CVE-2026-47261
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:
dir_descriptor.open_at(
PathFlags::empty(),
FILENAME,
OpenFlags::TRUNCATE,
DescriptorFlags::READ,
)
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:
if oflags.contains(OpenFlags::TRUNCATE) {
opts.truncate(true).write(true);
}
and the single line fix is:
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:
builder.preopened_dir("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.
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Exploitation outlook
- Low exploitation risk0.36% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 28% of all EPSS-scored CVEs.
Advisory coverage (1)
External references
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