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CVE-2026-26961 low CVSS 3.7

Vulnerability in rack (CVE-2026-26961)

Summary

Vulnerability in rack (CVE-2026-26961). Risk of unauthorized operations or information disclosure. Exploitable via boundary. Mitigation: upgrade to 3.2.6 or later.

Original description

The description as published by the source.

Summary

Rack::Multipart::Parser extracts the boundary parameter from multipart/form-data using a greedy regular expression. When a Content-Type header contains multiple boundary parameters, Rack selects the last one rather than the first.

In deployments where an upstream proxy, WAF, or intermediary interprets the first boundary parameter, this mismatch can allow an attacker to smuggle multipart content past upstream inspection and have Rack parse a different body structure than the intermediary validated.

Details

Rack identifies the multipart boundary using logic equivalent to:

MULTIPART = %r|\Amultipart/.*boundary=\"?([^\";,]+)\"?|ni

Because the expression is greedy, it matches the last boundary= parameter in a header such as:

Content-Type: multipart/form-data; boundary=safe; boundary=malicious

As a result, Rack parses the request body using malicious, while another component may interpret the same header using safe.

This creates an interpretation conflict. If an upstream WAF or proxy inspects multipart parts using the first boundary and Rack later parses the body using the last boundary, a client may be able to place malicious form fields or uploaded content in parts that Rack accepts but the upstream component did not inspect as intended.

This issue is most relevant in layered deployments where security decisions are made before the request reaches Rack.

Impact

Applications that accept multipart/form-data upl…

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-2026-26961 has been found in rack.
Attackers can target a specific entry point like boundary over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 3.7/10.
What to do: upgrade rack to 3.2.6 or later.
If unsure, ask your IT team or search "rack CVE-2026-26961" on the vendor's site.

CVE-2026-26961 (rack) — CWE-436 / CVSS v3 3.7
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: boundary / malicious / safe
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-436) exists in rack. Attackers reach the vulnerable code path via boundary without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of rack. Attack surface: boundary / malicious / safe.

🔥 Severity

Severity: Low (CVSS 3.7/10). 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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