Selection of Less-Secure Algorithm During Negotiation ('Algorithm Downgrade')

Overview

A protocol or its implementation supports interaction between multiple actors and allows those actors to negotiate which algorithm should be used as a protection mechanism such as encryption or authentication, but it does not select the strongest algorithm that is available to both parties.

When a security mechanism can be forced to downgrade to use a less secure algorithm, this can make it easier for attackers to compromise the product by exploiting weaker algorithm.

The victim might not be aware that the less secure algorithm is being used.

For example, if an attacker can force a communications channel to use cleartext instead of strongly-encrypted data, then the attacker could read the channel by sniffing, instead of going through extra effort of trying to decrypt the data using brute force techniques.

Recommendation

Check your APIs TLS libraries and configurations to ensure that legacy protocol are disabled.

General purpose web applications should default to TLS 1.3 (support TLS 1.2 if necessary) with all other protocols disabled.

In specific and uncommon situations where a web server is required to accommodate legacy clients that depend on outdated and unsecured browsers (like Internet Explorer 10), activating TLS 1.0 may be the only option. However, this approach should be exercised with caution and is generally not advised due to the security implications. Additionally, "TLS_FALLBACK_SCSV" extension should be enabled in order to prevent downgrade attacks against newer clients.

Note that PCI DSS forbids the use of legacy protocols such as TLS 1.0.

References

• Common Weakness Enumeration: CWE-757.