---
id: CVE-2023-31146
aliases:
  - GHSA-3p37-3636-q8wv
  - PYSEC-2023-77
title: >-
  Vyper vulnerable to OOB DynArray access when array is on both LHS and RHS of
  an assignment
summary: >-
  Vyper vulnerable to OOB DynArray access when array is on both LHS and RHS of
  an assignment
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N'
vendor: vyper
product: vyper
ecosystem: pip
affected:
  - vyper < 0.3.8
patched:
  - vyper 0.3.8
published: '2023-05-12'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T03:49:54.489764847Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-3p37-3636-q8wv'
references:
  - url: 'https://github.com/vyperlang/vyper/security/advisories/GHSA-3p37-3636-q8wv'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2023-31146'
  - url: >-
      https://github.com/vyperlang/vyper/commit/4f8289a81206f767df1900ac48f485d90fc87edb
  - url: >-
      https://github.com/pypa/advisory-database/tree/main/vulns/vyper/PYSEC-2023-77.yaml
  - url: 'https://github.com/vyperlang/vyper'
tags:
  - osv
  - pip
epss: 0.01241
epssPercentile: 0.67872
ingestedAt: '2026-09-12T03:13:01.647Z'
---

## Overview

### Impact
during codegen, the length word of a dynarray is written before the data, which can result in OOB array access in the case where the dynarray is on both the lhs and rhs of an assignment. here is a minimal example producing the issue:
```vyper
a:DynArray[uint256,3]
@external
def test() -> DynArray[uint256,3]:
    self.a = [1,2,3]
    self.a = empty(DynArray[uint256,3])
    self.a = [self.a[0],self.a[1],self.a[2]]
    return self.a # return [1,2,3]
```

and here is an example demonstrating the issue can cause data corruption across call frames:

```vyper
@external
def test() -> DynArray[uint256,3]:
    self.a()
    return self.b() # return [1,2,3]

@internal
def a():
    a: uint256 = 0    
    b: uint256 = 1    
    c: uint256 = 2    
    d: uint256 = 3

@internal
def b() -> DynArray[uint256,3]:
    a: DynArray[uint256,3] = empty(DynArray[uint256,3])
    a = [a[0],a[1],a[2]]
    return a
```

examples involving append and pop:
```vyper
@internal
def foo():
    c: DynArray[uint256, 1] = []
    c.append(c[0])
```

```vyper
@internal
def foo():
    c: DynArray[uint256, 1] = [1]
    c[0] = c.pop()
```

the expected behavior in all of the above cases is to revert due to oob array access.

### Patches
patched in 4f8289a81206f767df1900ac48f485d90fc87edb

### Workarounds
_Is there a way for users to fix or remediate the vulnerability without upgrading?_

### References
_Are there any links users can visit to find out more?_


## Affected packages

- `vyper < 0.3.8`

## Remediation

Upgrade to a patched release:

- `vyper 0.3.8`