Related issue

Closes #14489

Proposed changes

Problem Recap

Validating extremely large Base64-encoded strings using a regular expression directly can lead to performance issues or runtime errors such as RangeError: Maximum call stack size exceeded. This typically occurs with very large payloads, such as image data encoded in Base64 (e.g., screenshots with 4+ million characters).

Updated Approach

To address this, we implemented a scalable validation strategy in the isBase64Safe function. The key change is the use of adaptive chunking for large inputs to avoid overwhelming the regex engine.

How It Works:

For small strings (length ≤ BASE_64_SAFE_STRING_TO_PROCESS_LENGTH), the regular expression is applied directly to the entire string.

For larger strings:

The string’s total length is used to determine the number of digits (i.e., its order of magnitude).

The input is split into fixed-size chunks, where each chunk’s size is calculated as:
Math.floor(length / digitCount / 4) * 4
(ensuring chunk lengths are divisible by 4 for valid Base64 structure).

Each chunk is tested individually against the BASE_64_REGEX.

If any chunk fails validation, the entire string is considered invalid.

If all chunks pass, the string is considered valid.

Benefits

Scalable: Handles very large Base64 strings without risking stack overflows or regex engine limitations.

Efficient: Reduces validation cost by applying regex in smaller, manageable chunks.

Safe: Ensures accurate Base64 validation while preserving application stability, even for very large inputs.

This enhancement allows the system to safely validate large, complex Base64 strings—such as those generated from high-resolution screenshots—without compromising performance or reliability.

Types of changes

• Polish (an improvement for an existing feature)
• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update (improvements to the project's docs)
• Specification changes (updates to WebDriver command specifications)
• Internal updates (everything related to internal scripts, governance documentation and CI files)

Checklist

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation (if appropriate)
• I have added proper type definitions for new commands (if appropriate)

Backport Request

//: # (The current main branch is the development branch for WebdriverIO v9. If your change should be released to the current major version of WebdriverIO (v8), please raise another PR with the same changes against the v8 branch.)

• This change is solely for v9 and doesn't need to be back-ported
• Back-ported PR at #XXXXX

Further comments

Reviewers: @webdriverio/project-committers