---
id: "CVE-2021-48019"
kind: "vulnerability"
aliases: ["CVE-2021-48019"]
title: "Vyper: forwarder proxies misreport return size, corrupting large call results"
severity: "medium"
cvss: 6.3
epss: 0.00248
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["ethereum", "package", "smart-contracts", "vyper"]
not_affected_if: ["no-create-forwarder-to", "vyper-0.2.9-or-later"]
published_at: "2026-10-21T01:17:19+00:00"
first_seen_at: "2026-10-23T21:57:27+00:00"
updated_at: "2026-10-25T04:01:25+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2021-48019", "first_seen_at": "2026-10-23T21:57:27+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2021-48019", "first_seen_at": "2026-10-24T13:15:55+00:00"}]
---

# Vyper: forwarder proxies misreport return size, corrupting large call results

Vyper versions before 0.2.9 deploy forwarder proxies via create_forwarder_to that report a fixed 4096-byte RETURNDATASIZE instead of the real return size. Calls through these proxies can return corrupted data for functions returning more than 4096 bytes, and strict return-size checks such as SafeERC20.safeTransfer can fail.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the Vyper compiler version used in the project, for example with `vyper --version`, and look for `create_forwarder_to` in the Vyper source.
- Search deployed or source contracts for `create_forwarder_to` calls or forwarder proxies built by the Vyper compiler.

## Fix

- Upgrade Vyper to `0.2.9` or later and recompile contracts that use `create_forwarder_to`.

## Timeline

- 2026-10-23T21:57:27+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2021-48019
- 2026-10-24T13:15:55+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2021-48019

## From the sources

Vyper before 0.2.9 deploys forwarder proxies through the built-in create_forwarder_to function that report a fixed 4096-byte RETURNDATASIZE rather than the actual return data size. Calls through these proxies can yield corrupted data when a function returns more than 4096 bytes without return data sanitation, and fail strict return-size checks such as SafeERC20.safeTransfer.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_