ROBOT Return of Bleichenbacher Oracle Threat - How to Fix

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Table of Contents

Overview

ROBOT is the return of a 19-year-old vulnerability that allows performing RSA decryption and signing operations with the private key of a TLS server.

In 1998, Daniel Bleichenbacher discovered that the error messages given by SSL servers for errors in the PKCS #1 v1.5 padding allowed an adaptive-chosen ciphertext attack; this attack fully breaks the confidentiality of TLS when used with RSA encryption.

 

Mitigation

          OBOT only affects TLS cipher modes that use RSA encryption. Most modern TLS connections use an Elliptic Curve Diffie Hellman key exchange and need RSA only for signatures. We believe RSA encryption modes are so risky that the only safe course of action is to disable them. Apart from being risky these modes also lack forward secrecy.

 

By disabling RSA encryption we mean all ciphers that start with TLS_RSA. It does not include the ciphers that use RSA signatures and include DHE or ECDHE in their name. These ciphers are not affected by our attack.

Based on some preliminary data we also believe the compatibility costs of disabling RSA encryption modes are relatively low. Cloudflare shared with us that around one percent of their connections use the RSA encryption modes. Disabling these modes on the HTTPS server operated by one of the authors caused no notable problems.

 

Solution

 Apply the appropriate patch according to the July 2017 Oracle Critical Patch Update advisory here.

 

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