1. Is server vulnerable if it supports RSA key exchange ciphers?
  2. Can server be still vulnerable if it is running on unpatched vendor software but does not support RSA key exchange ciphers?
  3. After applying vendor patches, Should RSA key exchange ciphers also needs to be disabled?
  4. How to mitigate ROBOT (Return Of Bleichenbacher's Oracle Threat) vulnerability?

The server is vulnerable to Return Of Bleichenbacher's Oracle Threat (ROBOT) only if it is hosted on a vendor software with RSA implementation bug and server supports RSA key exchange ciphers.

See the list of known vulnerable vendors and patches https://robotattack.org/#patches

Note: Not all servers that support RSA key exchange are vulnerable. But it is recommended to disable RSA key exchange ciphers as it does not support forward secrecy.

Robot vulnerability Severity:

For hosts that are vulnerable and support only RSA encryption key exchanges, it's severe vulnerability. An attacker can passively record traffic and decrypt it later.

Mitigation steps:

Ideally, following both mitigation steps should be taken.

  1. Update your server; patches are provided by most of the vendors. If you patch your server your immune to ROBOT vulnerability.

Note: If the patch is not available from your vendor, you can mitigate with 2nd step.

  1. Disable RSA key exchange ciphers (Recommended): But if you want to keep support for RSA key exchange ciphers. Your server should at least support forward secrecy with modern browsers. (keep RSA ciphers last)

Identifying RSA key exchange ciphers:

  1. SSL Labs uses TLS_* format. So that means all ciphers starting with "TLS_RSA" on the report.
  2. OpenSSL uses the different naming convention. All ciphers with "Kx=RSA"

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