Wasmtime versions prior to 24.0.9, 36.0.10, and 44.0.2 contain an improper access control vulnerability (CWE-284) in the handling of filesystem preopens. When a preopen is configured with DirPerms::all() and FilePerms::READ but without FilePerms::WRITE, the access control can be bypassed by opening a file with the OpenFlags::TRUNCATE flag via wasip2 descriptor.open-at or wasip1 path_open interfaces. The root cause is that the code handling OpenFlags::TRUNCATE in crates/wasi/src/filesystem.rs (Dir::open_at) did not set the open_mode to include OpenMode::WRITE, which is used to check permissions. This omission allows opening files with truncation despite lacking write permissions. The fix adds the missing open_mode |= OpenMode::WRITE assignment, causing affected calls to fail with permission errors. Only wasmtime-wasi embeddings combining DirPerms::MUTATE with FilePerms::READ are vulnerable; wasmtime-cli is not affected as it sets FilePerms::all() for preopens. The vulnerability is fixed in versions 24.0.9, 36.0.10, and 44.0.2.

This vulnerability allows an attacker to bypass intended filesystem access controls and perform file truncation operations without proper write permissions in affected wasmtime-wasi embeddings. This can lead to unauthorized modification of files, impacting integrity. The CVSS 3.1 score is 7.5 (high severity), reflecting network attack vector, low attack complexity, no privileges required, no user interaction, unchanged scope, no confidentiality impact, high integrity impact, and no availability impact.

A fix is available in wasmtime versions 24.0.9, 36.0.10, and 44.0.2. Users should upgrade to one of these versions or later to remediate this vulnerability. The wasmtime-cli tool is not affected by this issue. Patch status is confirmed by the vendor advisory. No additional mitigations are indicated.