CVE-2026-109990 in Flying Scripts Delay JavaScript to Improve Site Speed & Performance Plugin
Summary
by MITRE • 10/16/2026
The Flying Scripts: Delay JavaScript to Improve Site Speed & Performance plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'comment' parameter in all versions up to, and including, 1.2.4 due to insufficient input sanitization and output escaping. This makes it possible for unauthenticated attackers to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page. The exploit bypasses WordPress core's save-time kses sanitization entirely because the payload uses only kses-allowed elements — an <abbr title> attribute and <code> text content — and the XSS is introduced post-save when the plugin's output-buffer rewriter substitutes a double-quote-bearing <code> body into the <abbr title> attribute via predictable, unsalted ___noise___ placeholder keys.
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Analysis
by VulDB Data Team • 10/16/2026
The Flying Scripts: Delay JavaScript to Improve Site Speed & Performance plugin for WordPress contains a critical stored cross-site scripting vulnerability affecting all versions up to and including 1.2.4. This flaw stems from insufficient input sanitization and output escaping mechanisms within the application logic, specifically targeting the comment parameter used during user interactions with the site. The severity of this issue is amplified by its ability to bypass WordPress core's standard save-time kses sanitization filters entirely. Attackers can exploit this weakness without requiring any form of authentication, allowing them to inject arbitrary web scripts into pages that are subsequently served to other users. When a victim accesses an infected page, the injected script executes in their browser context, potentially leading to session hijacking, defacement, or further malware distribution.
The technical mechanism behind this vulnerability is particularly sophisticated and demonstrates how attackers can circumvent standard security controls by adhering strictly to allowed syntax while manipulating output rendering processes. The payload utilizes only elements that are permitted by the kses filter, specifically an abbr tag with a title attribute and code text content. This compliance allows the malicious data to pass through initial validation checks undetected. However, the vulnerability materializes during the post-save phase when the plugin's output-buffer rewriter processes the stored comment. The rewriter substitutes a double-quote-bearing code body into the abbr title attribute using predictable, unsalted noise placeholder keys. Because these placeholders are not randomized or salted, an attacker can predict how their input will be transformed and injected into the HTML structure, effectively breaking out of the intended context and injecting executable JavaScript via the malformed attributes.
From a classification perspective, this vulnerability aligns with CWE-79 Improper Neutralization of Input During Web Page Generation commonly known as Cross-site Scripting. The specific nature of the attack involves stored persistence where malicious scripts are saved on the target server rather than being reflected in an immediate request. This behavior is further categorized under ATT&CK technique T1059 Command and Scripting Interpreter, specifically through browser-based scripting languages such as JavaScript. The ability to execute arbitrary code within a user's session poses significant risks including unauthorized access to sensitive information, manipulation of page content, and potential lateral movement if the compromised environment allows for broader network interaction.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. Administrators should immediately update the Flying Scripts plugin to version 1.2.5 or later where these issues have been resolved by implementing proper output escaping and ensuring that dynamic substitutions do not introduce unescaped quotes into HTML attributes. In addition to updating, it is advisable to implement a Web Application Firewall configured with rulesets capable of detecting anomalous patterns in comment submissions, particularly those involving unusual combinations of allowed tags like abbr and code used in non-standard ways. Furthermore developers should enforce strict output encoding for all dynamic content inserted into HTML attributes regardless of whether the input passed initial sanitization checks. Adopting Content Security Policy headers can also help mitigate the impact by restricting script execution sources and reducing the effectiveness of any successfully injected payloads.