CVE-2026-34945: Wasmtime Wasmtime Has Host Data Leakage Vulnerability

LOW
Advisory Identifier: GHSA-m9w2-8782-2946
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CVSS 6.5
CWE-681 CWE-200
Published
Advisory Summary

CVE-2026-34945 is a LOW (CVSS 6.5) wasmtime has host data leakage vulnerability (CWE-681: Incorrect Conversion between Numeric Types) affecting Wasmtime libraries in the crates.io ecosystem. The issue involves wasmtime has host data leakage with 64-bit tables and winch. Remediation is available by upgrading affected packages to version 36.0.7, 42.0.2, 43.0.1.

CVE-2026-34945 at a Glance

CVE Identifier CVE-2026-34945
GHSA Identifier GHSA-m9w2-8782-2946
Severity LOW
CVSS v3.1 Base Score 6.5
CWE Classification CWE-681: Incorrect Conversion between Numeric Types
Affected Product Wasmtime
Package Ecosystem crates.io
Published Date
Last Updated

Security Impact

CVSS: 6.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required Low (PR:L)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality High (C:H)
Integrity None (I:N)
Availability None (A:N)

How to Fix CVE-2026-34945

To remediate this vulnerability, upgrade all affected Wasmtime dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

wasmtime fixed: 36.0.7 
Upgrade wasmtime to 36.0.7
wasmtime fixed: 42.0.2 
Upgrade wasmtime to 42.0.2
wasmtime fixed: 43.0.1 
Upgrade wasmtime to 43.0.1
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

inspect dependencies for wasmtime

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-34945, inspect your project dependencies for Wasmtime:

inspect dependencies for wasmtime

Analyze the terminal output to locate any version of Wasmtime in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 36.0.7), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-681: Incorrect Conversion between Numeric Types, where when converting from one data type to another, such as long to integer, data can be omitted or translated in a way that produces unexpected values. if the resulting values are used in a sensitive context, then dangerous behaviors may occur. In Wasmtime, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### Impact Wasmtime's Winch compiler contains a bug where a 64-bit table, part of the memory64 proposal of WebAssembly, incorrectly translated the `table.size` instruction. This bug could lead to disclosing data on the host's stack to WebAssembly guests. The host's stack can possibly contain sensitive data related to other host-originating operations which is not intended to be disclosed to guests. This bug specifically arose from a mistake where the return value of `table.size` was statically typed as a 32-bit integer, as opposed to consulting the table's index type to see how large the returned register could be. When combined with details about Wnich's ABI, such as multi-value returns, this can be combined to read stack data from the host, within a guest. This information disclosure should not be possible in WebAssembly, violates spec semantics, and is a vulnerability in Wasmtime. ### Patches Wasmtime 36.0.7, 42.0.2, and 43.0.1 have been issued to fix this bug. Users are recommended to update to these patched versions of Wasmtime. ### Workarounds Users of Cranelift are not affected by this issue, but users of Winch have no workarounds other than disabling the `Config::wasm_memory64` proposal.

CVE-2026-34945 Affected Wasmtime Versions

3 component(s)
Fixed Versions:
36.0.742.0.243.0.1
Product / Package Ecosystem Affected Versions Fixed Version
wasmtime
 crates.io
≥ 25.0.0
< 36.0.7
36.0.7
wasmtime
 crates.io
≥ 37.0.0
< 42.0.2
42.0.2
wasmtime
 crates.io
≥ 43.0.0
< 43.0.1
All 1 versions
43.0.0
43.0.1

References

GitHub Advisory / Wiki ADVISORY
https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-m9w2-8782-2946
NVD — CVE-2026-34945 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-34945
Wasmtime Source Repository REPOSITORY
https://github.com/bytecodealliance/wasmtime
rustsec.org/advisories/rustsec-... WEB
https://rustsec.org/advisories/RUSTSEC-2026-0086.html

Frequently Asked Questions

What is CVE-2026-34945?
CVE-2026-34945 is a security vulnerability in Wasmtime involving Wasmtime has host data leakage with 64-bit tables and Winch under CWE-681: Incorrect Conversion between Numeric Types.
Which Wasmtime versions are affected?
Affected versions include releases prior to 36.0.7, 42.0.2, 43.0.1 across affected packages: wasmtime.
What version fixes CVE-2026-34945?
Upgrading to 36.0.7, 42.0.2, 43.0.1 (or the latest patched release for your respective branch) resolves the vulnerability.
Is wasmtime affected?
Yes, wasmtime is affected in releases prior to 36.0.7, 42.0.2, 43.0.1. Upgrade your dependency to 36.0.7, 42.0.2, 43.0.1 or later.
What is the CVSS score of CVE-2026-34945?
CVE-2026-34945 has a CVSS v3.1 base score of 6.5 (LOW) with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N.
What CWE is associated with CVE-2026-34945?
CVE-2026-34945 is classified under CWE-681: Incorrect Conversion between Numeric Types (When converting from one data type to another, such as long to integer, data can be omitted or translated in a way that produces unexpected values. If the resulting values are used in a sensitive context, then dangerous behaviors may occur.).
How can I check crates.io dependencies for CVE-2026-34945?
Run the dependency tree command: 'inspect dependencies for wasmtime'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-m9w2-8782-2946",
  "modified" : "2026-04-24T21:03:32Z",
  "published" : "2026-04-09T20:23:02Z",
  "aliases" : [ "CVE-2026-34945" ],
  "summary" : "Wasmtime has host data leakage with 64-bit tables and Winch",
  "details" : "### Impact\n\nWasmtime's Winch compiler contains a bug where a 64-bit table, part of the memory64 proposal of WebAssembly, incorrectly translated the `table.size` instruction. This bug could lead to disclosing data on the host's stack to WebAssembly guests. The host's stack can possibly contain sensitive data related to other host-originating operations which is not intended to be disclosed to guests.\n\nThis bug specifically arose from a mistake where the return value of `table.size` was statically typed as a 32-bit integer, as opposed to consulting the table's index type to see how large the returned register could be. When combined with details about Wnich's ABI, such as multi-value returns, this can be combined to read stack data from the host, within a guest. This information disclosure should not be possible in WebAssembly, violates spec semantics, and is a vulnerability in Wasmtime.\n\n### Patches\n\nWasmtime 36.0.7, 42.0.2, and 43.0.1 have been issued to fix this bug. Users are recommended to update to these patched versions of Wasmtime.\n\n### Workarounds\n\nUsers of Cranelift are not affected by this issue, but users of Winch have no workarounds other than disabling the `Config::wasm_memory64` proposal.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:L/VI:N/VA:N/SC:L/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "crates.io",
      "name" : "wasmtime"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "25.0.0"
      }, {
        "fixed" : "36.0.7"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "crates.io",
      "name" : "wasmtime"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "37.0.0"
      }, {
        "fixed" : "42.0.2"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "crates.io",
      "name" : "wasmtime"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "43.0.0"
      }, {
        "fixed" : "43.0.1"
      } ]
    } ],
    "versions" : [ "43.0.0" ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-m9w2-8782-2946"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-34945"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/bytecodealliance/wasmtime"
  }, {
    "type" : "WEB",
    "url" : "https://rustsec.org/advisories/RUSTSEC-2026-0086.html"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-200", "CWE-681" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-09T20:23:02Z",
    "nvd_published_at" : "2026-04-09T19:16:24Z"
  }
}