CVE-2026-46695

Boxlite is a sandbox service that allows users to create lightweight virtual machines (Boxes) and launch OCI containers within them to run untrusted code. Prior to version 0.9.0, Boxlite does not restrict the kernel capabilities available inside the container, malicious code can remount the directory in rw mode, thereby gaining write access to that directory. This allows malicious code to perform arbitrary write operations on directories that should be read-only. This issue has been patched in version 0.9.0.

Boxlite is a sandbox service that allows users to create lightweight virtual machines (Boxes) and launch OCI containers within them to run untrusted code. Prior to version 0.9.0, Boxlite does not restrict the kernel capabilities available inside the container, malicious code can remount the directory in rw mode, thereby gaining write access to that directory. This allows malicious code to perform arbitrary write operations on directories that should be read-only. This issue has been patched in version 0.9.0.

Summary

Boxlite is a sandbox service that allows users to create lightweight virtual machines (Boxes) and launch OCI containers within them to run untrusted code.

One of the core security features claimed by Boxlite is the ability to mount host directories in read-only mode (read_only=True) into the VM via the virtiofs protocol (a host-guest shared filesystem protocol designed specifically for virtual machines), so that untrusted code can only read but not modify host data. Since the underlying function of the lightweight VM library libkrun used by Boxlite does not support mounting in read-only mode, Boxlite chooses to implement read-only by adding the MS_RDONLY flag when mounting the directory after the VM starts.

However, because Boxlite does not restrict the kernel capabilities available inside the container, malicious code can remount the directory in rw mode, thereby gaining write access to that directory. This allows malicious code to perform arbitrary write operations on directories that should be read-only.

In typical usage scenarios of Boxlite, an attacker can leverage this vulnerability to gain code execution capability on the host. For example, in AI Agent scenarios, user code, virtual environments, credentials, configuration files, and other content are often mounted in read-only mode into the container. Malicious code inside the sandbox can modify this information, such as planting malicious code, to gain code execution capability on the host, which may further introduce supply chain risks.

Details

1. User-Facing API Documents Read-Only Guarantee

File: boxlite/src/runtime/options.rs Function: VolumeSpec (line 223) Code:

/// Filesystem mount specification.
pub struct VolumeSpec {
    pub host_path: String,
    pub guest_path: String,
    pub read_only: bool,  // <-- operator sets this to restrict guest write access
}

Issue: The read_only field is documented (and in user-facing guides) as preventing the guest from writing to the host directory. The guarantee is "Agent can read but not write." This expectation is not met.

2. read_only Stored in FsShare — Passed to krun Without Enforcement

File: boxlite/src/vmm/krun/engine.rs Function: Krun::create() (line 334) Code:

for share in config.fs_shares.shares() {
    let path_str = share.host_path.to_str().ok_or_else(|| { ... })?;

    tracing::info!(
        "  {} → {} ({})",
        share.tag,
        share.host_path.display(),
        if share.read_only { "ro" } else { "rw" }  // Logged but NOT passed to krun
    );
    ctx.add_virtiofs(&share.tag, path_str)?;  // <-- read_only silently dropped
}

Issue: share.read_only is logged as "ro" or "rw" but is never passed to add_virtiofs. The actual hypervisor call receives only tag and host path.

3. Hypervisor FFI Has No Read-Only Parameter

File: boxlite/src/vmm/krun/context.rs Function: add_virtiofs() (line 423) Code:

pub unsafe fn add_virtiofs(&self, mount_tag: &str, host_path: &str) -> BoxliteResult<()> {
    let host_path_c = CString::new(host_path)
        .map_err(|e| BoxliteError::Engine(format!("invalid host path: {e}")))?;
    let mount_tag_c = CString::new(mount_tag)
        .map_err(|e| BoxliteError::Engine(format!("invalid mount tag: {e}")))?;

    check_status("krun_add_virtiofs", unsafe {
        krun_add_virtiofs(self.ctx_id, mount_tag_c.as_ptr(), host_path_c.as_ptr())
        // No read_only parameter — libkrun exposes the share as read-write to the guest
    })
}

Issue: krun_add_virtiofs in the FFI (deps/libkrun-sys/src/lib.rs:35) takes only ctx_id, mount_tag, and host_path. There is no read-only flag. Libkrun exposes the virtiofs share to the guest with full read-write access at the device level.

