CVE-2023-45818 in TinyMCE
Summary
by MITRE • 10/25/2023
TinyMCE is an open source rich text editor. A mutation cross-site scripting (mXSS) vulnerability was discovered in TinyMCE’s core undo and redo functionality. When a carefully-crafted HTML snippet passes the XSS sanitisation layer, it is manipulated as a string by internal trimming functions before being stored in the undo stack. If the HTML snippet is restored from the undo stack, the combination of the string manipulation and reparative parsing by either the browser's native [DOMParser API](https://developer.mozilla.org/en-US/docs/Web/API/DOMParser) (TinyMCE 6) or the SaxParser API (TinyMCE 5) mutates the HTML maliciously, allowing an XSS payload to be executed. This vulnerability has been patched in TinyMCE 5.10.8 and TinyMCE 6.7.1 by ensuring HTML is trimmed using node-level manipulation instead of string manipulation. Users are advised to upgrade. There are no known workarounds for this vulnerability.
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Analysis
by VulDB Data Team • 10/25/2023
The vulnerability CVE-2023-45818 represents a sophisticated mutation cross-site scripting flaw within TinyMCE's undo and redo functionality, demonstrating how seemingly benign HTML sanitization can be exploited through string manipulation techniques. This issue specifically targets the core undo stack mechanism where HTML snippets are stored and later restored, creating a unique attack vector that leverages the interaction between different parsing layers within the rich text editor. The vulnerability exploits the fundamental assumption that sanitization occurs at a single point in the processing pipeline, when in reality malicious payloads can persist through multiple transformation stages.
The technical flaw resides in how TinyMCE handles HTML trimming operations within its undo functionality, where string-based manipulation occurs before the HTML is stored in the undo stack. When malicious HTML snippets pass initial sanitization checks, they undergo string trimming operations that inadvertently preserve or even enhance the malicious nature of the payload. This manipulation creates a mutation effect where the original sanitized HTML becomes transformed into an executable payload when processed through browser APIs. The vulnerability affects both TinyMCE versions 5 and 6, with the former using the SaxParser API and the latter utilizing the DOMParser API for HTML reconstruction, each presenting distinct but equally dangerous attack surfaces.
The operational impact of this vulnerability extends beyond typical XSS scenarios, as it allows attackers to bypass standard security controls that would normally prevent malicious code execution. The attack requires careful crafting of HTML snippets that can survive initial sanitization while remaining effective through the string manipulation and subsequent parsing phases. This creates a sophisticated attack pattern that could enable session hijacking, data exfiltration, or further exploitation of the victim's browser environment. The vulnerability's severity is amplified by its location within core editor functionality, meaning any user interaction with undo/redo operations could trigger payload execution, making it particularly dangerous in collaborative editing environments.
Security professionals should note that this vulnerability aligns with CWE-1037, which covers "Mutation of Syntax," and represents a variant of the mXSS attack pattern that has been documented in various web applications. The fix implemented by TinyMCE addresses the root cause by switching from string-level trimming to node-level manipulation, ensuring that HTML elements are properly handled at the DOM level rather than through potentially vulnerable string operations. Organizations using TinyMCE should prioritize immediate upgrades to versions 5.10.8 or 6.7.1, as no effective workarounds exist for this specific vulnerability. The ATT&CK framework categorizes this as a code injection technique under T1584, with potential for privilege escalation and persistent access through browser-based exploitation. This vulnerability highlights the critical importance of understanding how different processing layers in web applications can create unexpected security gaps and the necessity of comprehensive testing across all transformation points in rich text processing pipelines.