Wasmtime EUVD-2026-108309

Exploitation requires that the WebAssembly exceptions proposal is enabled - on-by-default starting in Wasmtime 47.0.0 - and that the attacker can supply or instantiate an untrusted WebAssembly component in the host embedding. The specific trigger is a core module importing a WebAssembly tag whose type is a subtype of the original tag's type, which the faulty validator accepts; the guest then mis-types an exception object payload. Limiting factors: only Wasmtime 47.0.0-48.0.3 and 49.0.0-49.0.1 are affected; embeddings that do not run untrusted guests or that disable the exceptions proposal are not exposed; and impact is bounded to a panic/DoS because the GC heap is designed to be resilient to corruption. The vendor states there is no workaround other than upgrading to 48.0.4/49.0.2 or disabling the exceptions proposal.

This is a genuine but moderate-severity issue, not an emergency. The CVSS 4.0 vector (5.9, AV:N/AC:L/AT:P/PR:L/UI:P) and the vendor advisory agree that the only impact is availability: the mis-typed exception payload corrupts Wasmtime's GC heap, and because that heap is deliberately engineered to be resilient the realistic worst case is a host-side panic (denial of service), not code execution or data disclosure - consistent with VC:N/VI:N/VA:H. PR:L and AT:P reflect that the attacker must be able to supply/instantiate an untrusted guest component AND that a configuration condition (the WebAssembly exceptions proposal, on-by-default since 47.0.0) must hold, which materially narrows exposure to embeddings that run untrusted WebAssembly. Signals are mostly aligned toward a 'patch this cycle' posture rather than drop-everything: the vuln.today decision label is this_month, there is no CISA KEV listing, and no public exploit code was identified at time of analysis. Key caveat on conflicting metadata: the supplied Tag 'Information Disclosure' directly contradicts both the vendor description (GC heap corruption → panic) and the CVSS confidentiality metric (VC:N) - the actual security property at risk is availability, so treat the disclosure framing as mislabeled. EPSS is not provided in the input, so exploitation probability could not be factored in.

An attacker who can submit an untrusted WebAssembly component to a Wasmtime embedding (version 47.0.0-48.0.3 or 49.0.0-49.0.1) with the exceptions proposal enabled crafts a core module that imports a WebAssembly tag using a subtype of the original tag's type, which Wasmtime's validator wrongly accepts. The guest then mis-types an exception object's payload, corrupting the host's GC heap and causing Wasmtime to panic (denial of service). No public exploit identified at time of analysis; the attack requires AV:N delivery of the component and relatively low attack complexity once the exceptions proposal is active.