它是安全的。

---

parse_string、JSONObject、JSONArray

通过查看本文末尾的实现可以看到，唯一可以执行的外部代码是：

发件人JSONObject：

• object_pairs_hook
• object_hook

发件人JSONArray：

• scan_once

---

object_pairs_hook，object_hook

默认情况下object_pairs_hook和object_hook定义为来自解码器初始值设定项的None：

def __init__(self, object_hook=None, parse_float=None,
        parse_int=None, parse_constant=None, strict=True,
        object_pairs_hook=None)

---

scan_once

scan_once定义为：

self.scan_once = scanner.make_scanner(self)

其源可以在Libjsonscanner.py中找到，从中我们可以看到它scan_once只是为JSON对象的每个部分调用了相应的解析器。

---

结论

从上面和参考实现可以看出，只要JSON解码器使用的扫描器是默认的，就不会执行任意代码，很可能通过使用自定义解码器的__init__参数来使其执行任意代码，但是我不这么认为。

---

实施

反斜杠

BACKSLASH = {
    '"': '"', '\': '\', '/': '/',
    'b': '', 'f': 'f', 'n': '
', 'r': '', 't': '	',
}

字符串

STRINGCHUNK = re.compile(r'(.*?)(["\x00-x1f])', FLAGS)

扫描字符串

def py_scanstring(s, end, strict=True,
        _b=BACKSLASH, _m=STRINGCHUNK.match):
    """Scan the string s for a JSON string. End is the index of the
    character in s after the quote that started the JSON string.
    Unescapes all valid JSON string escape sequences and raises ValueError
    on attempt to decode an invalid string. If strict is False then literal
    control characters are allowed in the string.

    Returns a tuple of the decoded string and the index of the character in s
    after the end quote."""
    chunks = []
    _append = chunks.append
    begin = end - 1
    while 1:
        chunk = _m(s, end)
        if chunk is None:
            raise ValueError(
                errmsg("Unterminated string starting at", s, begin))
        end = chunk.end()
        content, terminator = chunk.groups()
        # Content is contains zero or more unescaped string characters
        if content:
            _append(content)
        # Terminator is the end of string, a literal control character,
        # or a backslash denoting that an escape sequence follows
        if terminator == '"':
            break
        elif terminator != '\':
            if strict:
                #msg = "Invalid control character %r at" % (terminator,)
                msg = "Invalid control character {0!r} at".format(terminator)
                raise ValueError(errmsg(msg, s, end))
            else:
                _append(terminator)
                continue
        try:
            esc = s[end]
        except IndexError:
            raise ValueError(
                errmsg("Unterminated string starting at", s, begin))
        # If not a unicode escape sequence, must be in the lookup table
        if esc != 'u':
            try:
                char = _b[esc]
            except KeyError:
                msg = "Invalid \escape: {0!r}".format(esc)
                raise ValueError(errmsg(msg, s, end))
            end += 1
        else:
            uni = _decode_uXXXX(s, end)
            end += 5
            if 0xd800 <= uni <= 0xdbff and s[end:end + 2] == '\u':
                uni2 = _decode_uXXXX(s, end + 1)
                if 0xdc00 <= uni2 <= 0xdfff:
                    uni = 0x10000 + (((uni - 0xd800) << 10) | (uni2 - 0xdc00))
                    end += 6
            char = chr(uni)
        _append(char)
    return ''.join(chunks), end

scanstring = c_scanstring or py_scanstring

空格

WHITESPACE = re.compile(r'[ 	
]*', FLAGS)

WHITESPACE_STR

WHITESPACE_STR = ' 	
'

JSONObject

def JSONObject(s_and_end, strict, scan_once, object_hook, object_pairs_hook,
               memo=None, _w=WHITESPACE.match, _ws=WHITESPACE_STR):
    s, end = s_and_end
    pairs = []
    pairs_append = pairs.append
    # Backwards compatibility
    if memo is None:
        memo = {}
    memo_get = memo.setdefault
    # Use a slice to prevent IndexError from being raised, the following
    # check will raise a more specific ValueError if the string is empty
    nextchar = s[end:end + 1]
    # Normally we expect nextchar == '"'
    if nextchar != '"':
        if nextchar in _ws:
            end = _w(s, end).end()
            nextchar = s[end:end + 1]
        # Trivial empty object
        if nextchar == '}':
            if object_pairs_hook is not None:
                result = object_pairs_hook(pairs)
                return result, end + 1
            pairs = {}
            if object_hook is not None:
                pairs = object_hook(pairs)
            return pairs, end + 1
        elif nextchar != '"':
            raise ValueError(errmsg(
                "Expecting property name enclosed in double quotes", s, end))
    end += 1
    while True:
        key, end = scanstring(s, end, strict)
        key = memo_get(key, key)
        # To skip some function call overhead we optimize the fast paths where
        # the JSON key separator is ": " or just ":".
        if s[end:end + 1] != ':':
            end = _w(s, end).end()
            if s[end:end + 1] != ':':
                raise ValueError(errmsg("Expecting ':' delimiter", s, end))
        end += 1

