UPDATE the 2nd: This post is getting a bit of attention and was really only intended as a quick skim, so if you're curious whether all PowerPC chips are vulnerable in the same fashion and why, read on for a deeper dive.
If you've been under a rock the last couple days, then you should read about Meltdown and Spectre (especially if you are using an Intel CPU).
Meltdown is specific to x86 processors made by Intel; it does not appear to affect AMD. But virtually every CPU going back decades that has a feature called speculative execution is vulnerable to a variety of the Spectre attack. In short, for those processors that execute "future" code downstream in anticipation of what the results of certain branching operations will be, Spectre exploits the timing differences that occur when certain kinds of speculatively executed code changes what's in the processor cache. The attacker may not be able to read the memory directly, but (s)he can find out if it's in the cache by looking at those differences (in broad strokes, stuff in the cache is accessed more quickly), and/or exploit those timing changes as a way of signaling the attacking software with the actual data itself. Although only certain kinds of code can be vulnerable to this technique, an attacker could trick the processor into mistakenly speculatively executing code it wouldn't ordinarily run. These side effects are intrinsic to the processor's internal implementation of this feature, though it is made easier if you have the source code of the victim process, which is increasingly common.
There are ways to stop speculative execution using certain instructions the processor must serialize, but this can seriously harm performance: speculative execution, after all, is a way to keep the processor busy with (hopefully) useful work while it waits for previous instructions to complete. On PowerPC, cache manipulation instructions, some kinds of special-purpose register accesses and even instructions like b . (branch to the next instruction, essentially a no-op) can halt speculative execution with a sometimes notable time penalty. I think there may be some ways we can harden the TenFourFox JIT with these instructions used selectively to reduce their overhead, though as I say, I don't find such attacks very practical on our geriatric machines in general.
Anyway, you can sleep well, because everybody's all in the same boat. Perhaps it's time to dust off those old strict CPUs. The world needs a port of Classilla to the Commodore 64. :)