Reported

October 16, 2026

Issued

October 26, 2026

Package

zstd-safe (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

• >=8.2.0

Affected Functions

Version

zstd_safe::train_from_buffer

• <8.2.0

Description

ZDICT_trainFromBuffer is never told the length of the samples buffer, so train_from_buffer checked that the sample sizes add up to it. With overflow checks off, as in release builds, that sum wrapped around: sizes totalling 2^64 plus the buffer length passed the check, and the trainer read past the end of the buffer. zstd::dict::from_continuous relies on this check from zstd 0.4.10; older versions of zstd had their own, covered by a separate advisory.

This requires the sample sizes to come from an untrusted source. The flaw was corrected by summing the sizes with overflow checks.

Advisory available under CC0-1.0 license.