RUSTSEC-2026-0403: Uninitialized memory exposed when streaming decompression of a legacy frame fails #58

Uninitialized memory exposed when streaming decompression of a legacy frame fails

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.

See advisory page for additional details.