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CVE-2026-34831 medium CVSS 4.8

Vulnerability in rack (CVE-2026-34831)

Summary

Vulnerability in rack (CVE-2026-34831). Risk of unauthorized operations or information disclosure. Exploitable via body.size. Mitigation: upgrade to 3.2.6 or later.

Original description

The description as published by the source.

Summary

Rack::Files#fail sets the Content-Length response header using String#size instead of String#bytesize. When the response body contains multibyte UTF-8 characters, the declared Content-Length is smaller than the number of bytes actually sent on the wire.

Because Rack::Files reflects the requested path in 404 responses, an attacker can trigger this mismatch by requesting a non-existent path containing percent-encoded UTF-8 characters.

This results in incorrect HTTP response framing and may cause response desynchronization in deployments that rely on the incorrect Content-Length value.

Details

Rack::Files#fail constructs error responses using logic equivalent to:

def fail(status, body, headers = {})
  body += "\n"
  [
    status,
    {
      "content-type" => "text/plain",
      "content-length" => body.size.to_s,
      "x-cascade" => "pass"
    }.merge!(headers),
    [body]
  ]
end

Here, body.size returns the number of characters, not the number of bytes. For multibyte UTF-8 strings, this produces an incorrect Content-Length value.

Rack::Files includes the decoded request path in 404 responses. A request containing percent-encoded UTF-8 path components therefore causes the response body to contain multibyte characters, while the Content-Length header still reflects character count rather than byte count.

As a result, the server can send more bytes than declared in the response headers.

This violates HTTP message framin…

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-2026-34831 has been found in rack.
Attackers can target a specific entry point like body.size over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 4.8/10.
What to do: upgrade rack to 3.2.6 or later.
If unsure, ask your IT team or search "rack CVE-2026-34831" on the vendor's site.

CVE-2026-34831 (rack) — CWE-130 / CVSS v3 4.8
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: body.size
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-130) exists in rack. Attackers reach the vulnerable code path via body.size without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of rack. Attack surface: body.size.

🔥 Severity

Severity: Medium (CVSS 4.8/10). 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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