GHSA-6wgr-89rj-399p CVE-2026-34988 RUSTSEC-2026-0088
Wasmtime has data leakage between pooling allocator instances
- Severity
- 0.0 / 10
- Medium · CVSS 4.0
- Exploit chance (EPSS)
- 0.29%
- Chance of exploitation in the next 30 days. Higher than 21.17% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Weakness
- CWE-119 Improper Restriction of Operations within the Bounds of a Memory Buffer
- Fix
36.0.7- Upgrade wasmtime to this version or later
Description
Impact
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.
Exposing this bug requires specific configuration values to be used. If any of these configurations are not applicable then this bug does not happen:
- The pooling allocator must be in use.
- The
Config::memory_guard_sizeconfiguration option must be 0. - The
Config::memory_reservationconfiguration must be less than 4GiB. - The pooling allocator must be configured with
max_memory_sizethe same as thememory_reservationvalue.
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.
Patches
Wasmtime 36.0.7, 42.0.2, and 43.0.1 have been issued to fix this bug. Users are recommended to update to these patched versions of Wasmtime.
Workarounds
All four conditions above must be met to be vulnerable to this bug, and users can work around this bug by adjusting any of the above conditions. For example it is strongly recommended that guard pages are configured for linear memories which would make this bug not applicable.
Affected versions
crates.io
| Package | Vulnerable | Fixed in |
|---|---|---|
| wasmtime | >= 28.0.0, < 36.0.7 |
36.0.7 |
| wasmtime | >= 37.0.0, < 42.0.2 |
42.0.2 |
| wasmtime | >= 43.0.0, < 43.0.1 |
43.0.1 |
| wasmtime | >= 0.0.0-0, < 36.0.7>= 37.0.0, < 42.0.2>= 43.0.0, < 43.0.1 |
36.0.742.0.243.0.1 |
How to fix
Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.
wasmtime crates.io → 36.0.7
- cargo
cargo update -p wasmtime --precise 36.0.7
Severity breakdown
CVSS 4.0 0.0
CVSS:4.0/AV:N/AC:H/AT:P/PR:L/UI:N/VC:L/VI:N/VA:N/SC:L/SI:N/SA:N
- Attack vector
- Network
- Attack complexity
- High
- Attack requirements
- Present
- Privileges required
- Low
- User interaction
- None
- Confidentiality (vulnerable system)
- Low
- Integrity (vulnerable system)
- None
- Availability (vulnerable system)
- None
- Confidentiality (subsequent systems)
- Low
- Integrity (subsequent systems)
- None
- Availability (subsequent systems)
- None
CVSS 3.1 6.3
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:N
- Attack vector
- Network
- Attack complexity
- High
- Privileges required
- Low
- User interaction
- None
- Scope
- Changed
- Confidentiality
- High
- Integrity
- None
- Availability
- None
Weakness
CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer
The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to other variables, data structures, or internal program data.
How to prevent it
- Requirements: Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, many languages that perform their own memory management, such as Java and Perl, are not subject to buffer overflows. Other languages, such as Ada and C#, typically provide overflow protection, but the protection can be disabled by the programmer. Be wary that a language's interface to native code may still be subject to overflows, even if the language itself is theoretically safe.
- Architecture and Design: Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. Examples include the Safe C String Library (SafeStr) by Messier and Viega [REF-57], and the Strsafe.h library from Microsoft [REF-56]. These libraries provide safer versions of overflow-prone string-handling functions.
- Operation, Build and Compilation: Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking. D3-SFCV (Stack Frame Canary Validation) from D3FEND [REF-1334] discusses canary-based detection in detail.
- Implementation: Consider adhering to the following rules when allocating and managing an application's memory: Double check that the buffer is as large as specified. When using functions that accept a number of bytes to copy, such as strncpy(), be aware that if the destination buffer size is equal to the source buffer size, it may not NULL-terminate the string. Check buffer boundaries if accessing the buffer in a loop and make sure there is no danger of writing past the allocated space. If necessary, truncate all input strings to a reasonable length before passing them to the copy and concatenation functions.
- Operation, Build and Compilation: Run or compile the software using features or extensions that randomly arrange the positions of a program's executable and libraries in memory. Because this makes the addresses unpredictable, it can prevent an attacker from reliably jumping to exploitable code. Examples include Address Space Layout Randomization (ASLR) [REF-58] [REF-60] and Position-Independent Executables (PIE) [REF-64]. Imported modules may be similarly realigned if their default memory addresses conflict with other modules, in a process known as "rebasing" (for Windows) and "prelinking" (for Linux) [REF-1332] using randomly generated addresses. ASLR for libraries cannot be used in conjunction with prelink since it would require relocating the libraries at run-time, defeating the whole purpose of prelinking. For more information on these techniques see D3-SAOR (Segment Address Offset Randomization) from D3FEND [REF-1335].
- Operation: Use a CPU and operating system that offers Data Execution Protection (using hardware NX or XD bits) or the equivalent techniques that simulate this feature in software, such as PaX [REF-60] [REF-61]. These techniques ensure that any instruction executed is exclusively at a memory address that is part of the code segment. For more information on these techniques see D3-PSEP (Process Segment Execution Prevention) from D3FEND [REF-1336].
- Implementation: Replace unbounded copy functions with analogous functions that support length arguments, such as strcpy with strncpy. Create these if they are not available.
References
Advisories
Packages
Other links
Timeline
- Published
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| RUSTSEC-2026-0424 Unchecked memory usage in transferring borrowed resources between components | Not rated yet | Also affects wasmtime |
| RUSTSEC-2026-0423 Cross-engine type confusion with host-created struct-types-with-supertypes | Not rated yet | Also affects wasmtime |
| RUSTSEC-2026-0422 Typed functions with `i31` or `nullref` arguments may panic due to insufficient type-check | Low | Also affects wasmtime |
| RUSTSEC-2026-0379 Wasmtime: Cranelift backend on aarch64 can jump to incorrect target on large Wasm `br_table` instruction | Critical | Also affects wasmtime |
| GHSA-9pvw-hwfg-92h5 Wasmtime: Panic lowering a host-created component model stream into a guest that expects a different payload type | Medium | Also affects wasmtime |
| GHSA-897f-g5gm-fmxm Wasmtime: Bit-packing in Cranelift compiler causes silent overflow and miscompilation | Critical | Also affects wasmtime |
| RUSTSEC-2026-0364 Exception tags can cause host data structures to be unaligned | Not rated yet | Also affects wasmtime |
| RUSTSEC-2026-0363 Component-model handle tables are capped only per instance and charged to no limit | Medium | Also affects wasmtime |
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