With a token of length 32 and a character space of size 16 (e.g. hex), the attacker needs an average of approximately 1,000 request if no recovery mechanisms are needed. In practice, we have been able to recover CSRF tokens with fewer than 4,000 requests. A browser like Google Chrome or Internet Explorer is able to issue this number of requests in under 30 seconds, including callbacks to the attacker command & control center.
The core committee has decided that one of the security issues due to be fixed next week is sufficiently bad that we need to take extra measures to prevent it from becoming public before packages containing the fix are available. (This is a scenario we've discussed before, but never had to actually implement.)
// Decrypt to plaintext + mac + padding
$plaintext_mac_padding = decrypt($ciphertext);
if (NULL != $plaintext_mac_padding) {
// Now decode padding part
$plaintext_mac = decode_padding($plaintext_mac_padding, $padding_length);
if (NULL != $plaintext_mac) {
// Now check MAC part
$plaintext = check_mac(plaintext_mac);
if (NULL != $plaintext) {
// Now it's okay
}
}
}
In general, the best way to do this is to compute the MAC even if the padding is incorrect, and only then reject the packet. For instance, if the pad appears to be incorrect, the implementation might assume a zero-length pad and then compute the MAC.
This leaves a small timing channel, since MAC performance depends to some extent on the size of the data fragment, but it is not believed to be large enough to be exploitable, due to the large block size of existing MACs and the small size of the timing signal.
The attacks apply to all TLS and DTLS implementations that are compliant with TLS 1.1 or 1.2, or with DTLS 1.0 or 1.2. They also apply to implementations of SSL 3.0 and TLS 1.0 that incorporate countermeasures to previous padding oracle attacks. Variant attacks may also apply to non-compliant implementations.
