CVE-2026-34942: Wasmtime Panic When Transcoding Misaligned Utf-16 Vulnerability

MODERATE
Advisory Identifier: GHSA-jxhv-7h78-9775
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CVSS 6.5
CWE-129 CWE-119
Published
Advisory Summary

CVE-2026-34942 is a MODERATE (CVSS 6.5) panic when transcoding misaligned utf-16 vulnerability (CWE-129: Improper Validation of Array Index) affecting Wasmtime libraries in the crates.io ecosystem. The issue involves panic when transcoding misaligned utf-16 strings. Remediation is available by upgrading affected packages to version 24.0.7, 36.0.7, 42.0.2, 43.0.1.

CVE-2026-34942 at a Glance

CVE Identifier CVE-2026-34942
GHSA Identifier GHSA-jxhv-7h78-9775
Severity MODERATE
CVSS v3.1 Base Score 6.5
CWE Classification CWE-129: Improper Validation of Array Index
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:N/I:N/A:H).

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 None (C:N)
Integrity None (I:N)
Availability High (A:H)

How to Fix CVE-2026-34942

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: 24.0.7 
Upgrade wasmtime to 24.0.7
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-2026-34942, 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 24.0.7), 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-129: Improper Validation of Array Index, where the product uses untrusted input when calculating or using an array index, but the product does not validate or incorrectly validates the index to ensure the index references a valid position within the array. 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 implementation of transcoding strings into the Component Model's `utf16` or `latin1+utf16` encodings improperly verified the alignment of reallocated strings. This meant that unaligned pointers could be passed to the host for transcoding which would trigger a host panic. This panic is possible to trigger from malicious guests which transfer very specific strings across components with specific addresses. Host panics are considered a DoS vector in Wasmtime as the panic conditions are controlled by the guest in this situation. ### Patches Wasmtime 24.0.7, 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 There is no workaround for this bug. Hosts are recommended to updated to a patched version of Wasmtime.

CVE-2026-34942 Affected Wasmtime Versions

4 component(s)
Fixed Versions:
24.0.736.0.742.0.243.0.1
Product / Package Ecosystem Affected Versions Fixed Version
wasmtime
 crates.io
< 24.0.7
24.0.7
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-jxhv-7h78-9775
NVD — CVE-2026-34942 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-34942
Wasmtime Source Repository REPOSITORY
https://github.com/bytecodealliance/wasmtime
rustsec.org/advisories/rustsec-... WEB
https://rustsec.org/advisories/RUSTSEC-2026-0092.html

Frequently Asked Questions

What is CVE-2026-34942?
CVE-2026-34942 is a security vulnerability in Wasmtime involving Panic when transcoding misaligned utf-16 strings under CWE-129: Improper Validation of Array Index.
Which Wasmtime versions are affected?
Affected versions include releases prior to 24.0.7, 36.0.7, 42.0.2, 43.0.1 across affected packages: wasmtime.
What version fixes CVE-2026-34942?
Upgrading to 24.0.7, 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 24.0.7, 36.0.7, 42.0.2, 43.0.1. Upgrade your dependency to 24.0.7, 36.0.7, 42.0.2, 43.0.1 or later.
What is the CVSS score of CVE-2026-34942?
CVE-2026-34942 has a CVSS v3.1 base score of 6.5 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H.
What CWE is associated with CVE-2026-34942?
CVE-2026-34942 is classified under CWE-129: Improper Validation of Array Index (The product uses untrusted input when calculating or using an array index, but the product does not validate or incorrectly validates the index to ensure the index references a valid position within the array.).
How can I check crates.io dependencies for CVE-2026-34942?
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-jxhv-7h78-9775",
  "modified" : "2026-04-24T21:03:47Z",
  "published" : "2026-04-09T20:22:12Z",
  "aliases" : [ "CVE-2026-34942" ],
  "summary" : "Wasmtime: Panic when transcoding misaligned utf-16 strings",
  "details" : "### Impact\n\nWasmtime's implementation of transcoding strings into the Component Model's `utf16` or `latin1+utf16` encodings improperly verified the alignment of reallocated strings. This meant that unaligned pointers could be passed to the host for transcoding which would trigger a host panic. This panic is possible to trigger from malicious guests which transfer very specific strings across components with specific addresses. \n\nHost panics are considered a DoS vector in Wasmtime as the panic conditions are controlled by the guest in this situation.\n\n### Patches\n\nWasmtime 24.0.7, 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\nThere is no workaround for this bug. Hosts are recommended to updated to a patched version of Wasmtime.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:P/VC:N/VI:N/VA:H/SC:N/SI:N/SA:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "crates.io",
      "name" : "wasmtime"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "24.0.7"
      } ]
    } ]
  }, {
    "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-jxhv-7h78-9775"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-34942"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/bytecodealliance/wasmtime"
  }, {
    "type" : "WEB",
    "url" : "https://rustsec.org/advisories/RUSTSEC-2026-0092.html"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-119", "CWE-129" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-09T20:22:12Z",
    "nvd_published_at" : "2026-04-09T19:16:23Z"
  }
}