Wasmtime Patches Two Critical Sandbox Escape Flaws in Cranelift
Bytecode Alliance fixed two critical flaws in Wasmtime's Cranelift compiler: CVE-2026-115889, silent miscompilation on any architecture, and CVE-2026-115893, sandbox escape on ARM64. Both are rated CVSS 9.9; exploitation requires a vulnerable Wasmtime instance to compile an attacker-controlled WebAssembly module, and no exploitation in the wild has been reported. Update to Wasmtime 36.0.19, 48.0.7, 49.0.4, or 50.0.0, or Spin 4.2.4; unsupported lines, including 24.x LTS, get no fix.
What happened
Bytecode Alliance fixed two critical flaws (CVSS 9.9) in Wasmtime, a WebAssembly runtime, disclosed in advisories on October 19, 2026. A malicious WebAssembly module can make the compiler generate incorrect machine code and escape the runtime sandbox. No exploitation in the wild has been reported.
Why it matters
Services that accept and execute untrusted customer WebAssembly modules face the greatest risk, according to the disclosure. A sandbox escape gives the attacker access to host data and resources, with high impact on confidentiality, integrity, and availability. On ARM64, every version since support was introduced in 2020 is affected.
What to do
Update to Wasmtime 36.0.19, 48.0.7, 49.0.4, or 50.0.0, or Spin 4.2.4. Unsupported lines, including 24.x LTS, get no fix: plan a migration. Check ARM64 hosts on 36.0.7, 42.0.2, or 43.0.1, which remain affected by CVE-2026-115893.
For the technical team
- CVE
- CVE-2026-115889, CVE-2026-115893
- Affected
- Bytecode Alliance Wasmtime (compilador Cranelift) · CVE-2026-115889: wasmtime before 36.0.19, and 37.0.0 to before 43.0.0, any architecture. CVE-2026-115893: wasmtime before 36.0.19, 37.0.0 to before 48.0.7, and 49.0.0 to before 49.0.4, on aarch64; all versions supporting aarch64 since 2020, before Wasmtime v1.0. ARM64 hosts on 36.0.7, 42.0.2, or 43.0.1 (from the April Cranelift fix) remain affected by CVE-2026-115893. Unsupported lines, including 24.x LTS, get no fix.
- Exploitation
- No exploitation in the wild reported, per DarkWebInformer. Advisories published October 19, 2026; on October 26, 2026 Bytecode Alliance detailed affected versions and fixes and press attributed CVSS 9.9 to both flaws. No PoC cited in the sources.
- Fix
- wasmtime 36.0.19, 48.0.7, 49.0.4, and 50.0.0; Spin 4.2.4 (moves to Wasmtime 50.0.0). No fix for unsupported lines, such as 24.x LTS.
- Vector
- Flaw in Cranelift, Wasmtime's default compiler. CVE-2026-115889: bit-packing causes silent overflow and miscompilation on any architecture. CVE-2026-115893 (CWE-691, insufficient control flow management): on aarch64, a br_table instruction with more than ~256k targets compiles to an incorrect conditional branch target; the branch can jump to arbitrary code at a user-controlled offset, leading to arbitrary sandbox escapes. Exploitation requires a vulnerable Wasmtime instance to compile an attacker-controlled Wasm module. Vector: AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H.
- More details
- CVE-2026-115893 reported by ilyas-mallah; advisory GHSA-mghf-wjfp-p5xm published by pchickey on October 19, 2026. CVE-2026-115889 corresponds to advisory GHSA-897f-g5gm-fmxm. CVE-2026-115893 affects any Wasmtime engine on aarch64 with default settings. The Hacker News reported that both flaws could let malicious WebAssembly modules escape sandboxes.
Timeline
Disclosed details give both flaws a CVSS of 9.9 and note that exploitation requires a vulnerable Wasmtime instance.
Bytecode Alliance advisory details affected versions and fixes for Wasmtime and Spin.
Sources 3 sources
- GHSA-897f-g5gm-fmxm: Bit-packing in Cranelift compiler causes silent overflow and miscompilation Bytecode Alliance
- GHSA-mghf-wjfp-p5xm: Cranelift backend on aarch64 can jump to incorrect target on large Wasm br_table instruction Bytecode Alliance
- Two Critical Wasmtime Flaws Could Let Malicious WebAssembly Modules Escape Sandboxes The Hacker News
- @DarkWebInformer @DarkWebInformer
