Reported

October 16, 2026

Issued

October 26, 2026

Package

zstd (crates.io)

Type

INFO Unsound

Categories

• memory-exposure

Keywords

#integer-overflow #out-of-bounds #dictionary

References

• https://github.com/gyscos/zstd-rs/commit/781ea454191dbf178d09c1daf6fc896d15fbaa2f
• https://github.com/gyscos/zstd-rs/pull/384

Patched

• >=0.4.10

Unaffected

• <0.1.1

Description

ZDICT_trainFromBuffer is never told the length of the samples buffer. Before 0.4.10, zstd::dict::from_continuous (initially train_dict) called it directly: up to 0.2.1 without checking the sample sizes against the buffer at all, and from 0.3.0 with a sum that wraps around when overflow checks are off, as in release builds. Sizes that do not describe the buffer made the trainer read past its end.

This requires the sample sizes to come from an untrusted source. From 0.4.10, training goes through zstd-safe, where the same flaw is covered by the zstd-safe advisory for it, fixed in zstd-safe 8.2.0.

Advisory available under CC0-1.0 license.