{"id":"UBUNTU-CVE-2026-34987","details":"Wasmtime is a runtime for WebAssembly. From 25.0.0 to before 36.0.7, 42.0.2, and 43.0.1, Wasmtime with its Winch (baseline) non-default compiler backend may allow properly constructed guest Wasm to access host memory outside of its linear-memory sandbox. This vulnerability requires use of the Winch compiler (-Ccompiler=winch). By default, Wasmtime uses its Cranelift backend, not Winch. With Winch, the same incorrect assumption is present in theory on both aarch64 and x86-64. The aarch64 case has an observed-working proof of concept, while the x86-64 case is theoretical and may not be reachable in practice. This Winch compiler bug can allow the Wasm guest to access memory before or after the linear-memory region, independently of whether pre- or post-guard regions are configured. The accessible range in the initial bug proof-of-concept is up to 32KiB before the start of memory, or ~4GiB after the start of memory, independently of the size of pre- or post-guard regions or the use of explicit or guard-region-based bounds checking. However, the underlying bug assumes a 32-bit memory offset stored in a 64-bit register has its upper bits cleared when it may not, and so closely related variants of the initial proof-of-concept may be able to access truly arbitrary memory in-process. This could result in a host process segmentation fault (DoS), an arbitrary data leak from the host process, or with a write, potentially an arbitrary RCE. This vulnerability is fixed in 36.0.7, 42.0.2, and 43.0.1.","modified":"2026-05-20T16:12:55.194307955Z","published":"2026-04-09T19:16:00Z","upstream":["CVE-2026-34987"],"references":[{"type":"REPORT","url":"https://ubuntu.com/security/CVE-2026-34987"},{"type":"REPORT","url":"https://www.cve.org/CVERecord?id=CVE-2026-34987"},{"type":"REPORT","url":"https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-xx5w-cvp6-jv83"}],"affected":[{"package":{"name":"rust-wasmtime","ecosystem":"Ubuntu:24.04:LTS","purl":"pkg:deb/ubuntu/rust-wasmtime?arch=source&distro=noble"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["10.0.1+dfsg-7","15.0.1+dfsg-4","16.0.0+dfsg-2","16.0.0+dfsg-3"],"ecosystem_specific":{"binaries":[{"binary_version":"0.103.0+16.0.0+dfsg-3","binary_name":"librust-cranelift-dev"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-34987.json"}},{"package":{"name":"rust-wasmtime","ecosystem":"Ubuntu:25.10","purl":"pkg:deb/ubuntu/rust-wasmtime?arch=source&distro=questing"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["26.0.1+dfsg-3"],"ecosystem_specific":{"binaries":[{"binary_version":"0.113.1+26.0.1+dfsg-3","binary_name":"librust-cranelift-dev"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-34987.json"}},{"package":{"name":"rust-wasmtime","ecosystem":"Ubuntu:26.04:LTS","purl":"pkg:deb/ubuntu/rust-wasmtime?arch=source&distro=resolute"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["26.0.1+dfsg-3","26.0.1+dfsg-4","28.0.1+dfsg-3","29.0.1+dfsg-5","36.0.5+dfsg-1"],"ecosystem_specific":{"binaries":[{"binary_version":"0.123.5+36.0.5+dfsg-1","binary_name":"librust-cranelift-dev"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-34987.json"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V4","score":"CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H"},{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H"},{"type":"Ubuntu","score":"medium"}]}