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CVE-2026-56669: CVE-2026-56669: Remote Denial of Service via Algorithmic Complexity and Interpretation Conflict in Elysia

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CVE-2026-56669: Remote Denial of Service via Algorithmic Complexity and Interpretation Conflict in Elysia

Vulnerability ID: CVE-2026-56669
CVSS Score: 7.5
Published: 2026-09-23

CVE-2026-56669 is a high-severity vulnerability in the Elysia web framework (ElysiaJS) that combines Inefficient Algorithmic Complexity (CWE-407) and an Interpretation Conflict (CWE-436). It allows remote, unauthenticated attackers to cause a complete Denial of Service (DoS) via CPU resource exhaustion using specially crafted multipart or urlencoded payloads.

TL;DR

A high-severity denial-of-service vulnerability in Elysia prior to v1.4.29 allows remote, unauthenticated attackers to freeze the server's single-threaded event loop. By using an interpretation conflict to bypass request-size filters, attackers can submit thousands of unique keys that trigger quadratic processing times during form-data normalization.

⚠️ Exploit Status: POC

Technical Details

  • CWE ID: CWE-407, CWE-436
  • Attack Vector: Network (AV:N)
  • CVSS Score: 7.5 (High)
  • EPSS Score: 0.0063 (Percentile: 48.83%)
  • Impact: Denial of Service (DoS)
  • Exploit Status: Proof of Concept (PoC) Public
  • KEV Status: Not Listed

Affected Systems

  • ElysiaJS web applications running on Bun or Node.js runtimes using versions prior to 1.4.29
  • elysia: < 1.4.29 (Fixed in: 1.4.29)

Code Analysis

Commit: 8358ff9

Fix quadratic form parsing using Map grouping

diff --git a/src/dynamic-handle.ts b/src/dynamic-handle.ts
index ab165f5..8358ff9 100
--- a/src/dynamic-handle.ts
+++ b/src/dynamic-handle.ts
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Mitigation Strategies

  • Upgrade to Elysia version 1.4.29 or later.
  • Filter and sanitize malformed HTTP Content-Type headers at the reverse proxy or WAF layer.
  • Enforce limit controls on the maximum number of allowed keys in incoming HTTP requests.

Remediation Steps:

  1. Locate the package.json file of the Elysia project.
  2. Update the 'elysia' dependency to version '^1.4.29'.
  3. Run 'bun install' or 'npm install' to apply the update.
  4. Verify the deployment in a staging environment by testing standard form-data submissions.

References

Read the full report for CVE-2026-56669 on our website for more details including interactive diagrams and full exploit analysis.

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