CVE-2026-56669: Elysia Form Data Processing DoS Vulnerability

Technical summary

The vulnerability stems from Elysia's use of the getAll method during form data normalization for multipart/form-data endpoints. The getAll operation exhibits quadratic time complexity relative to the number of unique key-value pairs in the request. This algorithmic inefficiency means that processing time grows dramatically (not linearly) with each additional form field, enabling attackers to trigger CPU exhaustion with a relatively modest number of form fields. The flaw affects all versions of Elysia prior to 1.4.29. The issue is categorized under CWE-407 (inefficient algorithmic complexity) and CWE-436 (interpretation conflict), both common precursors to resource exhaustion attacks.

Business impact

Elysia-based services are vulnerable to remotely triggered availability attacks. An attacker sending a single malicious request with dozens or hundreds of form fields can cause significant CPU spikes, degrading or completely disabling the service. For organizations running Elysia in production—particularly SaaS platforms, APIs handling file uploads, or backend services with high availability requirements—this vulnerability poses a direct threat to service uptime and customer experience. The attack requires no credentials, no complex exploit infrastructure, and no user interaction, making it a straightforward denial-of-service vector that could be weaponized broadly once disclosed.

Affected systems

Elysia framework versions prior to 1.4.29 are affected. Organizations using Elysia for any multipart/form-data handling (file uploads, complex form submissions, etc.) should be considered at risk. This includes custom applications, microservices, and any TypeScript/Bun-based backends leveraging Elysia for request validation and routing. The vulnerability is network-accessible and does not require the attacker to be authenticated or have local access.

Exploitability

This vulnerability is highly exploitable. The attack is trivial to execute—an attacker need only craft an HTTP POST request with multipart/form-data encoding and include a large number of form fields (or deeply nested structures) to trigger quadratic algorithmic behavior. No special tooling, credentials, or user interaction is required. The attack surface is wide: any publicly exposed endpoint accepting multipart/form-data is a potential target. The low CVSS attack complexity (AC:L) and lack of privilege requirements (PR:N) reflect this ease of exploitation. However, the attack is limited to denial-of-service; it does not leak data or allow code execution.

Remediation

Upgrade Elysia to version 1.4.29 or later. This version replaces the inefficient getAll normalization with a more efficient algorithm, eliminating the quadratic complexity. Organizations should prioritize this upgrade for any production Elysia deployments, particularly those handling untrusted input or exposed to the internet. Testing after upgrade is recommended to ensure no breaking changes to existing form data handling logic.

Patch guidance

Apply Elysia 1.4.29 or later as soon as possible. Verify the version in your package.json or package-lock.json, and update using your package manager (npm, yarn, pnpm, etc.). If you are running Elysia as a dependency of another framework or library, check with the upstream maintainer for an updated release incorporating the patched Elysia version. No interim workarounds or configuration changes mitigate this flaw; patching is the only reliable fix. After patching, restart your application to ensure the new version is active.

Detection guidance

Monitor for incoming HTTP requests with multipart/form-data encoding that contain an unusually high number of form fields (e.g., >50–100 fields, depending on your baseline). Track CPU usage on Elysia application processes; sustained spikes correlated with form data requests may indicate exploitation attempts. Application Performance Monitoring (APM) tools can flag requests that take disproportionately long to process. Log unusual form submission patterns and consider rate-limiting or request size caps as temporary protective measures. Note that legitimate bulk form submissions may also trigger these signatures, so contextualize alerts accordingly. Post-patch, such monitoring can serve as forensic evidence of exploitation attempts before the upgrade was deployed.

Why prioritize this

This vulnerability warrants urgent patching despite its availability-only impact (no confidentiality or integrity risk). The combination of trivial exploitability, lack of authentication barriers, and direct threat to service availability makes it a prime candidate for rapid attack. Organizations with public-facing Elysia services or those processing untrusted file uploads should treat this as a critical operational risk. The fix is straightforward (a version bump), and there are no known significant breaking changes in the patch, reducing deployment friction.

Risk score, explained

The CVSS 3.1 score of 7.5 (HIGH) reflects a network-accessible, low-complexity attack requiring no privileges or user interaction that results in a complete availability impact. The attack vector is network-based (AV:N), attack complexity is low (AC:L), no privileges are needed (PR:N), and no user interaction is required (UI:N). The scope is unchanged (S:U), meaning the attack affects only the vulnerable application itself and not other systems. Confidentiality and integrity are not impacted (C:N/I:N), but availability is fully compromised (A:H). This score appropriately prioritizes the ease and breadth of exploitation against the limited scope of impact (denial-of-service only).

Frequently asked questions