- Qt 6.9
- Qt Overviews
- Best Practice Guides
- Handling Untrusted Data
- Qt 6.9.3 Reference Documentation

Restoring a Window's Geometry ◦ Porting Qt 5 to Qt 6

# Handling Untrusted Data

Advice on handling untrusted data with Qt.

## Untrusted Data

Untrusted data is data that comes from sources whose integrity the application cannot guarantee. Examples of untrusted data are user-provided data, network data, generated data, or data that the application does not directly create. Improper handling of untrusted data can lead to application crashes, exposed private data, and harm to users and hardware. Therefore, developers must be vigilant about how their code receives and processes untrusted data.

The responsibility of ensuring that the data is safe is ultimately on the application. However, applications can depend on certain parts of Qt being developed to safely process and validate data. For example, Qt modules can validate images, manipulate strings safely, and can process incoming network packets. Qt considers these types of functionality as security critical. Security critical APIs undergo extra scrutiny and testing during development.

The Qt Project has an extensive process for designing and testing Qt functionality. For more information about Qt's security policy, see Security in Qt.

## Mitigating Risks and Vulnerabilities with Qt

Using Qt decreases many of the risks of processing untrusted data. The following sections cover aspects of untrusted data handling in Qt applications.

### User Input Validation

The lack of proper input validation can cause erroneous data and unexpected behaviors. However, strict validation rules can also hinder users from entering data. For example, preventing certain characters can result in users inputting incorrect information or weak passwords.

Qt provides several validators for user input. See:

- QValidator and Line Edits Example - C++ validators and example
- Qt Quick Validators - QML validators

### Command Injection

A common security vulnerability is malicious command execution by an external process. Executing commands from untrusted data can lead to exposed user data, access to private resources, and other malicious activities. Applications may also combine untrusted data with trusted data, tainting the data.

A common mitigation is to validate and separate the text from possible executable commands. An explicit allow list of commands, using both server-side and client-side verification, and careful parsing of data can also help mitigate the risk. It is also recommended to use trusted plugins and libraries that execute system commands instead of executing text scripts.

### Decoding Images

Images from untrusted sources can cause system crashes or decreased performance. Ensure that the application sanitizes image paths, image types, and file extensions before loading or processing images. Also, applications should process large images and unexpected formats without degrading performance or crashing.

### Deserializing Data

Treat data for deserialization as untrusted data if it is coming from outside the trust boundary, where data must undergo extra scrutiny. The serialized data may be corrupt or contain malicious content. Deserializing these data can result in erroneous data, application crashes, or malicious code execution. Check the integrity of the contents, for example, by using encryption or hashing techniques. Also, when deserializing data, handle errors gracefully and prevent system crashes.

For more information, see

- Qt Serialization
- Corruption and Security in QDataStream.

### Qt Qml and JavaScript Sources

Qt Qml assumes that the application developer provides all code and resources. QML is a generally memory-safe language with basic protection. However, QML does not have the concept of separate privacy domains. This means any code, whether loaded remotely or locally, can access all data in its scope.

QML Applications should load resources from the local filesystem or through Qt's resource system. If there is a need for the network, fetch the resource over HTTPS or within a trusted network.