1.0
LOW CVSS 4.0
CVE-2025-61670
Wasmtime has memory leak in C API with `externref` and `anyref` types
Description

Wasmtime is a runtime for WebAssembly. Wasmtime 37.0.0 and 37.0.1 have memory leaks in the C/C++ API when using bindings for the `anyref` or `externref` WebAssembly values. This is caused by a regression introduced during the development of 37.0.0 and all prior versions of Wasmtime are unaffected. If `anyref` or `externref` is not used in the C/C++ API then embeddings are also unaffected by the leaky behavior. The `wasmtime` Rust crate is unaffected by this leak. Development of Wasmtime 37.0.0 included a refactoring in Rust of changing the old `ManuallyRooted<T>` type to a new `OwnedRooted<T>` type. This change was integrated into Wasmtime's C API but left the C API in a state which had memory leaks. Additionally the new ownership semantics around this type were not reflected into the C++ API, making it leak-prone. A short version of the change is that previously `ManuallyRooted<T>`, as the name implies, required manual calls to an "unroot" operation. If this was forgotten then the memory was still cleaned up when the `wasmtime_store_t` itself was destroyed eventually. Documentation of when to "unroot" was sparse and there were already situations prior to 37.0.0 where memory would be leaked until the store was destroyed anyway. All memory, though, was always bound by the store, and destroying the store would guarantee that there were no memory leaks. In migrating to `OwnedRooted<T>` the usage of the type in Rust changed. A manual "unroot" operation is no longer required and it happens naturally as a destructor of the `OwnedRooted<T>` type in Rust itself. These new resource ownership semantics were not fully integrated into the preexisting semantics of the C/C++ APIs in Wasmtime. A crucial distinction of `OwnedRooted<T>` vs `ManuallyRooted<T>` is that the `OwnedRooted<T>` type allocates host memory outside of the store. This means that if an `OwnedRooted<T>` is leaked then destroying a store does not release this memory and it's a permanent memory leak on the host. This led to a few distinct, but related, issues arising: A typo in the `wasmtime_val_unroot` function in the C API meant that it did not actually unroot anything. This meant that even if embedders faithfully call the function then memory will be leaked. If a host-defined function returned a `wasmtime_{externref,anyref}_t` value then the value was never unrooted. The C/C++ API no longer has access to the value and the Rust implementation did not unroot. This meant that any values returned this way were never unrooted. The goal of the C++ API of Wasmtime is to encode automatic memory management in the type system, but the C++ API was not updated when `OwnedRooted<T>` was added. This meant that idiomatic usage of the C++ API would leak memory due to a lack of destructors on values. These issues have all been fixed in a 37.0.2 release of Wasmtime. The implementation of the C and C++ APIs have been updated accordingly and respectively to account for the changes of ownership here. For example `wasmtime_val_unroot` has been fixed to unroot, the Rust-side implementation of calling an embedder-defined function will unroot return values, and the C++ API now has destructors on the `ExternRef`, `AnyRef`, and `Val` types. These changes have been made to the 37.0.x release branch in a non-API-breaking fashion. Changes to the 38.0.0 release branch (and `main` in the Wasmtime repository) include minor API updates to better accommodate the API semantic changes. The only known workaround at this time is to avoid using `externref` and `anyref` in the C/C++ API of Wasmtime. If avoiding those types is not possible then it's required for users to update to mitigate the leak issue.

INFO

Published Date :

Oct. 7, 2025, 7:15 p.m.

Last Modified :

Oct. 8, 2025, 7:38 p.m.

Remotely Exploit :

No
Affected Products

The following products are affected by CVE-2025-61670 vulnerability. Even if cvefeed.io is aware of the exact versions of the products that are affected, the information is not represented in the table below.

ID Vendor Product Action
1 Bytecodealliance wasmtime
CVSS Scores
The Common Vulnerability Scoring System is a standardized framework for assessing the severity of vulnerabilities in software and systems. We collect and displays CVSS scores from various sources for each CVE.
Score Version Severity Vector Exploitability Score Impact Score Source
CVSS 4.0 LOW [email protected]
Solution
Update Wasmtime to version 37.0.2 or later to fix memory leaks.
  • Update Wasmtime to version 37.0.2 or later.
  • Apply patches to the C/C++ API implementation.
  • Ensure destructors are present for relevant types.
  • Avoid externref and anyref if updating is not possible.
References to Advisories, Solutions, and Tools

Here, you will find a curated list of external links that provide in-depth information, practical solutions, and valuable tools related to CVE-2025-61670.

