CVE-2026-47261: Wasmtime: WASI path_open(TRUNCATE) bypasses `FilePerms::WRITE` host restriction

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 pathopen interfaces by opening a file with OpenFlags::TRUNCATE oflag only, for example:

rust dirdescriptor.openat( PathFlags::empty(), FILENAME, OpenFlags::TRUNCATE, DescriptorFlags::READ, )

rust wasip1::pathopen( dirfd, 0, FILENAME, wasip1::OFLAGSTRUNC, wasip1::RIGHTSFDREAD, 0, 0 )

The root cause is that the clause that considered OpenFlags::TRUNCATE did not set openmode |= 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 pathopen fail with error-code.not-permitted and ERRNOPERM respectively.

The bug in crates/wasi/src/filesystem.rs, Dir::openat, 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); openmode |= 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.preopeneddir("readonly", "readonly", DirPerms::READ | DirPerms::MUTATE, FilePerms::READ);

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.