CVE-2026-56858 in html-template
Summary
by MITRE • 08/14/2026
Previously, pathological inputs could close an unescaped '/' early, allowing for attack-controlled data to inject arbitrary content, potentially leading to XSS.
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Analysis
by VulDB Data Team • 08/14/2026
This vulnerability represents a classic input validation flaw that undermines the security of web applications through improper handling of special characters in user-supplied data. The issue occurs when the application fails to properly escape or sanitize forward slash characters during processing, creating an opportunity for attackers to manipulate input parsing logic. When pathological inputs containing unescaped slashes are processed, they can prematurely terminate string parsing operations, allowing malicious data to be injected into the application's output stream. This type of vulnerability directly enables cross-site scripting attacks by permitting attackers to inject arbitrary content that gets executed in users' browsers.
The technical implementation of this flaw typically involves applications that use string manipulation functions without proper sanitization of input data before processing. When a forward slash character is encountered in user-controlled input, the application's parsing logic incorrectly handles the character, potentially causing the system to interpret subsequent data as part of the original input stream rather than as separate entities. This parsing error creates a window where attacker-controlled content can be injected into contexts that should remain secure from user influence. The vulnerability is particularly dangerous because it leverages the fundamental way applications process and validate input data, making it difficult to detect during normal application testing.
The operational impact of this vulnerability extends beyond simple XSS exploitation to potentially enable more sophisticated attack vectors including session hijacking, credential theft, and privilege escalation within affected applications. Attackers can craft inputs that manipulate the parsing behavior to inject malicious scripts, redirect users to phishing sites, or perform actions on behalf of authenticated users. The attack surface is broad as this vulnerability can affect any application component that processes user input containing forward slash characters without proper validation. This includes web forms, API endpoints, and any interface where user data is accepted and processed, making the potential impact widespread across different application functionalities.
Security mitigations for this vulnerability should focus on implementing robust input sanitization and validation mechanisms that properly escape or encode special characters before processing user data. The most effective approach involves applying comprehensive sanitization routines that handle all potentially dangerous characters including forward slashes, backslashes, and other control characters that could interfere with parsing logic. Organizations should implement proper output encoding when rendering user-supplied content to prevent script execution in browser contexts. Additionally, regular security testing including fuzzing and input validation testing should be conducted to identify similar vulnerabilities in application code. This vulnerability aligns with CWE-79, which addresses cross-site scripting flaws, and maps to ATT&CK technique T1203 for exploiting input validation weaknesses in web applications.