SSL 2.0 has been officially deprecated as of March 2011 (RFC 6176).

SSL 3.0 has been officially deprecated as of June 2015 (RFC 7568).

DES is a cipher with an effective key length of 56 bits, which is now considered too weak by many agencies, including ECRYPT and NIST (ECRYPT 2016 report, NIST SP 800-57, Part 1, Rev. 3).

Application data is not encrypted when the NULL cipher is used, exposing it to eavesdroppers.

For legacy reasons, some TLS cipher suites are composed of export-grade cryptography, which is insecure by today's standards. Furthermore, servers that accept EXPORT cipher suites may be vulnerable to the FREAK (freakattack.com) or the Logjam (weakdh.org) downgrade attacks.

The hash function used on certificate signatures must be cryptographically secure in order for the certificate not to be forgeable. Hash functions MD2 and MD5 are considered to be broken. SHA-1 certificates are in the process of being deprecated because of their weaknesses although many browsers and websites still support them. The SHA-2 family of functions (SHA-224, SHA-256, ...) are a safe alternative.

TLS Fallback SCSV (RFC 7507) enables a server to determine whether a protocol version downgrade by the client is legitimate. If this mechanism is not supported, an attacker could make both endpoints choose a lower protocol version they both support, probably resulting in a less secure connection.

Forward secrecy is achieved when the security of session keys is not affected by a compromise of long-term keys.

In TLS, forward secrecy is enabled by DHE, ECDHE and DH_anon cipher suites. They protect past communications from a compromise of a long-term key (such as an RSA key).

A server is vulnerable to the "General DROWN" attack when it supports, willingly or not, weak SSLv2 cipher suites. A server is vulnerable to the even more powerful "Special DROWN" attack when it is affected by CVE-2016-0703. DROWN may affect any TLS server (even with TLS 1.2 and on a different machine) which uses the same RSA key as the vulnerable SSLv2 server (drownattack.com).

ROBOT (https://robotattack.org/) 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. It affects vulnerable server implementations that also allow the use of RSA encryption (e.g. TLS_RSA cipher suites).

Each certificate defines a validity period. Outside of this period, it is not valid. A validity start date in the future is a sign that the certificate is bogus or that the system that generated it has a desynchronized clock.

The CRIME vulnerability (also known as CVE-2012-4929) happens when TLS compression is enabled. An attacker can get information about sensitive data in pages by observing the size of compressed responses. Using this, it is possible to impersonate users by retrieving their session cookies. This exploit has been demonstrated on HTTPS, but the weakness is fundamental to compression in TLS and may be exploitable in non-HTTP services.

Each cipher suite describes how server authentication is done. Anonymous cipher suites tell the client not to authenticate the server. They should thus not be used unless server authentication is not required, as is usually the case for SMTP servers.

The OpenSSL Heartbleed bug (heartbleed.com, CVE-2014-0160) allows secrets including the server's private key to be extracted in the clear.

In some configurations of TLS, it is possible for an attacker with sufficient access to mount a MITM attack that gives him the ability to decrypt and modify all the traffic between the server and the client.

In some configurations of TLS a padding oracle might be present on the server. This makes it possible for an attacker with sufficient access to partially decrypt the information sent from the client to the server.

In some configurations of TLS, it is possible for an attacker with sufficient access to decrypt the Master Secret and thus the rest of the TLS stream.

The recommended SSH protocol is SSH-2, adopted as a standard in 2006 (RFC 4251) and identified as "SSH-2.0". SSH-1, designed in 1995, is now discouraged. Servers claiming compatibility with both versions use "SSH-1.99" in their version string.

DES is a cipher with an effective key length of 56 bits, which is now considered too weak by many agencies, including ECRYPT and NIST (ECRYPT 2016 report, NIST SP 800-57, Part 1, Rev. 3).

IDEA has a 64-bit block size, which can be insufficient for some applications, for example because of birthday attacks (sweet32.info). There seem to be no advantage to using it over more secure and more widely supported ciphers.

RC4 is a stream cipher in which significant weaknesses have been found. The use of this cipher in any protocol has been discouraged by ECRYPT as of 2014 (ECRYPT 2016 report).

