Chrome enabled TLS 1.3 in Chrome 70. However, due to bugs in some man-in-the-middle proxies, anti-downgrade enforcement was not enabled. The problematic proxies in question are duplicating a value in the TLS handshake from the origin server rather than randomly generating it themselves. Firstly, this means that they're implementing a slightly different protocol than TLS on their LAN side, the security properties of which are not clear. Secondly, it means that these proxies appear to TLS 1.3-enabled clients to be signaling that a downgrade attack is occurring because they're taking TLS 1.3-based values from the origin server and using them in a lower-version TLS handshake on the LAN side.
In Chrome 72, downgrade protection will be enabled for TLS connections that use certificates that chain to a public CA. This should not affect MITM proxies since they cannot use publicly-trusted certificates. However, in order to get to the point where this workaround can be removed, all affected MITM proxies will need to be updated. The following lists minimum firmware versions for affected products that we're aware of:
Palo Alto Networks:
- PAN-OS 8.1 must be ≥ 8.1.4
- PAN-OS 8.0 must be ≥ 8.0.14
- PAN-OS 7.1 must be ≥ 7.1.21
Cisco Firepower Threat Defense and ASA with FirePOWER Services when operating in “Decrypt - Resign mode/SSL Decryption Enabled” (advisory) :
- Firmware 6.2.3 must be ≥ 220.127.116.11
- Firmware 6.2.2 must be ≥ 18.104.22.168
- Firmware 6.1.0 must be ≥ 22.214.171.124
Administrators can test compatibility by flipping chrome://flags/#enforce-tls13-downgrade to Enabled
Please report problems on the administrator's forum.