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High, CVSS 7.5amended

linkify-it: Quadratic-complexity DoS via the mailto: validator scan-loop on attacker text

CVE-2026-59887GHSA-v245-v573-v5vm

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (linkify-it 5.0.2), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
EPSS
  • CVE-2026-59887: 0.47% estimated exploitation activity in the next 30 days; percentile 40.46 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary linkify-it's schema-scan loop (.test() / .match(), the documented public API) invokes the mailto: schema validator at every mailto: occurrence in the input text. For each occurrence the validator does text.slice(pos) (an O(n) copy) and runs an email regex whose local-part class srcemailname greedily scans the entire remaining tail (O(n)) before failing. With N mailto: occurrences that is N × O(n) = O(n²). Because linkify-it runs on arbitrary user text (markdown-it feeds it whole documents when linkify:true), an unauthenticated attacker can block the single-threaded event loop for many seconds with a small input. No length bound (unlike an HTTP header). Root cause — index.mjs + lib/re.mjs The while ((m = re.exec(text)) !== null) { …testSchemaAt… } scan loop calls the validator at each mailto: hit; srcemailname greedily consumes the whole tail (all chars are in its class) then fails for lack of @. http:/https: do NOT blow up — their validator requires the tail to start with //,…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from ghsa record GHSA-v245-v573-v5vm

  2. Revision 1 · created ·

    Published from osv record GHSA-v245-v573-v5vm

Agent brief

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# Patch Next advisory brief: CVE-2026-59887

Advisory: linkify-it: Quadratic-complexity DoS via the `mailto:` validator scan-loop on attacker text
IDs: CVE-2026-59887, GHSA-v245-v573-v5vm
Record: https://patchnext.com/advisories/cve-2026-59887 (revision 2, source checked 26 Oct 2026, 12:57 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: linkify-it 5.0.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm linkify-it: affected <= 5.0.1; fixed in 5.0.2

## Suggested inspection (read-only)
- `npm ls linkify-it --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on linkify-it.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-v245-v573-v5vm (OSV record GHSA-v245-v573-v5vm lists affected ranges: npm linkify-it >=0 <5.0.2 (fixed))
- Verified: https://github.com/advisories/GHSA-v245-v573-v5vm (GitHub-reviewed advisory GHSA-v245-v573-v5vm lists affected ranges: npm linkify-it >=0 <=5.0.1)

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