URL Resource
https://github.com/bytecodealliance/wasmtime/commit/adff9d9d0f09569203709d5687e5a7dc8e1ad0a3
https://github.com/bytecodealliance/wasmtime/releases/tag/v37.0.2
https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-vvp9-h8p2-xwfc
CWE - Common Weakness Enumeration

While CVE identifies specific instances of vulnerabilities, CWE categorizes the common flaws or weaknesses that can lead to vulnerabilities. CVE-2025-61670 is associated with the following CWEs:

Common Attack Pattern Enumeration and Classification (CAPEC)

Common Attack Pattern Enumeration and Classification (CAPEC) stores attack patterns, which are descriptions of the common attributes and approaches employed by adversaries to exploit the CVE-2025-61670 weaknesses.

We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).

Results are limited to the first 15 repositories due to potential performance issues.

The following list is the news that have been mention CVE-2025-61670 vulnerability anywhere in the article.

The following table lists the changes that have been made to the CVE-2025-61670 vulnerability over time.

Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.

  • New CVE Received by [email protected]

    Oct. 07, 2025

    Action Type Old Value New Value
    Added Description Wasmtime is a runtime for WebAssembly. Wasmtime 37.0.0 and 37.0.1 have memory leaks in the C/C++ API when using bindings for the `anyref` or `externref` WebAssembly values. This is caused by a regression introduced during the development of 37.0.0 and all prior versions of Wasmtime are unaffected. If `anyref` or `externref` is not used in the C/C++ API then embeddings are also unaffected by the leaky behavior. The `wasmtime` Rust crate is unaffected by this leak. Development of Wasmtime 37.0.0 included a refactoring in Rust of changing the old `ManuallyRooted<T>` type to a new `OwnedRooted<T>` type. This change was integrated into Wasmtime's C API but left the C API in a state which had memory leaks. Additionally the new ownership semantics around this type were not reflected into the C++ API, making it leak-prone. A short version of the change is that previously `ManuallyRooted<T>`, as the name implies, required manual calls to an "unroot" operation. If this was forgotten then the memory was still cleaned up when the `wasmtime_store_t` itself was destroyed eventually. Documentation of when to "unroot" was sparse and there were already situations prior to 37.0.0 where memory would be leaked until the store was destroyed anyway. All memory, though, was always bound by the store, and destroying the store would guarantee that there were no memory leaks. In migrating to `OwnedRooted<T>` the usage of the type in Rust changed. A manual "unroot" operation is no longer required and it happens naturally as a destructor of the `OwnedRooted<T>` type in Rust itself. These new resource ownership semantics were not fully integrated into the preexisting semantics of the C/C++ APIs in Wasmtime. A crucial distinction of `OwnedRooted<T>` vs `ManuallyRooted<T>` is that the `OwnedRooted<T>` type allocates host memory outside of the store. This means that if an `OwnedRooted<T>` is leaked then destroying a store does not release this memory and it's a permanent memory leak on the host. This led to a few distinct, but related, issues arising: A typo in the `wasmtime_val_unroot` function in the C API meant that it did not actually unroot anything. This meant that even if embedders faithfully call the function then memory will be leaked. If a host-defined function returned a `wasmtime_{externref,anyref}_t` value then the value was never unrooted. The C/C++ API no longer has access to the value and the Rust implementation did not unroot. This meant that any values returned this way were never unrooted. The goal of the C++ API of Wasmtime is to encode automatic memory management in the type system, but the C++ API was not updated when `OwnedRooted<T>` was added. This meant that idiomatic usage of the C++ API would leak memory due to a lack of destructors on values. These issues have all been fixed in a 37.0.2 release of Wasmtime. The implementation of the C and C++ APIs have been updated accordingly and respectively to account for the changes of ownership here. For example `wasmtime_val_unroot` has been fixed to unroot, the Rust-side implementation of calling an embedder-defined function will unroot return values, and the C++ API now has destructors on the `ExternRef`, `AnyRef`, and `Val` types. These changes have been made to the 37.0.x release branch in a non-API-breaking fashion. Changes to the 38.0.0 release branch (and `main` in the Wasmtime repository) include minor API updates to better accommodate the API semantic changes. The only known workaround at this time is to avoid using `externref` and `anyref` in the C/C++ API of Wasmtime. If avoiding those types is not possible then it's required for users to update to mitigate the leak issue.
    Added CVSS V4.0 AV:A/AC:L/AT:P/PR:L/UI:P/VC:N/VI:N/VA:L/SC:N/SI:N/SA:L/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
    Added CWE CWE-772
    Added Reference https://github.com/bytecodealliance/wasmtime/commit/adff9d9d0f09569203709d5687e5a7dc8e1ad0a3
    Added Reference https://github.com/bytecodealliance/wasmtime/releases/tag/v37.0.2
    Added Reference https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-vvp9-h8p2-xwfc
EPSS is a daily estimate of the probability of exploitation activity being observed over the next 30 days. Following chart shows the EPSS score history of the vulnerability.
Vulnerability Scoring Details
Base CVSS Score: 1.0
Attack Vector
Attack Complexity
Attack Requirements
Privileges Required
User Interaction
VS Confidentiality
VS Integrity
VS Availability
SS Confidentiality
SS Integrity
SS Availability