Wasmtime EUVD-2026-108302

Requires an embedder (or the Wasmtime CLI) running an affected version (47.0.0-48.0.3 or 49.0.0-49.0.1) that compiles and executes an attacker-supplied Wasm module using GC references, with exception support enabled - both GC and exceptions are on by default in Wasmtime v47 and above. The attacker must craft a module whose callee triggers a garbage collection while a GC reference is live across a try_call. LIMITING FACTORS: only deployments that execute untrusted/attacker-controlled Wasm are exposed; hosts that disable exception support (-W exceptions=n or Config::wasm_exceptions(false)) are not affected even before patching; and per CVSS PR:L/UI:P the attacker needs the ability to get a module loaded and executed by the host.

This is a medium-severity, context-dependent risk rather than a drop-everything emergency, and the signals are largely consistent. The vendor CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:H) scores 6.9 and rates ONLY availability impact as high, with no confidentiality or integrity impact and no scope/subsequent-system effect. There is no CISA KEV listing and no EPSS score in the provided data, so there is no evidence of active or probable widespread exploitation; the vuln.today decision label is 'this_month' (not act_now/emergency), matching a patch-promptly-but-not-panic posture. No public exploit code or PoC reference is present in the input - the only references are the GHSA and VulnCheck advisories - so exploitation is theoretical at time of analysis. The main caveat: the input tags include 'Information Disclosure', which conflicts with the vendor's VC:N (no confidentiality impact) rating - treat the confidentiality angle as unconfirmed and defer to the vendor vector. The practical risk is almost entirely a function of whether you run attacker-supplied Wasm: embedders executing untrusted guest modules with GC and exceptions enabled (the v47+ default) should treat this as a priority; everyone else has little exposure.

A service that embeds Wasmtime 47.0.0-48.0.3 or 49.0.0-49.0.1 to execute untrusted guest WebAssembly accepts and compiles an attacker-crafted module that uses GC references. The module is written so that a callee triggers garbage collection while a GC reference remains live across a try_call, leaving a stale/dangling reference and corrupting the GC heap, leading to incorrect execution or a crash within the runtime. No public exploit code is identified in the provided data; the attack is derived from the vendor advisory's description of the flaw.