        try:
            if s[end] in _ws:
                end += 1
                if s[end] in _ws:
                    end = _w(s, end + 1).end()
        except IndexError:
            pass

        try:
            value, end = scan_once(s, end)
        except StopIteration as err:
            raise ValueError(errmsg("Expecting value", s, err.value)) from None
        pairs_append((key, value))
        try:
            nextchar = s[end]
            if nextchar in _ws:
                end = _w(s, end + 1).end()
                nextchar = s[end]
        except IndexError:
            nextchar = ''
        end += 1

        if nextchar == '}':
            break
        elif nextchar != ',':
            raise ValueError(errmsg("Expecting ',' delimiter", s, end - 1))
        end = _w(s, end).end()
        nextchar = s[end:end + 1]
        end += 1
        if nextchar != '"':
            raise ValueError(errmsg(
                "Expecting property name enclosed in double quotes", s, end - 1))
    if object_pairs_hook is not None:
        result = object_pairs_hook(pairs)
        return result, end
    pairs = dict(pairs)
    if object_hook is not None:
        pairs = object_hook(pairs)
    return pairs, end

JSONArray

def JSONArray(s_and_end, scan_once, _w=WHITESPACE.match, _ws=WHITESPACE_STR):
    s, end = s_and_end
    values = []
    nextchar = s[end:end + 1]
    if nextchar in _ws:
        end = _w(s, end + 1).end()
        nextchar = s[end:end + 1]
    # Look-ahead for trivial empty array
    if nextchar == ']':
        return values, end + 1
    _append = values.append
    while True:
        try:
            value, end = scan_once(s, end)
        except StopIteration as err:
            raise ValueError(errmsg("Expecting value", s, err.value)) from None
        _append(value)
        nextchar = s[end:end + 1]
        if nextchar in _ws:
            end = _w(s, end + 1).end()
            nextchar = s[end:end + 1]
        end += 1
        if nextchar == ']':
            break
        elif nextchar != ',':
            raise ValueError(errmsg("Expecting ',' delimiter", s, end - 1))
        try:
            if s[end] in _ws:
                end += 1
                if s[end] in _ws:
                    end = _w(s, end + 1).end()
        except IndexError:
            pass

    return values, end

scanner.make_scanner

def py_make_scanner(context):
    parse_object = context.parse_object
    parse_array = context.parse_array
    parse_string = context.parse_string
    match_number = NUMBER_RE.match
    strict = context.strict
    parse_float = context.parse_float
    parse_int = context.parse_int
    parse_constant = context.parse_constant
    object_hook = context.object_hook
    object_pairs_hook = context.object_pairs_hook
    memo = context.memo

    def _scan_once(string, idx):
        try:
            nextchar = string[idx]
        except IndexError:
            raise StopIteration(idx)

        if nextchar == '"':
            return parse_string(string, idx + 1, strict)
        elif nextchar == '{':
            return parse_object((string, idx + 1), strict,
                _scan_once, object_hook, object_pairs_hook, memo)
        elif nextchar == '[':
            return parse_array((string, idx + 1), _scan_once)
        elif nextchar == 'n' and string[idx:idx + 4] == 'null':
            return None, idx + 4
        elif nextchar == 't' and string[idx:idx + 4] == 'true':
            return True, idx + 4
        elif nextchar == 'f' and string[idx:idx + 5] == 'false':
            return False, idx + 5

        m = match_number(string, idx)
        if m is not None:
            integer, frac, exp = m.groups()
            if frac or exp:
                res = parse_float(integer + (frac or '') + (exp or ''))
            else:
                res = parse_int(integer)
            return res, m.end()
        elif nextchar == 'N' and string[idx:idx + 3] == 'NaN':
            return parse_constant('NaN'), idx + 3
        elif nextchar == 'I' and string[idx:idx + 8] == 'Infinity':
            return parse_constant('Infinity'), idx + 8
        elif nextchar == '-' and string[idx:idx + 9] == '-Infinity':
            return parse_constant('-Infinity'), idx + 9
        else:
            raise StopIteration(idx)

    def scan_once(string, idx):
        try:
            return _scan_once(string, idx)
        finally:
            memo.clear()

    return _scan_once

make_scanner = c_make_scanner or py_make_scanner