The SSL RC4 Cipher Suites Supported (Bar Mitzvah) vulnerability when detected with a vulnerability scanner will report it as a CVSSv3 5.9.

CVSS is a scoring system for vulnerability systems, its an industry standard scoring system to mark findings against a specific number ranging from 0 to 10. They are shown as:

The remote host supports the use of RC4 in one or more cipher suites.
The RC4 cipher is flawed in its generation of a pseudo-random stream of bytes so that a wide variety of small biases are introduced into the stream, decreasing its randomness.
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If plaintext is repeatedly encrypted (e.g., HTTP cookies), and an attacker is able to obtain many (i.e., tens of millions) ciphertexts, the attacker may be able to derive the plaintext

Block cyphers are a type of symmetric algorithm that encrypts plaintext in blocks, as the name implies, rather than bit-by-bit. One of the characteristics of such cyphers is the block length; which determines the size of the chunks into which the plaintext is split and then encrypted. Importantly, the block length of the cypher is independent of the length of the key. So even if you choose a large key size for your encryption, the block length of the cypher can impose its own limitations, and in this case, vulnerabilities.

To help protect against this vulnerability, you need to disable some older cyphers by making changes in the SSL configuration file. The location of this file depends on what kind of service you are running for example Apache, Nginx etc.

The SSL configuration section or the file should have the SSL Cipher Suite section that should look similar to below:

SSLCipherSuite "EECDH+ECDSA+AESGCM EECDH+aRSA+AESGCM EECDH+ECDSA+SHA384 EECDH+ECDSA+SHA256 EECDH+aRSA+SHA384 EECDH+aRSA+SHA256 !EECDH+aRSA+RC4 EECDH EDH+aRSA !RC4 !aNULL !eNULL !LOW !3DES !MD5 !EXP !PSK !SRP !DSS !EDH-RSA-DES-CBC3-SHA !ECDHE-RSA-DES-CBC3-SHA !DES-CBC3-SHA !ECDHE-RSA-RC4-SHA !RC4-MD5 !RC4-SHA"

Note: A restart of the service and/or the server will be required after making these changes.