In SSH, RC4 is implemented as "arcfour", "arcfour128" and "arcfour256". "arcfour128" and "arcfour256" improve the original algorithm but are still considered weak (RFC 4345).

SSL 2.0 has been officially deprecated as of March 2011 (RFC 6176).

SSL 3.0 has been officially deprecated as of June 2015 (RFC 7568).

TLS 1.0 is discouraged by PCI-DSS and has been considered non-compliant since June 2018 (PCI-DSS v3.2).

Servers that support SSL 3.0 with cipher suites containing the CBC mode of operation can be vulnerable to the POODLE attack (RFC 7568). A possible countermeasure consists in enabling TLS Fallback SCSV along with more recent TLS protocols such as TLS 1.2.

DES is a cipher with an effective key length of 56 bits, which is now considered too weak by many agencies, including ECRYPT and NIST (ECRYPT 2016 report, NIST SP 800-57, Part 1, Rev. 3).

Three-key-3DES is a cipher with 168-bit keys but an effective key length of 112 bits because of a meet-in-the-middle attack. This is considered enough only for legacy. Furthermore, it has a 64-bit block size, which can be insufficient for some applications, for example because of birthday attacks (sweet32.info).

Application data is not encrypted when the NULL cipher is used, exposing it to eavesdroppers.

For legacy reasons, some TLS cipher suites are composed of export-grade cryptography, which is insecure by today's standards. Furthermore, servers that accept EXPORT cipher suites may be vulnerable to the FREAK (freakattack.com) or the Logjam (weakdh.org) downgrade attacks.

RC4 is a stream cipher in which significant weaknesses have been found. The use of this cipher in any protocol has been discouraged by ECRYPT as of 2014 (ECRYPT 2016 report).

In TLS, cipher suites using RC4 have been deprecated as of February 2015 (RFC 7465).

The hash function used on certificate signatures must be cryptographically secure in order for the certificate not to be forgeable. Hash functions MD2 and MD5 are considered to be broken. SHA-1 certificates are in the process of being deprecated because of their weaknesses although many browsers and websites still support them. The SHA-2 family of functions (SHA-224, SHA-256, ...) are a safe alternative.

Forward secrecy is achieved when the security of session keys is not affected by a compromise of long-term keys.

In TLS, forward secrecy is enabled by DHE, ECDHE and DH_anon cipher suites. They protect past communications from a compromise of a long-term key (such as an RSA key).

A server is vulnerable to the "General DROWN" attack when it supports, willingly or not, weak SSLv2 cipher suites. A server is vulnerable to the even more powerful "Special DROWN" attack when it is affected by CVE-2016-0703. DROWN may affect any TLS server (even with TLS 1.2 and on a different machine) which uses the same RSA key as the vulnerable SSLv2 server (drownattack.com).

ROBOT (https://robotattack.org/) 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. It affects vulnerable server implementations that also allow the use of RSA encryption (e.g. TLS_RSA cipher suites).

Each certificate defines a validity period. Outside of this period, it is not valid. A validity start date in the future is a sign that the certificate is bogus or that the system that generated it has a desynchronized clock.

The CRIME vulnerability (also known as CVE-2012-4929) happens when TLS compression is enabled. An attacker can get information about sensitive data in pages by observing the size of compressed responses. Using this, it is possible to impersonate users by retrieving their session cookies. This exploit has been demonstrated on HTTPS, but the weakness is fundamental to compression in TLS and may be exploitable in non-HTTP services.

Each cipher suite describes how server authentication is done. Anonymous cipher suites tell the client not to authenticate the server. They should thus not be used unless server authentication is not required, as is usually the case for SMTP servers.

The OpenSSL Heartbleed bug (heartbleed.com, CVE-2014-0160) allows secrets including the server's private key to be extracted in the clear.

In some configurations of TLS, it is possible for an attacker with sufficient access to mount a MITM attack that gives him the ability to decrypt and modify all the traffic between the server and the client.

In some configurations of TLS a padding oracle might be present on the server. This makes it possible for an attacker with sufficient access to partially decrypt the information sent from the client to the server.

In some configurations of TLS, it is possible for an attacker with sufficient access to decrypt the Master Secret and thus the rest of the TLS stream.