When decompression fails inside a legacy (zstd v0.4 to v0.7) frame, the bundled zstd library reports the output buffer as fully written although nothing was written. DCtx::decompress stream committed that position even on error, and for a Vec<u8> destination this extended the vector's length over uninitialized memory.

Legacy support is enabled by default. A 5- or 6-byte corrupt legacy frame is enough: through zstd::stream::write::Decoder, up to 32 KiB of stale heap memory is then passed to the wrapped writer on the next write() or flush(), or when an auto flush()/on flush() decoder is dropped. Whether this reaches an attacker depends on where the application sends the decompressed output. zstd::stream::read::Decoder, zstd::decode all and the bulk API are not affected.

Before zstd-safe 4.0.0, output buffers could only be initialized slices. The zstd crate's own writer had the same flaw from 0.4.22 to 0.7.x, which is covered by a separate advisory.

The flaw was corrected by only committing the buffer positions of a successful call.