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CVE-2026-41591 medium CVSS 6.4

Cross-Site Scripting (XSS) in marko (CVE-2026-41591)

Summary

Cross-site scripting in marko (CVE-2026-41591). Risk of unauthorized operations or information disclosure. Exploitable via _escape_script. Mitigation: upgrade to 5.38.36 or later.

Original description

The description as published by the source.

Summary

When dynamic text is interpolated into a or tag the Marko runtime failed to prevent tag breakout when the closing tag used non-lowercase casing.
An attacker able to place input inside a or block could break out of the tag with , , etc. and inject arbitrary HTML/JavaScript, resulting in cross-site scripting.

Details

The affected helpers used case-sensitive regular expressions to detect attempts at closing the surrounding tag:

// packages/runtime-tags/src/html/content.ts
const unsafeScriptReg = /<\/script/g;
const unsafeStyleReg  = /<\/style/g;

// packages/runtime-class/src/runtime/html/helpers/escape-script-placeholder.js
const unsafeCharsReg = /<\/script/g;

// packages/runtime-class/src/runtime/html/helpers/escape-style-placeholder.js
const unsafeCharsReg = /<\/style/g;

HTML tag names are case-insensitive in the browser parser, so inputs such as , , or `` were not matched by these regexes and passed through the helpers unchanged. A browser rendering the output treats the mixed-case end tag as a valid closing tag, terminating the script or style context, and then parses anything that follows as HTML.

The Marko compiler routes interpolated values inside and tags through these helpers automatically (see native-tag.ts:1080-1085), so application code following the framework's conventions had no way to detect or compensate for the gap.

PoC

$ const userCode = "alert(1)//";

  const data = ${JSON.stringify(u…

Source: NIST National Vulnerability Database NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-41591 has been found in marko.
Attackers can target a specific entry point like _escape_script over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 6.4/10.
What to do: upgrade marko to 5.38.36 or later.
If unsure, ask your IT team or search "marko CVE-2026-41591" on the vendor's site.

CVE-2026-41591 (marko) — CWE-79 / CVSS v3 6.4
Attack vector: remote (network-reachable) / no user interaction
Attack surface: _escape_script / _escape_style / _escape_comment
Patched: 5.38.36 — apply immediately
Workaround: Upgrade to the patched release.
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

Cross-site scripting (XSS) (CWE-79) exists in marko. Attackers reach the vulnerable code path via _escape_script.

📍 Affected scope

Check the vendor advisory for the affected versions of marko. Attack surface: _escape_script / _escape_style / _escape_comment.

🔥 Severity

Severity: Medium (CVSS 6.4/10). Risk of unauthorized operations or information disclosure.

🔧 How to fix

Update to 5.38.36.

🛡️ Workaround

Workaround: Upgrade to the patched release.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'marko' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'marko' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で marko を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Apply temporary workaround mitigate 
    Upgrade to the patched release.

    パッチが適用されるまでの応急処置として、Upgrade to the patched release. を実施。回避策の副作用 (機能低下) を確認した上で。

  3. 3
    Apply patch patch 
    Upgrade marko to 5.38.36

    ステージング環境で 5.38.36 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  4. 4
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

npm marko
[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"5.38.36"}]}]
npm @marko/runtime-tags
[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"6.0.164"}]}]

References

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