Critical Wasmtime Flaws Could Enable WebAssembly Sandbox Escapes
Summary
Two critical Cranelift flaws can let crafted Wasm modules escape Wasmtime’s sandbox: one affects all architectures, the other ARM64. No exploitation was reported; upgrade to fixed releases, with no workaround available.
Key points
- CVE-2026-115889 involves silent overflow of 24-bit SSA value indices and affects vulnerable Cranelift builds across architectures.
- CVE-2026-115893 affects ARM64 and can misdirect a br_table default branch when a table contains more than roughly 256,000 targets.
- The flaws could cause miscompilation, type confusion, or arbitrary code execution outside the Wasm sandbox; the article says no public post-escape chain has been demonstrated.
- Fixed releases include Wasmtime 36.0.19, 48.0.7, 49.0.4, and 50.0.0, depending on the release line; 24.x users must move to a supported line.
- No in-the-wild exploitation or public proof of concept was reported as of October 26, 2026.
- The advisories provide no workaround. The article recommends upgrading, rebuilding embedding applications, and recompiling cached modules.
- Until upgrades are deployed, teams can limit untrusted-module compilation and isolate Wasmtime processes with minimal privileges.
Article Details
- Vulnerability Types
- WebAssembly sandbox escape
- 24-bit SSA value index overflow
- Out-of-range default branch offset in large br_table sequences
- Severity
- Critical; both CVEs are rated 9.9 under CVSS v3.1
- Affected Versions
- Wasmtime versions earlier than 36.0.19, including 24.x (CVE-2026-115889 and CVE-2026-115893)
- Wasmtime 37.0.0 to 42.x (CVE-2026-115889 and CVE-2026-115893)
- Wasmtime 43.0.0 to 47.x on ARM64 (CVE-2026-115893)
- Wasmtime 48.0.0 to 48.0.6 on ARM64 (CVE-2026-115893)
- Wasmtime 49.0.0 to 49.0.3 on ARM64 (CVE-2026-115893)
- Exploitation Status
- not_reported
- Exploit Availability
- none_reported
- Patch Status
- available
- Workarounds
- Temporarily pause or limit compilation of modules from untrusted parties where feasible.
- Require authentication and review for module uploads; reject unusually large modules or functions.
- Run Wasmtime processes with minimal privileges and strong OS-level isolation.
CVE
CVE-2026-115889Two critical Wasmtime sandbox escape flaws, CVE-2026-115889 and CVE-2026-115893, let a crafted WebAssembly (Wasm) module push the Cranelift compiler into generating unsafe machine code.CVE-2026-115893Two critical Wasmtime sandbox escape flaws, CVE-2026-115889 and CVE-2026-115893, let a crafted WebAssembly (Wasm) module push the Cranelift compiler into generating unsafe machine code.CVE-2026-34971Some teams upgraded ARM64 hosts to 36.0.7, 42.0.2 or 43.0.1 to fix an earlier Cranelift sandbox escape, CVE-2026-34971.
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CraneliftTwo critical Wasmtime sandbox escape flaws, CVE-2026-115889 and CVE-2026-115893, let a crafted WebAssembly (Wasm) module push the Cranelift compiler into generating unsafe machine code.WasmtimeTwo critical Wasmtime sandbox escape flaws, CVE-2026-115889 and CVE-2026-115893, let a crafted WebAssembly (Wasm) module push the Cranelift compiler into generating unsafe machine code.