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GHSA-4873-36h9-wv49 CVE-2021-39216 CVE-2021-39218 CVE-2021-39219 GHSA-q879-9g95-56mx GHSA-v4cp-h94r-m7xf PYSEC-2021-320 PYSEC-2021-321 PYSEC-2021-322 RUSTSEC-2021-0110

Out-of-bounds read/write and invalid free with `externref`s and GC safepoints in Wasmtime

Medium PyPIcrates.io
Severity
0.0 / 10
Medium · CVSS 4.0
Exploit chance (EPSS)
0.36%
Chance of exploitation in the next 30 days. Higher than 29.93% of scored vulnerabilities.
Known exploited
No
Not in CISA's catalog
Weakness
CWE-125 Out-of-bounds Read
CWE-590 Free of Memory not on the Heap
CWE-787 Out-of-bounds Write
Fix
Available
2 packages have a fixed version

Description

Impact

There was an invalid free and out-of-bounds read and write bug when running Wasm that uses externrefs in Wasmtime.

To trigger this bug, Wasmtime needs to be running Wasm that uses externrefs, the host creates non-null externrefs, Wasmtime performs a garbage collection (GC), and there has to be a Wasm frame on the stack that is at a GC safepoint where

  • there are no live references at this safepoint, and
  • there is a safepoint with live references earlier in this frame's function.

Under this scenario, Wasmtime would incorrectly use the GC stack map for the safepoint from earlier in the function instead of the empty safepoint. This would result in Wasmtime treating arbitrary stack slots as externrefs that needed to be rooted for GC. At the next GC, it would be determined that nothing was referencing these bogus externrefs (because nothing could ever reference them, because they are not really externrefs) and then Wasmtime would deallocate them and run <ExternRef as Drop>::drop on them. This results in a free of memory that is not necessarily on the heap (and shouldn't be freed at this moment even if it was), as well as potential out-of-bounds reads and writes.

Even though support for externrefs (via the reference types proposal) is enabled by default, unless you are creating non-null externrefs in your host code or explicitly triggering GCs, you cannot be affected by this bug.

We have reason to believe that the effective impact of this bug is relatively small because usage of externref is currently quite rare.

Patches

This bug has been patched and users should upgrade to Wasmtime version 0.30.0.

Additionally, we have updated our primary externref fuzz target such that it better exercises these code paths and we can have greater confidence in their correctness going forward.

Workarounds

If you cannot upgrade Wasmtime at this time, you can avoid this bug by disabling the reference types proposal by passing false to wasmtime::Config::wasm_reference_types

References

For more information

If you have any questions or comments about this advisory:

Affected versions

PyPI

PackageVulnerableFixed in
wasmtime
>= 0.26.0, < 0.30.0
0.30.0
wasmtime
< 0.30.0
0.30.0
wasmtime
< 0.30.0
0.30.0
wasmtime
< 0.30.0
0.30.0
wasmtime
< 0.30.0
0.30.0

crates.io

PackageVulnerableFixed in
wasmtime
>= 0.26.0, < 0.30.0
0.30.0
wasmtime
< 0.30.0
0.30.0
wasmtime
>= 0.0.0-0, < 0.30.0
0.30.0

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 PyPI → 0.30.0

pip
pip install --upgrade "wasmtime>=0.30.0"
poetry
poetry add "wasmtime>=0.30.0"
uv
uv add "wasmtime>=0.30.0"

wasmtime crates.io → 0.30.0

cargo
cargo update -p wasmtime --precise 0.30.0

Severity breakdown

CVSS 4.0 0.0

CVSS:4.0/AV:L/AC:L/AT:P/PR:L/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N

Attack vector
Local
Attack complexity
Low
Attack requirements
Present
Privileges required
Low
User interaction
None
Confidentiality (vulnerable system)
None
Integrity (vulnerable system)
High
Availability (vulnerable system)
High
Confidentiality (subsequent systems)
None
Integrity (subsequent systems)
None
Availability (subsequent systems)
None

CVSS 3.1 6.3

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H

Attack vector
Local
Attack complexity
High
Privileges required
Low
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
High

Weakness

CWE-125: Out-of-bounds Read

The product reads data past the end, or before the beginning, of the intended buffer.

How to prevent it
  • Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright. To reduce the likelihood of introducing an out-of-bounds read, ensure that you validate and ensure correct calculations for any length argument, buffer size calculation, or offset. Be especially careful of relying on a sentinel (i.e. special character such as NUL) in untrusted inputs.
  • Architecture and Design: Use a language that provides appropriate memory abstractions.

CWE-590: Free of Memory not on the Heap

The product calls free() on a pointer to memory that was not allocated using associated heap allocation functions such as malloc(), calloc(), or realloc().

How to prevent it
  • Implementation: Only free pointers that you have called malloc on previously. This is the recommended solution. Keep track of which pointers point at the beginning of valid chunks and free them only once.
  • Implementation: Before freeing a pointer, the programmer should make sure that the pointer was previously allocated on the heap and that the memory belongs to the programmer. Freeing an unallocated pointer will cause undefined behavior in the program.
  • 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. For example, glibc in Linux provides protection against free of invalid pointers.
  • Architecture and Design: Use a language that provides abstractions for memory allocation and deallocation.

CWE-787: Out-of-bounds Write

The product writes data past the end, or before the beginning, of the intended buffer.

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

  1. Published
  2. Last updated by the source
  3. Latest EPSS score
AdvisorySeverityWhy 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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