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CVE-2026-34786 medium CVSS 5.3

Vulnerability in rack (CVE-2026-34786)

Summary

Vulnerability in rack (CVE-2026-34786). Risk of unauthorized operations or information disclosure. Exploitable via header_rules. Mitigation: upgrade to 3.2.6 or later.

Original description

The description as published by the source.

Summary

Rack::Static#applicable_rules evaluates several header_rules types against the raw URL-encoded PATH_INFO, while the underlying file-serving path is decoded before the file is served. As a result, a request for a URL-encoded variant of a static path can serve the same file without the headers that header_rules were intended to apply.

In deployments that rely on Rack::Static to attach security-relevant response headers to static content, this can allow an attacker to bypass those headers by requesting an encoded form of the path.

Details

Rack::Static#applicable_rules matches rule types such as :fonts, Array, and Regexp directly against the incoming PATH_INFO. For example:

when :fonts
  /\.(?:ttf|otf|eot|woff2|woff|svg)\z/.match?(path)
when Array
  /\.(#{rule.join('|')})\z/.match?(path)
when Regexp
  rule.match?(path)

These checks operate on the raw request path. If the request contains encoded characters such as %2E in place of ., the rule may fail to match even though the file path is later decoded and served successfully by the static file server.

For example, both of the following requests may resolve to the same file on disk:

/fonts/test.woff
/fonts/test%2Ewoff

but only the unencoded form may receive the headers configured through header_rules.

This creates a canonicalization mismatch between the path used for header policy decisions and the path ultimately used for file serving.

Impact

Applicati…

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-2026-34786 has been found in rack.
Attackers can target a specific entry point like header_rules over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 5.3/10.
What to do: upgrade rack to 3.2.6 or later.
If unsure, ask your IT team or search "rack CVE-2026-34786" on the vendor's site.

CVE-2026-34786 (rack) — CWE-180 / CVSS v3 5.3
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: header_rules / PATH_INFO / Array / Regexp
Patched: 3.2.6 — 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

A vulnerability (CWE-180) exists in rack. Attackers reach the vulnerable code path via header_rules without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of rack. Attack surface: header_rules / PATH_INFO / Array / Regexp.

🔥 Severity

Severity: Medium (CVSS 5.3/10). Risk of unauthorized operations or information disclosure.

🔧 How to fix

Update to 3.2.6.

🛡️ 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 'rack' . 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 'rack' . | 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 rack 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 rack to 3.2.6

    ステージング環境で 3.2.6 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

rubygems rack
[{"type":"ECOSYSTEM","events":[{"introduced":"3.2.0"},{"fixed":"3.2.6"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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