RUSTSEC-2026-0402: Heap buffer overflow in CCtx::try_clone on a multithreaded context #37

Heap buffer overflow in CCtx::try_clone on a multithreaded context

CCtx::try_clone (experimental feature, deprecated) copies tables sized
from the context's applied parameters. Starting a multithreaded frame
(zstdmt feature) replaces those parameters with the workers' without
resizing the context's own tables, so after an abandoned single-threaded
frame, the copy wrote up to 512 KiB past a heap allocation, or read past
one. This became reachable with the bundled zstd 1.5.6.

The flaw was corrected by refusing try_clone (and the unsafe
compress_block) once NbWorkers was set on the context.

See advisory page for additional details.