GHSA-f984-pcp8-v2p7 CVE-2026-35186 RUSTSEC-2026-0094
Wasmtime has improperly masked return value from `table.grow` with Winch compiler backend
- Severity
- 0.0 / 10
- High · CVSS 4.0
- Exploit chance (EPSS)
- 0.21%
- Chance of exploitation in the next 30 days. Higher than 11.68% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Weakness
- CWE-789 Memory Allocation with Excessive Size Value
- Fix
36.0.7- Upgrade wasmtime to this version or later
Description
Impact
Wasmtime's Winch compiler backend contains a bug where translating the table.grow operator causes the result to be incorrectly typed. For 32-bit tables this means that the result of the operator, internally in Winch, is tagged as a 64-bit value instead of a 32-bit value. This invalid internal representation of Winch's compiler state compounds into further issues depending on how the value is consumed.
One example can be seen when the result of table.grow is used as the address of a load operation. The load operation is tricked into thinking the address is a 64-bit value, not a 32-bit value, which means that the final address to load from is calculated incorrectly. This can lead to a situation where the bytes before the start of linear memory can be loaded/stored to.
The primary consequence of this bug is that bytes in the host's address space can be stored/read from. This is only applicable to the 16 bytes before linear memory, however, as the only significant return value of table.grow that can be misinterpreted is -1. The bytes before linear memory are, by default, unmapped memory. Wasmtime will detect this fault and abort the process, however, because wasm should not be able to access these bytes.
Overall this this bug in Winch represents a DoS vector by crashing the host process, a correctness issue within Winch, and a possible leak of up to 16-bytes before linear memory. Wasmtime's default compiler is Cranelift, not Winch, and Wasmtime's default settings are to place guard pages before linear memory. This means that Wasmtime's default configuration is not affected by this issue, and when explicitly choosing Winch Wasmtime's otherwise default configuration leads to a DoS. Disabling guard pages before linear memory is required to possibly leak up to 16-bytes of host data.
Patches
Wasmtime 43.0.1, 42.0.2, and 36.0.7 have been released with fixes for this issue.
Workaround
There are no workarounds within the Winch compiler backend while using the affected versions. Users of Wasmtime are encouraged either to upgrade to patched versions or, if that is not possible, use the Cranelift compiler backend.
Affected versions
crates.io
| Package | Vulnerable | Fixed in |
|---|---|---|
| wasmtime | >= 25.0.0, < 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:L/VA:H/SC:N/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)
- Low
- Availability (vulnerable system)
- High
- Confidentiality (subsequent systems)
- None
- Integrity (subsequent systems)
- None
- Availability (subsequent systems)
- None
CVSS 3.1 7.5
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- Attack vector
- Network
- Attack complexity
- Low
- Privileges required
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
Weakness
CWE-789: Memory Allocation with Excessive Size Value
The product allocates memory based on an untrusted, large size value, but it does not ensure that the size is within expected limits, allowing arbitrary amounts of memory to be allocated.
How to prevent it
- Implementation, Architecture and Design: Perform adequate input validation against any value that influences the amount of memory that is allocated. Define an appropriate strategy for handling requests that exceed the limit, and consider supporting a configuration option so that the administrator can extend the amount of memory to be used if necessary.
- Operation: Run your program using system-provided resource limits for memory. This might still cause the program to crash or exit, but the impact to the rest of the system will be minimized.
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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