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CVE-2026-45623 high CVSS 7.5

Path Traversal in postcss (CVE-2026-45623)

Summary

Path traversal in postcss (CVE-2026-45623). Confidential information can be exposed externally. Exploitable via PreviousMap. Mitigation: upgrade to 8.5.12 or later.

Original description

The description as published by the source.

Summary

PostCSS's PreviousMap parses the /*# sourceMappingURL=PATH */ comment from any CSS string passed to process() and dereferences PATH against the local filesystem with no scheme, allowlist, or traversal check. An attacker who controls the CSS input can cause the host process to read any file readable by Node and leak the first ~10 bytes of its content through the resulting JSON.parse SyntaxError message. The bug also yields a precise file-existence oracle and a controllable-read primitive that may be combined with large-file targets for DoS. The behaviour is triggered with PostCSS's default options — no from, no map, no plugins required — and is therefore reachable from any pipeline that runs untrusted CSS through PostCSS (CMS themes, user-uploaded styles, browser-extension/userstyle processors, build pipelines for third-party packages, blog comment renderers, etc.).

Details

The dangerous chain lives in lib/previous-map.js and is wired into every Input construction at lib/input.js:70-77.

Input constructor (lib/input.js:70-77):

if (pathAvailable && sourceMapAvailable) {
  let map = new PreviousMap(this.css, opts)
  if (map.text) {
    this.map = map
    let file = map.consumer().file
    if (!this.file && file) this.file = this.mapResolve(file)
  }
}

PreviousMap constructor (lib/previous-map.js:17-29):

constructor(css, opts) {
  if (opts.map === false) return
  this.loadAnnotation(css)
  this.inline = this.startW…

Source: GitHub Security Advisories (Global) 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-2026-45623 has been found in postcss.
Attackers can target a specific entry point like PreviousMap over the network to misuse the product.
Confidential information can be exposed externally. CVSS score: 7.5/10.
What to do: upgrade postcss to 8.5.12 or later.
If unsure, ask your IT team or search "postcss CVE-2026-45623" on the vendor's site.

CVE-2026-45623 (postcss) — CWE-22 / CVSS v3 7.5
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: PreviousMap / PATH / JSON.parse / SyntaxError
Patched: 8.5.12 — apply immediately
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

Path traversal (CWE-22) exists in postcss. Attackers reach the vulnerable code path via PreviousMap without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of postcss. Attack surface: PreviousMap / PATH / JSON.parse / SyntaxError.

🔥 Severity

Severity: High (CVSS 7.5/10). Confidential information can be exposed externally.

🔧 How to fix

Update to 8.5.12.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'postcss' . 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 'postcss' . | 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 postcss 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 patch patch 
    Upgrade postcss to 8.5.12

    ステージング環境で 8.5.12 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。

  3. 3
    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 postcss
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"8.5.12"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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