CVE-2023-42443 is a high-severity (CVSS 8.1) Out-of-bounds Write vulnerability in vyper. 1 public exploit reference exists, so weaponization risk is real. A fix is available for vyper — see the affected versions and patch details below.
Vyper vulnerable to memory corruption in certain builtins utilizing `msize`
Exploitation Status
No confirmed exploitation observed yet
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2023-42443.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2023-42443 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,636 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
vyperReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Impact
In certain conditions, the memory used by the builtins raw_call, create_from_blueprint and create_copy_of can be corrupted.
- For
raw_call, the argument buffer of the call can be corrupted, leading to incorrectcalldatain the sub-context. - For
create_from_blueprintandcreate_copy_of, the buffer for the to-be-deployed bytecode can be corrupted, leading to deploying incorrect bytecode.
Below are the conditions that must be fulfilled for the corruption to happen for each builtin:
raw_call
- memory is not fully initialized, ex. all parameters to an external function live in calldata and
- The
dataargument of the builtin ismsg.data. and - The
to,valueorgaspassed to the builtin is some complex expression that results in writing to uninitialized memory (e.g. calling an internal function)
create_copy_of
- memory is not fully initialized, ex. all parameters to an external function live in calldata and
- The
valueorsaltpassed to the builtin is some complex expression that results in writing to uninitialized memory (e.g. calling an internal function)
create_from_blueprint
- memory is not fully initialized, ex. all parameters to an external function live in calldata and
- Either no constructor parameters are passed to the builtin or
raw_argsis set to True. and - The
valueorsaltpassed to the builtin is some complex expression that results in writing to uninitialized memory (e.g. calling an internal function)
Note: When the builtin is being called from an internal function f from a function g, the issue is not present provided that g has written to memory before calling f.
Examples
raw_call
In the following contract, calling bar(1,1) will return:
ae42e95100000000000000000000000000000000000000000000000000000000ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff00000001
instead of:
ae42e95100000000000000000000000000000000000000000000000000000000000000010000000000000000000000000000000000000000000000000000000000000001
identity: constant(address) = 0x0000000000000000000000000000000000000004
@external
def foo():
pass
@internal
@view
def get_address()->address:
a:uint256 = max_value(uint256) # 0xfff...fff
return identity
@external
def bar(f:uint256, u:uint256) -> Bytes[100]:
a: Bytes[100] = raw_call(self.get_address(), msg.data, max_outsize=100)
return a
create_copy_of
In the following contract, after calling test(), the code deployed at self.created_address does not match the bytecode at target.
created_address: public(address)
@external
def test(target: address) -> address:
# The expression in salt= is complex and will require to store to memory
self.created_address = create_copy_of(target, salt = keccak256(_abi_encode(target)))
return self.created_address
create_from_blueprint
In the following contract, after calling test(), the init bytecode used to create the contract deployed at the address self.created_address will not match the blueprint bytecode stored at target.
created_address: public(address)
salt: constant(bytes32) = keccak256("kebab")
@external
@payable
def test(target: address):
# The expression in salt= is complex and will require to store to memory
self.created_address = create_from_blueprint(target, code_offset=0, salt=keccak256(_abi_encode(target)))
Patches
issue tracking in https://github.com/vyperlang/vyper/issues/3609, patched in #3610
Workarounds
The complex expressions that are being passed as kwargs to the builtin should be cached in memory prior to the call to the builtin. For the last example above, it would be:
created_address: public(address)
salt: constant(bytes32) = keccak256("kebab")
@external
@payable
def test(target: address):
salt: bytes32 = keccak256(_abi_encode(target))
self.created_address = create_from_blueprint(target, code_offset=0, salt=salt)
References
Are there any links users can visit to find out more?
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | vyper | ≥ 0.3.4&&< 0.3.10 | 0.3.10pip install --upgrade 'vyper==0.3.10' |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for vyper, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update vyper to 0.3.10 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2023-42443 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2023-42443 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2023-42443. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2023-42443 in your dependencies?
O3 Security finds CVE-2023-42443 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.