Wasmtime fixes two Cranelift flaws that could escape its sandbox

Two critical Wasmtime sandbox escape flaws have been patched in Cranelift, CVE-2026-115889 and CVE-2026-115893. The Bytecode Alliance said a crafted WebAssembly module could cause Cranelift to generate unsafe native code, undermining Wasmtime’s isolation. The fixes were released on 19 October 2026, with both advisories rating the flaws as 9.9 on CVSS v3.1.

The first flaw, CVE-2026-115889, is a silent overflow in Cranelift’s 24-bit SSA value tracking that could misdirect the compiler and lead to type confusion or other side effects. The second, CVE-2026-115893, concerns an out-of-range default branch in very large br_table sequences, which on ARM64 could allow a user-controlled Wasm module to force execution of arbitrary code. Both flaws are considered high severity, with CVSS scores of 9.9.

The affected and fixed Wasmtime versions vary by release line and architecture. CVE-2026-115889 affects all architectures prior to 36.0.19, with safe releases beginning at 36.0.19; CVE-2026-115893 affects ARM64 and requires updating ARM64 hosts to a fixed release. The guidance notes that long-term support 24.x lines remain exposed and should move to 36.0.19 or newer, while ARM64 hosts already upgraded to earlier Cranelift fixes still need the 115893 patch.

No exploitation, PoC, or KEV listings had been reported as of 26 October 2026, and the advisories describe the attack as network-reachable with low complexity and no user interaction. The researchers behind the reports are credited as jaydns (for the silent overflow) and ilyas-mallah (for the ARM64 flaw), with no public PoCs at the time.