Some microprocessors from Intel and AMD are vulnerable to a newly discovered speculative execution attack that can covertly leak password data and other sensitive material, sending both chipmakers scrambling once again to contain what is proving to be a stubbornly persistent vulnerability.

Researchers from ETH Zurich have named their attack Retbleed because it exploits a software defense known as retpoline, which was introduced in 2018 to mitigate the harmful effects of speculative execution attacks. Speculative execution attacks, including one known as Spectre, exploit the fact that when modern CPUs encounter a direct or indirect instruction branch, they predict the address for the next instruction they’re about to receive and automatically execute it before the prediction is confirmed. Spculative execution attacks works by tricking the CPU into executing an instruction that accesses sensitive data in memory that would normally be off-limits to a low-privileged application. Retbleed then extracts the data after the operation is canceled.

Is it a trampoline or a slingshot?

Retpoline works by using a series of return operations to isolate indirect branches from speculative execution attacks, in effect erecting the software equivalent of a trampoline that causes them to safely bounce. Stated differently, a retpoline works by replacing indirect jumps and calls with returns, which many researchers presumed weren’t susceptible. The defense was designed to counter variant 2 of the original speculative execution attacks from January 2018. Abbreviated as BTI, the variant forces an indirect branch to execute so-called “gadget” code, which in turn creates data to leak through a side channel.

Some researchers have warned for years that retpoline isn’t sufficient to mitigate speculative execution attacks because the returns retpoline used were susceptible to BTI. Linux creator Linus Torvalds famously rejected such warnings, arguing that such exploits weren’t practical.