CVE-2008-2545 in Skype
Summary
by MITRE
Skype 3.6.0.248, and other versions before 3.8.0.139, uses a case-sensitive comparison when checking for dangerous extensions, which allows user-assisted remote attackers to bypass warning dialogs and possibly execute arbitrary code via a file: URI with a dangerous extension that uses a different case.
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Analysis
by VulDB Data Team • 08/11/2019
The vulnerability identified as CVE-2008-2545 represents a critical security flaw in Skype versions prior to 3.8.0.139, specifically affecting version 3.6.0.248 and earlier. This issue stems from an insufficient input validation mechanism that fails to properly handle case-insensitive file extension comparisons. The flaw exists within the application's URI handling system where Skype performs checks to determine whether file extensions pose potential security risks. When processing file: URIs, the software employs a case-sensitive comparison method that can be easily circumvented by attackers who manipulate the case of file extensions to evade detection mechanisms.
The technical implementation of this vulnerability resides in the application's security validation logic where dangerous file extensions such as .exe, .bat, .scr, or .msi are checked against a predefined list of malicious extensions. The case-sensitive nature of this comparison allows attackers to bypass the security warnings by simply altering the case of the extension characters. For instance, while a file named "malicious.EXE" would trigger a warning dialog, the same file named "malicious.exe" might not be properly detected due to the flawed comparison algorithm. This weakness directly maps to CWE-20, which addresses improper input validation, and specifically relates to CWE-706, which deals with use of unsafe functions or libraries.
The operational impact of this vulnerability extends beyond simple bypass of warning dialogs, as it potentially enables arbitrary code execution through social engineering attacks. An attacker could craft malicious file URIs with carefully manipulated extensions that appear benign to the user interface while maintaining their malicious functionality. The user-assisted nature of this attack means that a victim must actively interact with the malicious URI, typically through clicking on a link in an email, instant message, or web page. However, the low barrier to exploitation makes this particularly dangerous in phishing campaigns and targeted attacks where the attacker can leverage the victim's trust in the Skype application. The vulnerability also aligns with ATT&CK technique T1193, which covers Spearphishing Attachments, and T1059, which addresses Command and Scripting Interpreter.
The security implications of this flaw demonstrate a fundamental weakness in the application's defense-in-depth strategy, where a single validation error can compromise the entire security posture. The vulnerability essentially creates a false sense of security for users who might trust Skype's warning dialogs, leading them to believe that potentially dangerous files are being blocked when they are actually being allowed to execute. This flaw particularly affects enterprise environments where Skype is commonly used for business communications, as it could serve as a vector for malware delivery and privilege escalation attacks. Organizations using older versions of Skype should immediately implement mitigation strategies including software updates, network-based filtering, and user education to prevent exploitation of this vulnerability. The issue highlights the importance of proper input validation and case-insensitive comparison operations in security-critical applications, particularly those handling external data inputs from untrusted sources.