CVE-2026-34988: Cross-Guest Memory Leak in Wasmtime Pooling Allocator

Wasmtime utilizes a pooling allocator to manage and reuse pre-allocated memory slots for WebAssembly guest instances. This component improves instantiation performance by recycling virtual memory regions instead of continuously allocating new ones via the host operating system. When an instance terminates, the allocator returns its slot to the pool and resets the memory permissions to prevent data leakage between distinct guests.

CVE-2026-34988 represents a flaw in this permission reset mechanism. Under specific non-default configurations, the allocator fails to re-apply the correct virtual memory protections to the slack space of a reused memory slot. A subsequent guest instance can access residual data left in this memory region by the previous tenant. This behavior violates the fundamental isolation guarantees established by the WebAssembly sandbox.

The vulnerability is classified as CWE-119, representing an improper restriction of operations within memory bounds. It requires a highly specific configuration involving a zero-sized memory guard and a fixed memory reservation matching the maximum allowed size. While the scope is strictly limited to data leakage within the bounds of a shared slot, the flaw highlights the complexity of synchronizing compiler assumptions with runtime memory management logic.

The root cause of CVE-2026-34988 is a predicate mismatch between the Cranelift compiler's optimization assumptions and the runtime pooling allocator's memory-reset behavior. To maximize execution speed, Cranelift elides explicit bounds-check instructions if it expects the host operating system to trap out-of-bounds accesses via hardware page faults (SIGSEGV). The compiler bases this optimization decision on the memory_reservation and memory_guard_size configuration parameters.

When reusing a memory slot, the pooling allocator must reset virtual memory protections using mprotect. It applies PROT_NONE to unmapped regions to ensure out-of-bounds accesses trigger the expected hardware traps. The allocator implementation contained a logic error where it used the can_elide_bounds_check predicate to determine if a permission reset was required for the slack region between the initial memory size and the total reservation.

The vulnerability manifests when Config::memory_guard_size is zero, Config::memory_reservation is under 4GiB, and the maximum memory size equals the reservation. Under these precise conditions, the allocator incorrectly concludes that a permission reset is unnecessary because the reservation is fixed and bounds checks are not elided. Consequently, the host virtual memory pages remain mapped with PROT_READ|PROT_WRITE permissions during slot reuse.

The vulnerability resides in crates/wasmtime/src/runtime/vm/cow.rs, specifically within the internal logic handling the initialization and reset of memory slots. The vulnerable code relied on the compiler-specific can_elide_bounds_check function to dictate runtime memory management. This architectural coupling caused the allocator to skip the mprotect call when the compiler decided not to elide bounds checks.

// Vulnerable Code Path
if initial_size_bytes_page_aligned < self.accessible
    && (tunables.memory_guard_size > 0
        || ty.can_elide_bounds_check(tunables, host_page_size_log2))
{ 
    // Apply PROT_NONE to the slack region
}

The patch resolves the vulnerability by replacing the can_elide_bounds_check predicate with can_use_virtual_memory. This change decouples the runtime protection logic from the compiler's optimization strategies. The allocator now correctly resets virtual memory permissions whenever the system relies on virtual memory mapping, regardless of the bounds-check elision status.

// Patched Code Path
if initial_size_bytes_page_aligned < self.accessible
    && (tunables.memory_guard_size > 0
        || ty.can_use_virtual_memory(tunables, host_page_size_log2))
{ 
    // Apply PROT_NONE to the slack region
}

Exploiting CVE-2026-34988 requires the attacker to control the WebAssembly module executed within the vulnerable Wasmtime runtime environment. The runtime must be explicitly configured with the pooling allocator enabled, a memory_guard_size of 0, and a memory_reservation identical to the max_memory_size. These prerequisites significantly restrict the attack surface, as most standard Wasmtime deployments utilize default configurations that are not vulnerable to this issue.

An attacker initiates the exploit sequence by provisioning a malicious WebAssembly guest module designed to perform out-of-bounds memory reads. This module must be instantiated into a pooling allocator slot previously occupied by a different, potentially benign, guest instance. The previous instance must have grown its heap into the reserved slack space and populated it with sensitive data before termination.

Upon execution, the attacker's module performs memory load operations targeting offsets beyond its initial memory size but within the bounds of the total memory reservation. Because the pooling allocator failed to reset the PROT_NONE protections on the reused virtual memory pages, the host operating system does not generate a hardware trap. The load operations succeed, allowing the attacker's module to extract residual plaintext data left by the previous tenant.

The primary security impact of CVE-2026-34988 is the unauthorized disclosure of information across WebAssembly trust boundaries. An attacker successfully exploiting this vulnerability gains read access to residual data left in linear memory by a previous guest instance. This data leakage undermines the core sandbox isolation properties that WebAssembly runtimes are designed to enforce at the host level.

The exposed data depends entirely on the workload of the previous instance assigned to the same pooling allocator slot. If the previous tenant processed cryptographic keys, user session tokens, or other sensitive application data within its heap, this information becomes directly accessible to the attacker. The vulnerability does not grant the ability to modify the memory of active instances or achieve arbitrary code execution on the underlying host system.

The CVSS v4.0 score of 2.3 reflects the low overall severity and the stringent requirements necessary for successful exploitation. The attack complexity is rated High due to the necessity of a specific, non-default Wasmtime configuration. Furthermore, the attacker must reliably land their instance in a newly recycled memory slot containing valuable residual data, adding an element of probabilistic complexity to the exploit chain.

The primary remediation for CVE-2026-34988 is upgrading the Wasmtime runtime to a patched version. The Bytecode Alliance has released versions 36.0.7, 42.0.2, and 43.0.1, which contain the corrected predicate logic in the pooling allocator reset routine. Administrators operating multi-tenant environments utilizing the pooling allocator must prioritize deploying these updates to fully restore WebAssembly sandbox isolation.

Systems unable to apply the software updates immediately can mitigate the vulnerability through strategic configuration changes. Disabling the pooling allocator entirely eliminates the attack vector, as instances will rely on standard operating system allocations that guarantee zeroed memory. However, administrators must note that this approach incurs a significant performance penalty during module instantiation.

Alternatively, administrators can retain the pooling allocator but adjust the memory configuration parameters to avoid the specific vulnerable state. Configuring Config::memory_guard_size to any non-zero value forces the allocator to apply the necessary memory protections during slot reset. Ensuring that the memory_reservation value differs from the max_memory_size also prevents the runtime from entering the vulnerable execution path.