CVE-2026-34988: CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer in bytecodealliance wasmtime
Wasmtime is a runtime for WebAssembly. From 28.0.0 to before 36.0.7, 42.0.2, and 43.0.1, Wasmtime's implementation of its pooling allocator contains a bug where in certain configurations the contents of linear memory can be leaked from one instance to the next. The implementation of resetting the virtual memory permissions for linear memory used the wrong predicate to determine if resetting was necessary, where the compilation process used a different predicate. This divergence meant that the pooling allocator incorrectly deduced at runtime that resetting virtual memory permissions was not necessary while compile-time determine that virtual memory could be relied upon. The pooling allocator must be in use, Config::memory_guard_size configuration option must be 0, Config::memory_reservation configuration must be less than 4GiB, and pooling allocator must be configured with max_memory_size the same as the memory_reservation value in order to exploit this vulnerability. If all of these conditions are applicable then when a linear memory is reused the VM permissions of the previous iteration are not reset. This means that the compiled code, which is assuming out-of-bounds loads will segfault, will not actually segfault and can read the previous contents of linear memory if it was previously mapped. This represents a data leakage vulnerability between guest WebAssembly instances which breaks WebAssembly's semantics and additionally breaks the sandbox that Wasmtime provides. Wasmtime is not vulnerable to this issue with its default settings, nor with the default settings of the pooling allocator, but embeddings are still allowed to configure these values to cause this vulnerability. This vulnerability is fixed in 36.0.7, 42.0.2, and 43.0.1.
AI Analysis
Technical Summary
Wasmtime versions from 28.0.0 up to but not including 36.0.7, 37.0.0 up to 42.0.2, and 43.0.0 up to 44.0.1 contain a vulnerability in the pooling allocator implementation. The allocator incorrectly determines at runtime whether to reset virtual memory permissions for linear memory due to a mismatch in predicates used at compile-time versus runtime. When the pooling allocator is used with Config::memory_guard_size set to 0, Config::memory_reservation less than 4GiB, and max_memory_size equal to memory_reservation, the allocator fails to reset VM permissions on memory reuse. This allows compiled WebAssembly code to read previous linear memory contents, violating memory isolation and sandboxing. Default Wasmtime configurations are not vulnerable. The issue is fixed in versions 36.0.7, 42.0.2, and 43.0.1.
Potential Impact
The vulnerability allows data leakage between isolated WebAssembly instances running in Wasmtime when specific non-default configurations are used. This breaks the memory isolation guarantees of Wasmtime's sandbox and WebAssembly semantics, potentially exposing sensitive data from one instance to another. The impact is limited by the requirement for specific configuration settings that are not enabled by default. The CVSS 4.0 base score is 2.3, indicating low severity. There are no known exploits in the wild.
Mitigation Recommendations
This vulnerability is fixed in Wasmtime versions 36.0.7, 42.0.2, and 43.0.1. Users should upgrade to one of these fixed versions to remediate the issue. Wasmtime's default configuration and default pooling allocator settings are not vulnerable. If upgrading is not immediately possible, ensure that the pooling allocator is not used with Config::memory_guard_size set to 0, Config::memory_reservation less than 4GiB, and max_memory_size equal to memory_reservation simultaneously, as these conditions enable the vulnerability. Patch status is not explicitly confirmed beyond the fixed versions; check the vendor advisory for the latest remediation guidance.
CVE-2026-34988: CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer in bytecodealliance wasmtime
Description
Wasmtime is a runtime for WebAssembly. From 28.0.0 to before 36.0.7, 42.0.2, and 43.0.1, Wasmtime's implementation of its pooling allocator contains a bug where in certain configurations the contents of linear memory can be leaked from one instance to the next. The implementation of resetting the virtual memory permissions for linear memory used the wrong predicate to determine if resetting was necessary, where the compilation process used a different predicate. This divergence meant that the pooling allocator incorrectly deduced at runtime that resetting virtual memory permissions was not necessary while compile-time determine that virtual memory could be relied upon. The pooling allocator must be in use, Config::memory_guard_size configuration option must be 0, Config::memory_reservation configuration must be less than 4GiB, and pooling allocator must be configured with max_memory_size the same as the memory_reservation value in order to exploit this vulnerability. If all of these conditions are applicable then when a linear memory is reused the VM permissions of the previous iteration are not reset. This means that the compiled code, which is assuming out-of-bounds loads will segfault, will not actually segfault and can read the previous contents of linear memory if it was previously mapped. This represents a data leakage vulnerability between guest WebAssembly instances which breaks WebAssembly's semantics and additionally breaks the sandbox that Wasmtime provides. Wasmtime is not vulnerable to this issue with its default settings, nor with the default settings of the pooling allocator, but embeddings are still allowed to configure these values to cause this vulnerability. This vulnerability is fixed in 36.0.7, 42.0.2, and 43.0.1.
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
Wasmtime versions from 28.0.0 up to but not including 36.0.7, 37.0.0 up to 42.0.2, and 43.0.0 up to 44.0.1 contain a vulnerability in the pooling allocator implementation. The allocator incorrectly determines at runtime whether to reset virtual memory permissions for linear memory due to a mismatch in predicates used at compile-time versus runtime. When the pooling allocator is used with Config::memory_guard_size set to 0, Config::memory_reservation less than 4GiB, and max_memory_size equal to memory_reservation, the allocator fails to reset VM permissions on memory reuse. This allows compiled WebAssembly code to read previous linear memory contents, violating memory isolation and sandboxing. Default Wasmtime configurations are not vulnerable. The issue is fixed in versions 36.0.7, 42.0.2, and 43.0.1.
Potential Impact
The vulnerability allows data leakage between isolated WebAssembly instances running in Wasmtime when specific non-default configurations are used. This breaks the memory isolation guarantees of Wasmtime's sandbox and WebAssembly semantics, potentially exposing sensitive data from one instance to another. The impact is limited by the requirement for specific configuration settings that are not enabled by default. The CVSS 4.0 base score is 2.3, indicating low severity. There are no known exploits in the wild.
Mitigation Recommendations
This vulnerability is fixed in Wasmtime versions 36.0.7, 42.0.2, and 43.0.1. Users should upgrade to one of these fixed versions to remediate the issue. Wasmtime's default configuration and default pooling allocator settings are not vulnerable. If upgrading is not immediately possible, ensure that the pooling allocator is not used with Config::memory_guard_size set to 0, Config::memory_reservation less than 4GiB, and max_memory_size equal to memory_reservation simultaneously, as these conditions enable the vulnerability. Patch status is not explicitly confirmed beyond the fixed versions; check the vendor advisory for the latest remediation guidance.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- GitHub_M
- Date Reserved
- 2026-03-31T19:38:31.617Z
- Cvss Version
- 4.0
- State
- PUBLISHED
- Remediation Level
- null
Threat ID: 69d7f88c1cc7ad14da0c1709
Added to database: 4/9/2026, 7:05:48 PM
Last enriched: 4/9/2026, 7:21:38 PM
Last updated: 5/25/2026, 2:43:02 AM
Views: 107
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