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The strength of the hash algorithm used in signing a digital certificate is a critical element of the security of the certificate. Weaknesses in hash algorithms can lead to situations in which attackers can create or obtain fraudulent certificates. As new attacks are found and improvements in available technology make attacks more feasible, the use of older algorithms is discouraged and support for them is eventually removed.
SHA-1 certificates will no longer be treated as secure by major browser manufacturers beginning in 2017.
Support for MD5 based signatures was removed in early 2012.
- Mozilla Security Blog post on the deprecation of SHA-1