dbcve.org
WasmtimeApplication · Bytecodealliance

CVE-2026-34945

MEDIUM · 6.5 CVSS v3.1 Published 2026-04-09
Fix available
A fix is available. Upgrade to 36.0.7 / 42.0.2 or later.
See remediation →
71/100
Remediation priority · Elevated
Remotely reachable Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Wasmtime is a runtime for WebAssembly. From 25.0.0 to before 36.0.7, 42.0.2, and 43.0.1, 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 vulnerability is fixed in 36.0.7, 42.0.2, and 43.0.1.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Wasmtime's Winch compiler incorrectly typed the return value of the table.size instruction as 32-bit instead of using the table's index type for 64-bit tables in the memory64 proposal. This allowed WebAssembly guests to read host stack data by exploiting the ABI mismatch between the 32-bit return value and the actual 64-bit table size, potentially exposing sensitive host operation data.

MitigationUpgrade Wasmtime to version 36.0.7, 42.0.2, or 43.0.1 or later. If upgrading is not immediately feasible, disable the Winch compiler or ensure no untrusted WebAssembly code using memory64 is executed with the affected versions.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
WasmtimeApplication
Affected:>= 25.0.0, < 36.0.7>= 37.0.0, < 42.0.2>= 43.0.0, < 43.0.1

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify Wasmtime version
    Run `wasmtime --version` or check the Wasmtime library version in your dependency manifest
    Affected if The version falls within any of these ranges: >=25.0.0 and <36.0.7, >=37.0.0 and <42.0.2, or >=43.0.0 and <43.0.1
  2. Confirm Winch compiler is in use
    Inspect your Wasmtime configuration or runtime settings to determine if the Winch compiler is enabled (check for 'winch' in compiler selection flags or config)
    Affected if Winch is selected as the compiler for WebAssembly modules
  3. Verify memory64 proposal is enabled
    Check if your WebAssembly module or runtime configuration enables the memory64 proposal (look for 'memory64' in Wasm features or module configuration)
    Affected if memory64 is enabled for WebAssembly modules being executed
  4. Confirm untrusted Wasm code execution
    Review your application architecture to determine whether untrusted or third-party WebAssembly code is executed using the affected Wasmtime instance
    Affected if Untrusted WebAssembly code that uses memory64 is executed with the vulnerable Wasmtime version

You are affected if your Wasmtime version is in the vulnerable ranges, Winch compiler is enabled, memory64 is in use, and untrusted WebAssembly code is executed with this configuration.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 36.0.7 / 42.0.2 / 43.0.1 or later
Fixed in 36.0.742.0.243.0.1
Interim mitigation

Upgrade Wasmtime to version 36.0.7, 42.0.2, or 43.0.1 or later. If upgrading is not immediately feasible, disable the Winch compiler or ensure no untrusted WebAssembly code using memory64 is executed with the affected versions.

Recommended fix High confidence

Upgrade to 36.0.7 (for 25.x-35.x), 42.0.2 (for 37.x-41.x), or 43.0.1 (for 43.0.0) - choose the appropriate version based on your current branch

  1. Identify the current Wasmtime version in use by checking your project's dependencies or build configuration
  2. Determine which version branch you are on based on the current version (25.x-35.x, 37.x-41.x, or 43.0.0)
  3. Upgrade Wasmtime to the appropriate fixed version: 36.0.7 or later for 25.x-35.x branches, 42.0.2 or later for 37.x-41.x branches, or 43.0.1 for the 43.0.0 branch
  4. Verify the upgrade by running your test suite to ensure the runtime functions correctly after the version change
Caveat Review Wasmtime release notes between your current version and the target version for any breaking changes in the runtime API or behavior

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Wasmtime Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing8.0 h
  • Review / QA4.0 h
18.0 hours of engineering $3,040
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $4,864.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-34945 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data