[update] fix (unexploitable) BB'06 vulnerability in rsa_verify
The rsa_verify code was vulnerable to a BB'06 attack, allowing to forge signatures for arbitrary messages if and only if the public key exponent is 3. Since the updates key is hardcoded to 65537, there is no risk for youtube-dl, but I don't want vulnerable code in the wild. The new function adopts a way safer approach of encoding-and-comparing to replace the dangerous parsing code.
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#!/usr/bin/env python
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from __future__ import unicode_literals
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# Allow direct execution
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import os
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import sys
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import unittest
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sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
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import json
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from youtube_dl.update import rsa_verify
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class TestUpdate(unittest.TestCase):
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def test_rsa_verify(self):
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UPDATES_RSA_KEY = (0x9d60ee4d8f805312fdb15a62f87b95bd66177b91df176765d13514a0f1754bcd2057295c5b6f1d35daa6742c3ffc9a82d3e118861c207995a8031e151d863c9927e304576bc80692bc8e094896fcf11b66f3e29e04e3a71e9a11558558acea1840aec37fc396fb6b65dc81a1c4144e03bd1c011de62e3f1357b327d08426fe93, 65537)
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with open(os.path.join(os.path.dirname(os.path.abspath(__file__)), 'versions.json'), 'rb') as f:
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versions_info = f.read().decode()
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versions_info = json.loads(versions_info)
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signature = versions_info['signature']
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del versions_info['signature']
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self.assertTrue(rsa_verify(
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json.dumps(versions_info, sort_keys=True).encode('utf-8'),
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signature, UPDATES_RSA_KEY))
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if __name__ == '__main__':
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unittest.main()
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{
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"latest": "2013.01.06",
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"signature": "72158cdba391628569ffdbea259afbcf279bbe3d8aeb7492690735dc1cfa6afa754f55c61196f3871d429599ab22f2667f1fec98865527b32632e7f4b3675a7ef0f0fbe084d359256ae4bba68f0d33854e531a70754712f244be71d4b92e664302aa99653ee4df19800d955b6c4149cd2b3f24288d6e4b40b16126e01f4c8ce6",
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"versions": {
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"2013.01.02": {
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"bin": [
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"http://youtube-dl.org/downloads/2013.01.02/youtube-dl",
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"f5b502f8aaa77675c4884938b1e4871ebca2611813a0c0e74f60c0fbd6dcca6b"
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],
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"exe": [
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"http://youtube-dl.org/downloads/2013.01.02/youtube-dl.exe",
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"75fa89d2ce297d102ff27675aa9d92545bbc91013f52ec52868c069f4f9f0422"
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],
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"tar": [
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"http://youtube-dl.org/downloads/2013.01.02/youtube-dl-2013.01.02.tar.gz",
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"6a66d022ac8e1c13da284036288a133ec8dba003b7bd3a5179d0c0daca8c8196"
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]
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},
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"2013.01.06": {
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"bin": [
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"http://youtube-dl.org/downloads/2013.01.06/youtube-dl",
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"64b6ed8865735c6302e836d4d832577321b4519aa02640dc508580c1ee824049"
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],
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"exe": [
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"http://youtube-dl.org/downloads/2013.01.06/youtube-dl.exe",
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"58609baf91e4389d36e3ba586e21dab882daaaee537e4448b1265392ae86ff84"
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],
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"tar": [
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"http://youtube-dl.org/downloads/2013.01.06/youtube-dl-2013.01.06.tar.gz",
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"fe77ab20a95d980ed17a659aa67e371fdd4d656d19c4c7950e7b720b0c2f1a86"
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]
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}
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}
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}
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@ -15,33 +15,17 @@ from .version import __version__
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def rsa_verify(message, signature, key):
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from struct import pack
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from hashlib import sha256
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assert isinstance(message, bytes)
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block_size = 0
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n = key[0]
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while n:
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block_size += 1
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n >>= 8
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signature = pow(int(signature, 16), key[1], key[0])
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raw_bytes = []
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while signature:
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raw_bytes.insert(0, pack("B", signature & 0xFF))
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signature >>= 8
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signature = (block_size - len(raw_bytes)) * b'\x00' + b''.join(raw_bytes)
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if signature[0:2] != b'\x00\x01':
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byte_size = (len(bin(key[0])) - 2 + 8 - 1) // 8
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signature = ('%x' % pow(int(signature, 16), key[1], key[0])).encode()
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signature = (byte_size * 2 - len(signature)) * b'0' + signature
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asn1 = b'3031300d060960864801650304020105000420'
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asn1 += sha256(message).hexdigest().encode()
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if byte_size < len(asn1) // 2 + 11:
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return False
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signature = signature[2:]
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if b'\x00' not in signature:
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return False
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signature = signature[signature.index(b'\x00') + 1:]
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if not signature.startswith(b'\x30\x31\x30\x0D\x06\x09\x60\x86\x48\x01\x65\x03\x04\x02\x01\x05\x00\x04\x20'):
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return False
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signature = signature[19:]
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if signature != sha256(message).digest():
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return False
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return True
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expected = b'0001' + (byte_size - len(asn1) // 2 - 3) * b'ff' + b'00' + asn1
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return expected == signature
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def update_self(to_screen, verbose, opener):
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