TLS_KRB5_WITH_RC4_128_SHA

What each component means

A cipher suite names the algorithms a client and server agree on in one handshake. TLS_KRB5_WITH_RC4_128_SHA combines KRB5 key exchange, KRB5 authentication, RC4-128 encryption and SHA1 integrity protection. Each part is explained below with its known weaknesses; for the bigger picture see how cipher suites work.

Key exchange and authentication: Kerberos (RFC 2712). Instead of a certificate exchange, the client presents a Kerberos service ticket and the premaster secret is protected with the ticket's session key, so TLS can reuse existing Kerberos infrastructure. These suites were only ever defined with DES, 3DES, RC4, IDEA and 40-bit export ciphers, so no modern combination exists, and OpenSSL 3.5 implements none of them.

Encryption: RC4, a stream cipher whose keystream is measurably biased. AlFardan, Bernstein, Paterson, Poettering and Schuldt showed in 2013 that a secret such as a cookie can be recovered from enough encryptions of it, and RFC 7465 (2015) then prohibited RC4 suites outright: clients MUST NOT offer them and servers MUST NOT select them.

Integrity: HMAC-SHA1. SHA-1 collisions are practical (SHAttered, 2017), but HMAC does not rely on collision resistance, so HMAC-SHA1 record MACs are not considered broken; this suite's weak points lie elsewhere. When TLS 1.2 is negotiated, keys are derived with the SHA-256 PRF, which RFC 5246 (2008) applies to every cipher suite defined before it.

Why this rating

ssl.sh rating: Insecure. The reasons:

• Uses RC4, which RFC 7465 prohibits in all TLS versions because of practical biases in its keystream.

How ssl.sh rates cipher suites

Which TLS libraries support it

Support for TLS_KRB5_WITH_RC4_128_SHA as read from each library's own source code or official documentation on 2026-11-10. “Enabled by default” means the library negotiates it with no configuration; “off by default” means it is implemented but has to be switched on.

None of the 9 libraries tracked here enables it by default. None of them implements it.

NSS shows the library's own defaults; Firefox switches every NSS suite off at start-up and re-enables only those with a security.ssl3.* or security.tls13.* preference. Java defaults are derived from the jdk.tls.disabledAlgorithms value Oracle documents for JDK 21, which later updates may extend. The OpenSSL and GnuTLS rows only show whether the suite is implemented.

History and standards status

TLS_KRB5_WITH_RC4_128_SHA (0x00,0x20) is registered in the IANA TLS Cipher Suites registry with RFC 2712, “Addition of Kerberos Cipher Suites to Transport Layer Security (TLS)” (October 1999, Standards Track) as its specification. DTLS 1.2 (RFC 6347, January 2012) had already said RC4 MUST NOT be used with DTLS. RFC 7465 (2015) prohibits RC4 cipher suites in every TLS version: clients MUST NOT offer them and servers MUST NOT select them. The registry marks it D (discouraged), a value introduced by RFC 9847 (2025).

Enable or disable it in Nginx and Apache

OpenSSL 3.5 doesn't implement this suite, so stock Nginx and Apache httpd builds can't offer it and you don't need to disable it.

How to test whether a server accepts it

OpenSSL 3.5 cannot offer TLS_KRB5_WITH_RC4_128_SHA, so openssl s_client can't test it directly; use a scanner that implements the suite itself.

nmap's ssl-enum-ciphers script connects repeatedly, trying one suite at a time, and lists every suite the server accepts with a grade from A to F. Look for TLS_KRB5_WITH_RC4_128_SHA in its output:

nmap --script ssl-enum-ciphers -p 443 example.com

To decode a name or hex code, use the cipher suite lookup; the SSL checker shows which protocol versions a live server accepts and the cipher suite it negotiates.