Outdated JavaScript Utility Library Can Be Used to Slow Down Your App

Vulnerability Explanation

Lodash versions prior to 4.17.21 contain inefficient regular expressions in the toNumber, trim, and trimEnd functions. These regexes exhibit catastrophic backtracking when evaluated against specially crafted input strings — for example, a long string of spaces followed by a non-matching character. An attacker who can influence input passed to any of these three functions can cause the JavaScript event loop to block for an extended period, degrading or halting service for all users.

Root Cause

The affected functions used regular expressions with patterns susceptible to exponential worst-case complexity (CWE-1333). The regex engine's backtracking behaviour means that for certain malformed inputs, the number of steps required grows exponentially with input length, consuming excessive CPU. This was fixed in 4.17.21 by replacing the problematic regex patterns with more efficient string-processing logic.

Technical Impact

An unauthenticated remote attacker can cause a denial-of-service condition by sending crafted input to any endpoint that passes user-controlled data into lodash's toNumber(), _.trim(), or _.trimEnd() functions. This blocks the Node.js event loop, making the application unresponsive to all concurrent users for the duration of the attack. No data confidentiality or integrity impact.

Severity Justification

CVSS 3.1 score of 5.3 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L) per NVD and GitHub Advisory Database. Impact is limited to availability (low), with no confidentiality or integrity impact. Exploitability requires the attacker to control input reaching the specific affected functions, which is not guaranteed in all deployments.

Affected Components

• lodash >= 4.0.0, < 4.17.21

Remediation Steps

1. Update lodash to 4.17.21 or later: run `npm update lodash` or `yarn upgrade lodash` in your project root.
2. If lodash is a transitive (indirect) dependency, identify the parent package with `npm ls lodash` and update that package, or use `npm audit fix` to resolve it automatically.
3. If an immediate upgrade is blocked by a dependency conflict, add an `overrides` (npm 8.3+) or `resolutions` (Yarn) entry to force the patched version: see code example below.
4. Run your test suite after upgrading to verify no regressions.
5. Verify the installed version is patched using `npm list lodash` — confirm output shows 4.17.21 or higher.

Verification Steps

1. Run `npm list lodash` (or `yarn list --pattern lodash`) and confirm all instances report version 4.17.21 or later.
2. Run `npm audit` and confirm CVE-2020-28500 no longer appears in the output.
3. If you have a staging environment, send a test request with a large whitespace-padded string to any endpoint that uses lodash trim/toNumber and confirm the response time is normal (under 100ms).

Code Examples (json)

// package.json — pinned to a vulnerable version
{
  "dependencies": {
    "lodash": "^4.17.15"
  }
}

// Option 1 — direct dependency: bump the version
{
  "dependencies": {
    "lodash": "^4.17.21"
  }
}

// Option 2 — transitive dependency: force the version via overrides (npm 8.3+)
{
  "overrides": {
    "lodash": "^4.17.21"
  }
}

// Option 2 — Yarn: use resolutions
{
  "resolutions": {
    "lodash": "^4.17.21"
  }
}

Best Practices

• Enable automated dependency scanning (GitHub Dependabot, Snyk, or npm audit in CI) to catch vulnerable library versions before they reach production.
• Pin or range-lock critical dependencies and review the lock file (`package-lock.json` / `yarn.lock`) in code review.
• Validate and limit the length of user-supplied strings before passing them to any string-processing utility function.
• Periodically audit transitive dependencies — not just direct ones — since vulnerabilities often arrive through indirect packages.

References

• github.com/advisories/GHSA-29mw-wpgm-hmr9
• security.snyk.io/vuln/SNYK-JS-LODASH-1018905
• www.cve.org/CVERecord
• github.com/lodash/lodash/commit/c4847ebe7d14540bb28a8b932a9ce1b9ecbfee1a