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Wasmtime is vulnerable to panic when dropping a `[Typed]Func::call_async` future

Moderate severity GitHub Reviewed Published Feb 24, 2026 in bytecodealliance/wasmtime • Updated Feb 27, 2026

Package

cargo wasmtime (Rust)

Affected versions

>= 39.0.0, < 40.0.4
>= 41.0.0, < 41.0.4

Patched versions

40.0.4
41.0.4

Description

The affected versions of Wasmtime can panic if the host embedder drops the future returned by wasmtime::component::[Typed]Func::call_async before it resolves.

Details

Starting with Wasmtime 39.0.0, the component-model-async feature became the default, which brought with it a new implementation of [Typed]Func::call_async which made it capable of calling async-typed guest export functions. However, that implementation had a bug leading to a panic under certain circumstances:

  1. The host embedding calls [Typed]Func::call_async on a function exported by a component, polling the returned Future once.
  2. The component function yields control to the async runtime (e.g. Tokio), e.g. due to a call to host function registered using LinkerInstance::func_wrap_async which yields, or due an epoch interruption.
  3. The host embedding drops the Future after polling it once. This leaves the component instance in a non-reenterable state since the call never had a chance to complete.
  4. The host embedding calls [Typed]Func::call_async again, polling the returned Future. Since the component instance cannot be entered at this point, the call traps, but not before allocating a task and thread for the call.
  5. The host embedding ignores the trap and drops the Future. This panics due to the runtime attempting to dispose of the task created above, which panics since the thread has not yet exited.

Impact

When a host embedder using the affected versions of Wasmtime calls wasmtime::component::[Typed]Func::call_async on a guest export and then drops the returned future without waiting for it to resolve, and then does so again with the same component instance, Wasmtime will panic. Embeddings that have the component-model-async compile-time feature disabled are unaffected.

Patches

Wasmtime 40.0.4 and 41.0.4 have been patched to fix this issue. Versions 42.0.0 and later are not affected.

Workarounds

If an embedding is not actually using any component-model-async features then disabling the component-model-async Cargo feature can work around this issue. This issue can also be worked around by either ensuring every call_async future is awaited until it completes or refraining from using the Store again after dropping a not-yet-resolved call_async future.

Resources

This was first reported in https://bytecodealliance.zulipchat.com/#narrow/channel/206238-general/topic/Panic.20in.20Wasmtime.2041.2E0.2E3.20.28runtime.2Fconcurrent.2Fcomponent.29

References

@alexcrichton alexcrichton published to bytecodealliance/wasmtime Feb 24, 2026
Published to the GitHub Advisory Database Feb 24, 2026
Reviewed Feb 24, 2026
Published by the National Vulnerability Database Feb 24, 2026
Last updated Feb 27, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity High
Attack Requirements Present
Privileges Required Low
User interaction Passive
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability High
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability High

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:H/AT:P/PR:L/UI:P/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(17th percentile)

Weaknesses

Improper Handling of Exceptional Conditions

The product does not handle or incorrectly handles an exceptional condition. Learn more on MITRE.

CVE ID

CVE-2026-27195

GHSA ID

GHSA-xjhv-v822-pf94

Source code

Credits

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