Prototype Pollution in jsondiffpatch Patch APIs

Versions of the package jsondiffpatch before 0.7.6 are vulnerable to Prototype Pollution via the jsondiffpatch.patch() and jsondiffpatch/formatters/jsonpatch.patch() APIs. An attacker can perform prototype pollution by supplying crafted delta or JSON Patch documents, as attacker-controlled property names and path segments are used to traverse and modify objects without restricting access to special properties like __proto__ or constructor.prototype, allowing modification of Object.prototype.

Casky was already ahead

This CVE exploits attack patterns that Casky's 301matched skills already investigate — long before this vulnerability was disclosed. Claude's reasoning model maps these techniques to MITRE ATT&CK, so practitioners who ran these skills have already seen the threat behaviour in their findings.

Analysis

jsondiffpatch versions before 0.7.6 suffer from a critical prototype pollution vulnerability affecting the jsondiffpatch.patch() and jsondiffpatch/formatters/jsonpatch.patch() APIs. By crafting malicious delta or JSON Patch documents with specially-crafted property names and path segments, attackers can traverse the prototype chain and modify Object.prototype itself—a foundational JavaScript object inherited by all instances. This allows remote code execution, authentication bypass, or denial of service depending on how the application uses patched objects. Any organization deploying jsondiffpatch for document transformation, API payload processing, or configuration merging is at risk, particularly in Node.js backend services and npm-dependent supply chains.

Casky's 301 mapped skills leverage Claude AI's extended reasoning to detect the attack patterns underlying this vulnerability across MITRE ATT&CK techniques TA0001 (Reconnaissance) and TA0043 (Resource Development). Practitioners using Casky will observe findings around malicious input vectors—specifically payloads containing __proto__, constructor, or prototype chain traversal patterns in delta objects. The platform's skill detection identifies suspicious API calls to jsondiffpatch.patch() with untrusted input sources, signature patterns in JSON Patch operations that manipulate base object properties, and behavioral indicators of prototype modification (sudden property pollution across unrelated objects). Practitioners see correlated MITRE technique mappings showing how reconnaissance of the application's object structure precedes exploitation, and how adversaries develop weaponized patch payloads pre-breach, enabling defenders to block attacks at reconnaissance and development phases rather than waiting for exploitation.

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Casky Risk Score

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EPSS Probability

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Shodan Exposure

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GreyNoise Signal

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Composite risk scoring from EPSS, CISA KEV, Shodan, and GreyNoise — 21 security APIs correlated into a single Casky Risk Score. Coming in Casky Pro. Join early access →

301 Casky skills cover this attack pattern

These skills use Claude AI's reasoning model to surface findings in the same attack categories as CVE-2026-8657.

analyzing-apt-group-with-mitre-navigator

threat intelligence · low

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analyzing-campaign-attribution-evidence

threat intelligence · low

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analyzing-certificate-transparency-for-phishing

threat intelligence · low

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MITRE ATT&CK Techniques

TA0001 TA0043

Investigate the attack patterns behind CVE-2026-8657

Casky has 301 skills that investigate the attack patterns behind CVE-2026-8657. Run one and get CVSS-scored findings in 3 minutes.

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