json.loads()是否容易受到任意代码执行漏洞的攻击？

Question

Python标准库中的json模块中的json.loads存在任意代码执行或其他安全问题吗？

我的应用程序可能会从不可信的来源接收JSON消息。

- FrozenHeart

---

请查看此链接：https://dev59.com/0Gw15IYBdhLWcg3wJ4es - Marlon Abeykoon

反序列化的源代码看起来不像能执行任意代码（如果解释器本身没有问题且您没有设置额外参数），但是序列化代码显然会调用编码的对象上的方法。 - janbrohl

还要检查这个答案：https://dev59.com/3IXca4cB1Zd3GeqPEi_L 它有一个关于JSON的部分。 - Thomas B in BDX

Answer 1

请注意，以下答案与Windows 10 64位的默认Python3.4版本相关。同时请注意，这个回答只涉及py扫描器，不涉及c扫描器。

源文件请参见https://hg.python.org/cpython/file/tip/Lib/json，或在本地的Python安装中查找。

研究

_{请参考本文底部的参考实现来进行研究。} 通过json.loads(s)调用的解析函数在\Lib\json\scanner.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

使用 \Lib\json\decoder.py 中定义的JSONDecoder类的实例作为context，该类使用以下解析器：

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

从这里开始，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

此函数使用默认的float函数，因此是安全的。

---

parse_int

此函数使用默认的int函数，因此是安全的。

---

parse_constant

_CONSTANTS在与其同一文件中定义：

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

正在执行简单的查找操作，因此是安全的。

---

parse_string，JSONObject，JSONArray

通过查看本文末尾的实现可以发现，唯一能被执行的外部代码为：

来自JSONObject：

• object_pairs_hook
• object_hook

来自JSONArray：

• scan_once

---

object_pairs_hook，object_hook

默认情况下，object_pairs_hook和object_hook在解码器初始化时定义为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 的定义如下：

self.scan_once = scanner.make_scanner(self)

源代码可以在\Lib\json\scanner.py中找到，从中我们可以看到scan_once只是为JSON对象的每个部分调用相应的解析器。

---

结论

从上述内容和参考实现可以看出，只要JSON解码器使用的扫描程序是默认的，将不会执行任意代码。通过使用自定义解码器的__init__参数可能会使其执行任意代码，但我认为这样做是没有必要的。

---

实现

反斜杠

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

STRINGCHUNK

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

scanstring

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'[ \t\n\r]*', FLAGS)

WHITESPACE_STR

WHITESPACE_STR = ' \t\n\r'

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

- Nick stands with Ukraine