A crafted .zst input under 128 KB can force ruzstd to allocate several gigabytes at once, crashing the process with an out-of-memory condition. Since ruzstd is a decoder for untrusted input, a single small file is enough to take down anything decoding it.

The compressed size of a block was capped at 128 KB, but a compressed block's decompressed size was never checked against Block Maximum Size = min(Window Size, 128 KiB) (RFC 8878). In execute sequences the output was built with no running bound and no flush between blocks, so one block could expand to gigabytes — bypassing both the configured window-size limit and the block-by-block streaming that would otherwise release memory. (A related u32 counter overflow on such a block caused an additional panic.)

This is a denial of service only; memory access stays in bounds (no corruption, no RCE).

ruzstd is also built as a dependency of the Rust standard library via rustc-dep-of-std; in the toolchain it is used only to load zstd-compressed debug symbols, so the toolchain-side impact is limited.

Fixed in 0.9.1 (commit d3907c9), which enforces the per-block output bound. Upgrade to ruzstd >= 0.9.1.