CVE-2026-57439: CyberChef Prototype Pollution via Series Chart CSV Parsing

Technical summary

CyberChef versions prior to 11.2.0 fail to sanitize the `__proto__` property when the Series Chart operation parses CSV input. In JavaScript, `__proto__` is a special property that allows direct access to an object's prototype chain. When this unsanitized input is processed, it pollutes the prototype of the Object class, affecting all objects created subsequently. An attacker can chain this with operations such as Parse UDP to inject arbitrary JavaScript into generated HTML output. The vulnerability stems from insufficient input validation in CSV parsing logic and is resolved by upgrading to version 11.2.0 or later, which implements proper prototype pollution protections.

Business impact

While CyberChef is primarily a developer and security analyst tool, organizations using it in shared or multi-user environments face risk. A malicious CSV file could be weaponized to execute JavaScript in a user's browser, potentially leading to session hijacking, credential theft, or exfiltration of sensitive data being processed within the application. The impact is heightened if CyberChef processes confidential information such as cryptographic keys, decoded secrets, or sensitive logs. The requirement for user interaction (file upload/viewing) limits mass exploitation but makes it suitable for targeted attacks against specific analysts or teams.

Affected systems

CyberChef versions before 11.2.0 are affected. The vulnerability requires the Series Chart operation to be used with attacker-controlled CSV input. Downstream operations that generate HTML output (such as Parse UDP with HTML rendering) can amplify the risk. Organizations should determine their current CyberChef deployment version and assess whether users have access to untrusted data sources or whether CSV files could be smuggled into workflows.

Exploitability

Exploitation requires user interaction—specifically, a user must open or process a malicious CSV file through the Series Chart operation. The attack is not remotely triggered or worm-like; it demands deliberate action by the victim. However, the technical barrier is low: an attacker simply needs to craft a CSV with `__proto__` as a key and inject JavaScript. Social engineering can increase likelihood (e.g., sharing a 'sample data' CSV). No authentication bypass or elevated privileges are required, and the attacker does not need network access if the file is transferred offline. Given these factors, the attack is moderately exploitable in targeted scenarios but unlikely to achieve widespread impact without campaign effort.

Remediation

Upgrade CyberChef to version 11.2.0 or later immediately. This version includes patches to validate and reject `__proto__` and other prototype pollution vectors during CSV parsing. Organizations should enforce this update across all instances, especially in shared environments or on systems where users process external data. Additionally, educate users to avoid uploading untrusted CSV files and to be cautious of data sources offered through informal channels. Consider network segmentation or sandboxing of CyberChef if it processes particularly sensitive materials.

Patch guidance

CyberChef version 11.2.0 fixes this vulnerability. Verify the current version in your deployment (typically visible in the application footer or settings). If running an earlier version, obtain version 11.2.0 or later from the official CyberChef repository or distribution channel. Test the update in a non-production environment first to ensure compatibility with existing workflows. No configuration changes are required post-patch; the fix is transparent to users. For air-gapped or offline deployments, verify patch availability and plan a maintenance window to update all instances.

Detection guidance

Monitor HTTP logs or network traffic for uploads of CSV files to CyberChef instances, particularly those destined for the Series Chart operation. Inspect uploaded CSV files for suspicious patterns, such as `__proto__` keys, or unusual field names that deviate from normal data schemas. Review CyberChef application logs (if available) for errors or anomalies during CSV parsing. Implement file-upload scanning to flag CSVs containing `__proto__` or similar prototype pollution indicators. Browser developer console logs on affected versions may show JavaScript errors or unexpected prototype chain modifications if an attack is underway. Endpoint Detection and Response (EDR) tools should monitor for abnormal JavaScript execution or DOM manipulation originating from CyberChef.

Why prioritize this

While the CVSS score is moderate (5.0), this vulnerability warrants prompt attention because it affects a security-focused tool used by analysts and developers who handle sensitive data. The combination of prototype pollution and JavaScript injection creates a direct confidentiality and integrity risk. The presence of CWE-1321 (Improper Restriction of Rendered UI Layers or Frames) and CWE-79 (Cross-Site Scripting) reinforces the severity. Organizations relying on CyberChef for cryptographic or security operations should treat this as high-priority for their specific context, even if the generic CVSS score appears moderate.

Risk score, explained

CVSS 3.1 score of 5.0 (MEDIUM) reflects a local or adjacent attack vector (AV:L), low complexity (AC:L), no privilege requirement (PR:N), but user interaction (UI:R). The scope is changed (S:C), meaning the vulnerability can affect resources beyond the security scope of the affected component. Confidentiality and integrity are partially impacted (C:L, I:L), but availability is not (A:N). The score does not account for the sensitivity of data often processed by CyberChef or the reputational risk of a security tool being compromised; organizations should apply contextual risk assessment to their own use cases.

Frequently asked questions