4. Read-Only Enforcement Is Delegated to Guest Agent (Zone 0)

File: boxlite/src/volumes/guest_volume.rs Function: build_guest_mounts() (line 184) Code: …

Boxlite is a sandbox service that allows users to create lightweight virtual machines (Boxes) and launch OCI containers within them to run untrusted code. Prior to version 0.9.0, Boxlite does not restrict the kernel capabilities available inside the container, malicious code can remount the directory in rw mode, thereby gaining write access to that directory. This allows malicious code to perform arbitrary write operations on directories that should be read-only. This issue has been patched in version 0.9.0.

Boxlite is a sandbox service that allows users to create lightweight virtual machines (Boxes) and launch OCI containers within them to run untrusted code. Prior to version 0.9.0, Boxlite does not restrict the kernel capabilities available inside the container, malicious code can remount the directory in rw mode, thereby gaining write access to that directory. This allows malicious code to perform arbitrary write operations on directories that should be read-only. This issue has been patched in version 0.9.0.

Summary

Boxlite is a sandbox service that allows users to create lightweight virtual machines (Boxes) and launch OCI containers within them to run untrusted code.

One of the core security features claimed by Boxlite is the ability to mount host directories in read-only mode (read_only=True) into the VM via the virtiofs protocol (a host-guest shared filesystem protocol designed specifically for virtual machines), so that untrusted code can only read but not modify host data. Since the underlying function of the lightweight VM library libkrun used by Boxlite does not support mounting in read-only mode, Boxlite chooses to implement read-only by adding the MS_RDONLY flag when mounting the directory after the VM starts.

However, because Boxlite does not restrict the kernel capabilities available inside the container, malicious code can remount the directory in rw mode, thereby gaining write access to that directory. This allows malicious code to perform arbitrary write operations on directories that should be read-only.

In typical usage scenarios of Boxlite, an attacker can leverage this vulnerability to gain code execution capability on the host. For example, in AI Agent scenarios, user code, virtual environments, credentials, configuration files, and other content are often mounted in read-only mode into the container. Malicious code inside the sandbox can modify this information, such as planting malicious code, to gain code execution capability on the host, which may further introduce supply chain risks.

Details

1. User-Facing API Documents Read-Only Guarantee

File: boxlite/src/runtime/options.rs Function: VolumeSpec (line 223) Code:

/// Filesystem mount specification.
pub struct VolumeSpec {
    pub host_path: String,
    pub guest_path: String,
    pub read_only: bool,  // <-- operator sets this to restrict guest write access
}

Issue: The read_only field is documented (and in user-facing guides) as preventing the guest from writing to the host directory. The guarantee is "Agent can read but not write." This expectation is not met.

2. read_only Stored in FsShare — Passed to krun Without Enforcement

File: boxlite/src/vmm/krun/engine.rs Function: Krun::create() (line 334) Code:

for share in config.fs_shares.shares() {
    let path_str = share.host_path.to_str().ok_or_else(|| { ... })?;

    tracing::info!(
        "  {} → {} ({})",
        share.tag,
        share.host_path.display(),
        if share.read_only { "ro" } else { "rw" }  // Logged but NOT passed to krun
    );
    ctx.add_virtiofs(&share.tag, path_str)?;  // <-- read_only silently dropped
}

Issue: share.read_only is logged as "ro" or "rw" but is never passed to add_virtiofs. The actual hypervisor call receives only tag and host path.

3. Hypervisor FFI Has No Read-Only Parameter

File: boxlite/src/vmm/krun/context.rs Function: add_virtiofs() (line 423) Code:

pub unsafe fn add_virtiofs(&self, mount_tag: &str, host_path: &str) -> BoxliteResult<()> {
    let host_path_c = CString::new(host_path)
        .map_err(|e| BoxliteError::Engine(format!("invalid host path: {e}")))?;
    let mount_tag_c = CString::new(mount_tag)
        .map_err(|e| BoxliteError::Engine(format!("invalid mount tag: {e}")))?;

    check_status("krun_add_virtiofs", unsafe {
        krun_add_virtiofs(self.ctx_id, mount_tag_c.as_ptr(), host_path_c.as_ptr())
        // No read_only parameter — libkrun exposes the share as read-write to the guest
    })
}

Issue: krun_add_virtiofs in the FFI (deps/libkrun-sys/src/lib.rs:35) takes only ctx_id, mount_tag, and host_path. There is no read-only flag. Libkrun exposes the virtiofs share to the guest with full read-write access at the device level.

4. Read-Only Enforcement Is Delegated to Guest Agent (Zone 0)

File: boxlite/src/volumes/guest_volume.rs Function: build_guest_mounts() (line 184) Code: …