Retbleed is a speculative execution attack on x86-64 and ARM processors, including some recent Intel and AMD chips.
[1][2] First made public in 2022, it is a variant of the Spectre vulnerability which exploits retpoline, which was a mitigation for speculative execution attacks.
[3] According to the researchers, Retbleed mitigations require extensive changes to the system which results in up to 14% and 39% performance loss on Linux for affected AMD and Intel CPU respectively.
[2] Windows is not vulnerable because the existing mitigations already tackle it.
[5][6] The 32-bit Linux kernel, which is vulnerable, will not receive updates to fix the issue.