Wasmtime is a runtime for WebAssembly. In versions prior to 24.0.9, 36.0.10, and 44.0.2, when a filesystem preopen is given DirPerms::all() and FilePerms::READ without FilePerms::WRITE, this access control mechanism can be bypassed via the wasip2 descriptor.open-at or wasip1 path_open interfaces by opening a file with only the OpenFlags::TRUNCATE oflag. The root cause is that the clause handling OpenFlags::TRUNCATE in crates/wasi/src/filesystem.rs (Dir::open_at, lines 967–969) did not set open_mode |= OpenMode::WRITE;, which is later used for the access control check against FilePerms to determine whether opening the file is permitted; the single-line fix adds that missing assignment, after which the affected calls correctly fail with error-code.not-permitted and ERRNO_PERM respectively. Only wasmtime-wasi embeddings that combine DirPerms::MUTATE with FilePerms::READ are affected by this bug. In particular, the Wasmtime project's wasmtime-cli's use of wasmtime-wasi is not affected, because it always sets FilePerms::all() for all preopens. This issue has been fixed in versions 24.0.9, 36.0.10 and44.0.2.

Affected: crates.io: wasmtime-wasi

## Summary

In `wasmtime-wasi`, when a filesystem preopen is given `DirPerms::all()` and `FilePerms::READ` without `FilePerms::WRITE`, this wasmtime-wasi enforced access control mechanism can be bypassed by using the wasip2 `descriptor.open-at` or wasip1 `path_open` interfaces by opening a file with `OpenFlags::TRUNCATE` oflag only, for example:

```rust dir_descriptor.open_at( PathFlags::empty(), FILENAME, OpenFlags::TRUNCATE, DescriptorFlags::READ, ) ```

```rust wasip1::path_open( dir_fd, 0, FILENAME, wasip1::OFLAGS_TRUNC, wasip1::RIGHTS_FD_READ, 0, 0 ) ```

The root cause is that the clause that considered `OpenFlags::TRUNCATE` did not set `open_mode |= OpenMode::WRITE;`, used later in that function for the access control check against `FilePerms` for whether opening that file is permitted. With the bug corrected, these calls to `open-at` and `path_open` fail with `error-code.not-permitted` and `ERRNO_PERM` respectively.

The bug in `crates/wasi/src/filesystem.rs`, `Dir::open_at`, lines 967–969:

```rust if oflags.contains(OpenFlags::TRUNCATE) { opts.truncate(true).write(true); } ``` and the single line fix is: ```rust if oflags.contains(OpenFlags::TRUNCATE) { opts.truncate(true).write(true); open_mode |= OpenMode::WRITE; } ```

Only wasmtime-wasi embeddings that use a combination of DirPerms::MUTATE with FilePerms::READ are affected by this bug, e.g. those that use in the `WasiCtxBuilder`: ```rust builder.preopened_di…

Affected: crates.io: wasmtime-wasi

This is an entry in the RustSec database for the Wasmtime security advisory located at https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-2r75-cxrj-cmph For more information see the GitHub-hosted security advisory.

Affected: Ubuntu:24.04:LTS: rust-wasmtime, Ubuntu:25.10: rust-wasmtime, Ubuntu:26.04:LTS: rust-wasmtime

Wasmtime is a runtime for WebAssembly. In versions prior to 24.0.9, 36.0.10, and 44.0.2, when a filesystem preopen is given DirPerms::all() and FilePerms::READ without FilePerms::WRITE, this access control mechanism can be bypassed via the wasip2 descriptor.open-at or wasip1 path_open interfaces by opening a file with only the OpenFlags::TRUNCATE oflag. The root cause is that the clause handling OpenFlags::TRUNCATE in crates/wasi/src/filesystem.rs (Dir::open_at, lines 967–969) did not set open_mode |= OpenMode::WRITE;, which is later used for the access control check against FilePerms to determine whether opening the file is permitted; the single-line fix adds that missing assignment, after which the affected calls correctly fail with error-code.not-permitted and ERRNO_PERM respectively. Only wasmtime-wasi embeddings that combine DirPerms::MUTATE with FilePerms::READ are affected by this bug. In particular, the Wasmtime project's wasmtime-cli's use of wasmtime-wasi is not affected, because it always sets FilePerms::all() for all preopens. This issue has been fixed in versions 24.0.9, 36.0.10 and44.0.2.