CVE-2020-13893 in EasyPay
Summary
by MITRE • 10/19/2020
Multiple stored cross-site scripting (XSS) vulnerabilities in Sage EasyPay 10.7.5.10 allow authenticated attackers to inject arbitrary web script or HTML via multiple parameters through Unicode Transformations (Best-fit Mapping), as demonstrated by the full-width variants of the less-than sign (%EF%BC%9C) and greater-than sign (%EF%BC%9E).
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Analysis
by VulDB Data Team • 11/21/2020
The vulnerability CVE-2020-13893 represents a critical stored cross-site scripting flaw affecting Sage EasyPay version 10.7.5.10, demonstrating the persistent challenges organizations face when implementing proper input validation and output encoding mechanisms. This vulnerability specifically targets the application's handling of Unicode character transformations, particularly utilizing Best-fit Mapping techniques that allow attackers to bypass conventional security filters through the manipulation of full-width Unicode variants. The flaw enables authenticated attackers to inject malicious scripts into the application's database, which then get executed whenever legitimate users access the affected pages, creating a persistent threat vector that can compromise user sessions and data integrity.
The technical exploitation of this vulnerability relies on the application's insufficient sanitization of user input containing Unicode characters, specifically leveraging the full-width representations of HTML angle brackets. Attackers can utilize the encoded sequences %EF%BC%9C and %EF%BC%9E which represent the full-width versions of less-than and greater-than signs respectively. These Unicode transformations exploit the application's failure to properly normalize and validate input data, allowing malicious payloads to be stored in the database and subsequently executed in the context of other users' browsers. The vulnerability manifests through multiple parameters within the application's interface, indicating a systemic weakness in the input validation framework rather than isolated code flaws.
The operational impact of this vulnerability extends beyond simple script execution, as it provides attackers with the capability to perform session hijacking, steal sensitive user credentials, and potentially escalate privileges within the application environment. The stored nature of the vulnerability means that once successfully exploited, the malicious scripts persist indefinitely until manually removed from the database, creating a long-term threat that can affect multiple users over extended periods. This type of vulnerability directly violates security principles outlined in CWE-79, which addresses cross-site scripting flaws, and represents a significant deviation from the expected behavior of secure web applications as defined by industry standards.
Mitigation strategies should focus on implementing comprehensive input validation and output encoding mechanisms that properly handle Unicode normalization, particularly addressing Best-fit Mapping transformations that can bypass traditional security measures. Organizations should deploy proper character encoding standards and ensure that all user input is normalized and validated against a strict whitelist of acceptable characters. The implementation of Content Security Policy headers and proper HTML encoding of output data can provide additional defense layers. Furthermore, regular security testing including Unicode-based vulnerability assessments should be integrated into the application lifecycle to identify similar transformation-based bypasses that may exist in other components. This vulnerability demonstrates the critical importance of considering internationalization and localization aspects during security design phases, as outlined in ATT&CK technique T1068 which addresses application security weaknesses.