> For the complete documentation index, see [llms.txt](https://reports.immunefi.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://reports.immunefi.com/fuel-network-or-attackathon/attackathon-_-fuel-network-32730-smart-contract-low-the-sway-compiler-currently-disallows-read-acces.md).

# Attackathon \_ Fuel Network 32730 - \[Smart Contract - Low] The Sway compiler currently disallows read

Submitted on Mon Jul 01 2024 01:21:53 GMT-0400 (Atlantic Standard Time) by @rbz for [Attackathon | Fuel Network](https://immunefi.com/bounty/fuel-network-attackathon/)

Report ID: #32730

Report type: Smart Contract

Report severity: Low

Target: <https://github.com/FuelLabs/sway/tree/7b56ec734d4a4fda550313d448f7f20dba818b59>

Impacts:

* Griefing (e.g. no profit motive for an attacker, but damage to the users or the protocol)
* If contract A interacts with contract B and the interaction fails, causing contract B to revert, contract A may also revert as a consequence;

## Description

## Brief/Intro

The Sway compiler's current state Reverts any transaction that performs read access to storage if the call is made within the `fallback` function.

## Vulnerability Details

I adapted an existing collection of tests targetting `fallback` functionality (can found at <https://github.com/FuelLabs/sway/tree/7b56ec734d4a4fda550313d448f7f20dba818b59/test/src/sdk-harness/test\\_projects/run\\_external\\_proxy> and <https://github.com/FuelLabs/sway/tree/7b56ec734d4a4fda550313d448f7f20dba818b59/test/src/sdk-harness/test\\_projects/run\\_external\\_target>) to demonstrate an issue with the way compiler handles storage access in `fallback` function. While the compiler documentation suggests READ access to storage should be possible within fallback functions (since it's a common characteristic of any Sway function), the current state of the compiler prevents this. This difference between the documentation and the codebase's actual behavior raises doubts about whether fallback functions work as intended.

However, WRITE operations to storage within fallback functions are working as expected, indicating a potential inconsistency in how storage interactions are handled.

## Impact Details

* A malicious contract with a faulty fallback function could be deployed, deliberately hiding crucial storage access features within it. This could trick less technical users into thinking the contract is legitimate because it compiles successfully. This can potentially to griefing;
* If contract A interacts with contract B and the interaction fails, causing contract B to revert, contract A may also revert as a consequence;

## References

<https://github.com/FuelLabs/sway/blob/7b56ec734d4a4fda550313d448f7f20dba818b59/docs/book/src/blockchain-development/calling\\_contracts.md?plain=1#L174>

## Proof of concept

Please see following gists: <https://gist.github.com/0xZRA/c25f0ee9ef89d44adde573609c5553f3> <https://gist.github.com/0xZRA/19a9079f97f60a5143115e76a0eb84a9> <https://gist.github.com/0xZRA/18d8f25e6b3a97025de946868a230765> <https://gist.github.com/0xZRA/6e1684f538ad85dff5a39b25bb54523f> <https://gist.github.com/0xZRA/8875643f14e88092ac72e400079fd849>


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation by asking a question.

Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters:

```
GET https://reports.immunefi.com/fuel-network-or-attackathon/attackathon-_-fuel-network-32730-smart-contract-low-the-sway-compiler-currently-disallows-read-acces.md?ask=<question>&goal=<user_goal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.