Summary
CKEditor 5 can process crafted style attribute values through a vulnerable prototype-pollution path in its es-toolkit dependency. When malicious editor content is processed in an affected browser context, an attacker may achieve unauthorized JavaScript execution.
CKEditor 5 can process crafted style attribute values through a vulnerable prototype-pollution path in its es-toolkit dependency. When malicious editor content is processed in an affected browser context, an attacker may achieve unauthorized JavaScript execution.
Affected context
The issue occurs in CKEditor 5 deployments using affected releases where incoming editor content can reach style attribute processing. Exploitation depends on crafted content being accepted and processed in a browser context where the resulting JavaScript execution can affect the editor application or its users. The vulnerable dependency path is present in releases before the relevant fixes in the 47.x and 48.x release lines.
The issue occurs in CKEditor 5 deployments using affected releases where incoming editor content can reach style attribute processing. Exploitation depends on crafted content being accepted and processed in a browser context where the resulting JavaScript execution can affect the editor application or its users. The vulnerable dependency path is present in releases before the relevant fixes in the 47.x and 48.x release lines.
Exploit preconditions
- Attack surface: CKEditor 5 content processing in a web browser - Required access: Ability to submit, inject, or cause processing of crafted
editor content
- Trigger point: Handling of incoming style attribute values - Why exploitation is possible: A prototype-pollution behavior in the
underlying toolkit can influence processing of attacker-controlled data and
lead to script execution
- Attack surface: CKEditor 5 content processing in a web browser - Required access: Ability to submit, inject, or cause processing of crafted
editor content
- Trigger point: Handling of incoming style attribute values - Why exploitation is possible: A prototype-pollution behavior in the
underlying toolkit can influence processing of attacker-controlled data and
lead to script execution
Why this happens
The underlying bug combines unsafe handling of attacker-influenced style attribute values with prototype-pollution behavior in a dependency used by the editor engine. Prototype pollution can modify object properties or inherited behavior in ways that later security-sensitive code does not expect. In this case, the resulting behavior can enable client-side script execution, mapping to CWE-79, Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting').
The underlying bug combines unsafe handling of attacker-influenced style attribute values with prototype-pollution behavior in a dependency used by the editor engine. Prototype pollution can modify object properties or inherited behavior in ways that later security-sensitive code does not expect. In this case, the resulting behavior can enable client-side script execution, mapping to CWE-79, Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting').
Potential impact
- An attacker may execute JavaScript in a victim's browser context
- Editor users' session information or accessible application data could be exposed
- Application actions could be performed with the victim's permissions
- Stored or reflected malicious content could affect other editor users
- The issue could contribute to account compromise or content integrity loss
- An attacker may execute JavaScript in a victim's browser context
- Editor users' session information or accessible application data could be exposed
- Application actions could be performed with the victim's permissions
- Stored or reflected malicious content could affect other editor users
- The issue could contribute to account compromise or content integrity loss
General mitigation guidance
Mitigation generally focuses on removing unsafe prototype-pollution paths and reducing trust in rich-text input. Use maintained dependencies with secure object-handling behavior, constrain style attributes through an allowlist, and sanitize content before rendering or persistence. Prefer safe-by-default editor configuration, avoid merging untrusted objects into privileged application state, and apply output encoding appropriate to the rendering context. Run the editor with least privilege and isolate sensitive operations from client-controlled content.
Mitigation generally focuses on removing unsafe prototype-pollution paths and reducing trust in rich-text input. Use maintained dependencies with secure object-handling behavior, constrain style attributes through an allowlist, and sanitize content before rendering or persistence. Prefer safe-by-default editor configuration, avoid merging untrusted objects into privileged application state, and apply output encoding appropriate to the rendering context. Run the editor with least privilege and isolate sensitive operations from client-controlled content.
How teams detect issues like this
Detection can include static analysis for unsafe object merging, dynamic property assignment, and dependency versions known to expose prototype-pollution behavior. Code review should examine style attribute handling, object construction, sanitizer configuration, and any path from submitted content to DOM updates. Content-security policies, dependency allowlists, and automated security review tools can provide additional policy enforcement and monitoring. Browser telemetry may also help identify unexpected script execution originating from editor content.
Detection can include static analysis for unsafe object merging, dynamic property assignment, and dependency versions known to expose prototype-pollution behavior. Code review should examine style attribute handling, object construction, sanitizer configuration, and any path from submitted content to DOM updates. Content-security policies, dependency allowlists, and automated security review tools can provide additional policy enforcement and monitoring. Browser telemetry may also help identify unexpected script execution originating from editor content.
