sindresorhus/eslint-plugin-unicorn

Rule proposal: `.replace` or `.replaceAll` with non-literal replacement

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#2,309 opened on Apr 5, 2024

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help wantednew rule

Description

Description

One might think that a function like this generates safe HTML because the argument is HTML-escaped.

function imageLink(url) {
  const IMAGE_TEMPLATE = '<img src="{url}">';
  return IMAGE_TEMPLATE.replace('{url}', htmlEscape(url));
}

But in fact there’s a very obscure cross-site scripting vulnerability here, abusing the $` replacement sequence interpreted by String.prototype.replace and .replaceAll!

imageLink("$` onerror=alert(1) ")
//=> '<img src="<img src=" onerror=alert(1) ">'

To protect against this mistake, it would be nice to have an ESLint rule that forbids use of .replace and .replaceAll where the second argument isn’t a string literal or a function.

Fail

IMAGE_TEMPLATE.replace('{url}', htmlEscape(url))

Pass

IMAGE_TEMPLATE.replace('{url}', () => htmlEscape(url))

IMAGE_TEMPLATE.replace('{url}', "https://example.com")

Additional Info

No response

Contributor guide

Tech stack
javascriptnodejs
Domain
toolingdeveloper experiencesecurity
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
Basic JavaScriptESLint rule development basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65
Research direction
This issue proposes a new ESLint rule. To implement, study existing rules in the eslint plugin unicorn repository, particularly rules that analyze String.prototype.replace calls. Look at the rule structure in the 'rules' directory and test files in 'test'. The rule should check if the second argument to .replace or .replaceAll is a string literal or a function. The maintainers may have additional guidance in comments; review the discussion to ensure alignment.