Two Critical Wasmtime Flaws Could Let Malicious Code Break Out of Its Sandbox

Maintainers of Wasmtime, a runtime that runs WebAssembly code outside web browsers, have disclosed two critical flaws that could let a malicious module escape the sandbox meant to contain it. Both bugs are in Cranelift, the compiler Wasmtime uses by default, and the Bytecode Alliance rated each 9.9 out of 10 on the CVSS scale. Fixed versions were released on October 19 and 20. The maintainers have not reported any attacks, and neither flaw was in CISA's Known Exploited Vulnerabilities catalog as of October 26.

An attacker would first need a vulnerable Wasmtime build to compile a module they control, so the risk is highest for services that compile code written by other people. Fastly, for example, says its Compute platform runs customer code using Wasmtime. One of the flaws affects only ARM64 systems, and ARM64 hosts that updated in April to fix an earlier sandbox escape are still affected by CVE-2026-115893. The advisories do not name affected services or say what an attacker could access after escaping.

Only the 36, 48, 49 and 50 release lines are still supported. Other lines, including the 24.x LTS line, received no fix. Supported users should update to 36.0.19, 48.0.7, 49.0.4 or 50.0.0. Because Wasmtime can be built into other programs as a library, updating the command-line tool is not enough: each program that includes it must be rebuilt with a fixed version.