CVE-2026-35195: Bytecode Alliance Wasmtime out-of-bounds write in string transcoding

CVE-2026-35195 · Severity: medium · CVSS 5.4 · Published 2026-04-09

Wasmtime is a runtime for executing WebAssembly code, often used to run untrusted code in a secure sandbox. A flaw in how it handles text conversion between different components allows a malicious guest program to write data to memory locations it should not be able to access. In most cases, this will cause the application to crash, but in specific configurations, it could lead to the corruption of sensitive data or other programs running on the same system.

Technologies: Bytecodealliance Wasmtime, wasmtime (crates.io). Vendors: Bytecodealliance, crates.io.

Quick answers

Is CVE-2026-35195 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-35195 being exploited in the wild.
How severe is CVE-2026-35195?
Medium severity, with a CVSS score of 5.4, published 9 April 2026.
Which products and versions are affected?
wasmtime (crates.io) SEMVER:introduced=0,fixed=24.0.7, SEMVER:introduced=0.0.0-0,fixed=24.0.7,introduced=25.0.0,fixed=36.0.7,introduced=37.0.0,fixed=42.0.2,introduced=43.0.0,fixed=43.0.1.
Is there a fix for CVE-2026-35195?
Yes. wasmtime (crates.io): upgrade to 24.0.7, 43.0.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-35195 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-35195?
Check whether you run Bytecodealliance Wasmtime or wasmtime (crates.io) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • wasmtime (crates.io): upgrade to 24.0.7, 43.0.1 or later
    cargo update -p wasmtime

Technical details

A vulnerability exists in Wasmtime's implementation of string transcoding between components. The root cause is a failure to validate the return value of a guest component's 'realloc' function before the host attempts to write transcoded bytes through the resulting pointer. An attacker controlling a guest component can trigger an out-of-bounds write of arbitrary transcoded string bytes to locations up to 4GiB away from the base of linear memory. While default configurations (4GiB virtual memory reservation) typically result in a process abort due to an unhandled fault (DoS), configurations with smaller reservations or no guard pages may allow corruption of host data structures or other guest memories. Patches are available in versions 24.0.7, 36.0.7, 42.0.2, and 43.0.1.

Affected products

  • Bytecode Alliance wasmtime < 24.0.7, >= 25.0.0 < 36.0.7, >= 37.0.0 < 42.0.2, 43.0.0

Identifiers

  • CVE-2026-35195
  • GHSA-394w-hwhg-8vgm
  • RUSTSEC-2026-0091

Timeline

  • 2026-04-09: disclosed
  • 2026-04-09: patched
  • 2026-04-09: advisory

References

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