SSRF (Server-Side Request Forgery) in css_parser (CVE-2026-53727)
Summary
SSRF in css_parser (CVE-2026-53727). Confidential information can be exposed externally. Exploitable via css_parser. Mitigation: upgrade to 3.0.0 or later.
Original description
The description as published by the source.
css_parser is a Ruby CSS parser. From 2.2.0 until 3.0.0, CssParser::Parser#read_remote_file in lib/css_parser/parser.rb, and therefore load_uri! and the @import-following branch of add_block!, issued HTTP and HTTPS requests against any host, port, and URI without a scheme allowlist, host or IP filtering, or protection against link-local, loopback, or RFC-1918 addresses. Location: redirects were followed recursively back into the same function, which also serviced file:// URIs, so a single attacker-controlled HTTP redirect could upgrade the bug from SSRF to arbitrary local file disclosure. Any consumer of css_parser that hands it attacker-influenced CSS together with a base_uri: option is exposed. This issue is fixed in version 3.0.0.
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-2026-53727 has been found in css_parser.
Attackers can target a specific entry point like css_parser over the network to misuse the product.
Confidential information can be exposed externally. CVSS score: 8.6/10.
What to do: upgrade css_parser to 3.0.0 or later.
If unsure, ask your IT team or search "css_parser CVE-2026-53727" on the vendor's site.
CVE-2026-53727 (css_parser) — CWE-918 / CVSS v3 8.6
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: css_parser / uri.host / uri.scheme / Location
Patched: 3.0.0 — 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.
SSRF (CWE-918) exists in css_parser. Attackers reach the vulnerable code path via css_parser without authentication.
Check the vendor advisory for the affected versions of css_parser. Attack surface: css_parser / uri.host / uri.scheme / Location.
Severity: High (CVSS 8.6/10). Confidential information can be exposed externally.
Update to 3.0.0.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'css_parser' . 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
-
1Identify exposure identify
grep -r 'css_parser' . | 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
css_parserを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply patch patch
Upgrade css_parser to 3.0.0ステージング環境で 3.0.0 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。
-
3Post-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
Fix status by distribution
Fixed packages published by each distribution for this CVE
- Debian DEBIAN-CVE-2026-53727 ruby-css-parser
- Ubuntu UBUNTU-CVE-2026-53727 ruby-css-parser
References
- advisory github.com
- advisory github.com
- advisory nvd.nist.gov
- package github.com
- patch github.com
- patch github.com
- web github.com
- web cve.org
- web github.com
- web github.com