CVE-2026-34941: CWE-125: Out-of-bounds Read in bytecodealliance wasmtime
Wasmtime is a runtime for WebAssembly. Prior to 24.0.7, 36.0.7, 42.0.2, and 43.0.1, Wasmtime contains a vulnerability where when transcoding a UTF-16 string to the latin1+utf16 component-model encoding it would incorrectly validate the byte length of the input string when performing a bounds check. Specifically the number of code units were checked instead of the byte length, which is twice the size of the code units. This vulnerability can cause the host to read beyond the end of a WebAssembly's linear memory in an attempt to transcode nonexistent bytes. In Wasmtime's default configuration this will read unmapped memory on a guard page, terminating the process with a segfault. Wasmtime can be configured, however, without guard pages which would mean that host memory beyond the end of linear memory may be read and interpreted as UTF-16. A host segfault is a denial-of-service vulnerability in Wasmtime, and possibly being able to read beyond the end of linear memory is additionally a vulnerability. Note that reading beyond the end of linear memory requires nonstandard configuration of Wasmtime, specifically with guard pages disabled. This vulnerability is fixed in 24.0.7, 36.0.7, 42.0.2, and 43.0.1.
AI Analysis
Technical Summary
Wasmtime versions before 24.0.7, 36.0.7, 42.0.2, and 43.0.1 contain a CWE-125 out-of-bounds read vulnerability during UTF-16 to latin1+utf16 transcoding. The flaw is due to an incorrect bounds check that compares the number of code units instead of the byte length, which is twice as large. This can cause the host to read beyond the WebAssembly linear memory. In default configurations with guard pages, this results in a segfault and denial-of-service. Without guard pages, it may lead to reading arbitrary host memory as UTF-16. The issue is fixed in the specified versions. The CVSS 4.0 score is 6.9 (medium severity).
Potential Impact
The primary impact is a denial-of-service caused by a host process crash due to out-of-bounds memory reads when transcoding strings. In nonstandard configurations without guard pages, there is an additional risk of reading host memory beyond the intended boundary, which could lead to information disclosure. There are no reports of active exploitation in the wild.
Mitigation Recommendations
This vulnerability is fixed in Wasmtime versions 24.0.7, 36.0.7, 42.0.2, and 43.0.1. Users should upgrade to one of these fixed versions or later to remediate the issue. If upgrading is not immediately possible, ensure that Wasmtime is running with its default configuration that includes guard pages to prevent reading arbitrary host memory, though this does not prevent denial-of-service via segfault. Patch status is not explicitly confirmed in the vendor advisory, but the fixed versions are clearly specified.
CVE-2026-34941: CWE-125: Out-of-bounds Read in bytecodealliance wasmtime
Description
Wasmtime is a runtime for WebAssembly. Prior to 24.0.7, 36.0.7, 42.0.2, and 43.0.1, Wasmtime contains a vulnerability where when transcoding a UTF-16 string to the latin1+utf16 component-model encoding it would incorrectly validate the byte length of the input string when performing a bounds check. Specifically the number of code units were checked instead of the byte length, which is twice the size of the code units. This vulnerability can cause the host to read beyond the end of a WebAssembly's linear memory in an attempt to transcode nonexistent bytes. In Wasmtime's default configuration this will read unmapped memory on a guard page, terminating the process with a segfault. Wasmtime can be configured, however, without guard pages which would mean that host memory beyond the end of linear memory may be read and interpreted as UTF-16. A host segfault is a denial-of-service vulnerability in Wasmtime, and possibly being able to read beyond the end of linear memory is additionally a vulnerability. Note that reading beyond the end of linear memory requires nonstandard configuration of Wasmtime, specifically with guard pages disabled. This vulnerability is fixed in 24.0.7, 36.0.7, 42.0.2, and 43.0.1.
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
Wasmtime versions before 24.0.7, 36.0.7, 42.0.2, and 43.0.1 contain a CWE-125 out-of-bounds read vulnerability during UTF-16 to latin1+utf16 transcoding. The flaw is due to an incorrect bounds check that compares the number of code units instead of the byte length, which is twice as large. This can cause the host to read beyond the WebAssembly linear memory. In default configurations with guard pages, this results in a segfault and denial-of-service. Without guard pages, it may lead to reading arbitrary host memory as UTF-16. The issue is fixed in the specified versions. The CVSS 4.0 score is 6.9 (medium severity).
Potential Impact
The primary impact is a denial-of-service caused by a host process crash due to out-of-bounds memory reads when transcoding strings. In nonstandard configurations without guard pages, there is an additional risk of reading host memory beyond the intended boundary, which could lead to information disclosure. There are no reports of active exploitation in the wild.
Mitigation Recommendations
This vulnerability is fixed in Wasmtime versions 24.0.7, 36.0.7, 42.0.2, and 43.0.1. Users should upgrade to one of these fixed versions or later to remediate the issue. If upgrading is not immediately possible, ensure that Wasmtime is running with its default configuration that includes guard pages to prevent reading arbitrary host memory, though this does not prevent denial-of-service via segfault. Patch status is not explicitly confirmed in the vendor advisory, but the fixed versions are clearly specified.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- GitHub_M
- Date Reserved
- 2026-03-31T17:27:08.660Z
- Cvss Version
- 4.0
- State
- PUBLISHED
- Remediation Level
- null
Threat ID: 69d843791cc7ad14da3fb68c
Added to database: 4/10/2026, 12:25:29 AM
Last enriched: 4/10/2026, 12:37:04 AM
Last updated: 4/10/2026, 8:16:07 AM
Views: 6
Community Reviews
0 reviewsCrowdsource mitigation strategies, share intel context, and vote on the most helpful responses. Sign in to add your voice and help keep defenders ahead.
Want to contribute mitigation steps or threat intel context? Sign in or create an account to join the community discussion.
Actions
Updates to AI analysis require Pro Console access. Upgrade inside Console → Billing.
External Links
Need more coverage?
Upgrade to Pro Console for AI refresh and higher limits.
For incident response and remediation, OffSeq services can help resolve threats faster.
Latest Threats
Check if your credentials are on the dark web
Instant breach scanning across billions of leaked records. Free tier available.