Have you seen JPEG XL source code? I like the format, but the reference implementation in C++ looked pretty bad at least 2 years ago. I hope they rewrote it, because it surely looked like a security issue waiting to happen.
I think Google are aiming to replace all of Chromiums decoders with memory-safe ones anyway, even for relatively simple formats.
It's not the format, it's the C / C++ unfortunate baggage.
At this point, in 2025, any substantial (non-degenerative) image processing written in C++ is a security issue waiting to happen. That's not specific to JPEG XL.
https://chromium.googlesource.com/chromium/src/+/main/docs/s...
No new code goes in that violates the rule, and ideally no code at all goes in that is both unsafe and parses untrusted data (regardless of sandboxing) and old code doing both gets replaced.
A giant pile of C++ can be used for rendering, not parsing untrusted data. A giant pile of C++ can sit behind a validator: a memory-safe JSON validator can vet a stream, before an C++ library deserializes it. Etc.
I don't think it's irrational to be upset when a (near-)monopoly browser holds back useful features. Even if said browser is provided for free.
Google's position, on the other hand, has been a flat-out "no, we will not ship JXL". That's what has been met with criticism. Not an imagined reluctance to shipping a C++ JXL implementation.
Why this quality poses security issues?
Nonetheless, we should really bear in mind how entrenched Cpp is. If you normalize CVEs by language popularity Java looks downright dangerous!