wasmtime-wasi: WASI path_open(TRUNCATE) bypasses `FilePerms::WRITE` host restriction
Date: 2026-06
Affected: rust/wasmtime-wasi, rust/wasmtime-wasi, rust/wasmtime-wasi
Attack vector: auth-bypass
Category: vulnerability-disclosure
Severity: High
CVE: CVE-2026-47261
CWE: CWE-284
CVSS: 7.5 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N)

## 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_dir


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