A significant security vulnerability has been revealed in the URL parsing function of Python, posing a serious risk where attackers could exploit it to circumvent domain and protocol filtering mechanisms that rely on blocklists. This could lead to unauthorized file readings and arbitrary command executions.
The CERT Coordination Center (CERT/CC) issued an advisory on this issue, highlighting that “urlparse has a parsing problem when the entire URL starts with blank characters.” This flaw notably impacts the parsing of both the hostname and scheme, rendering any blocklisting methods ineffective.
The vulnerability has been assigned the code CVE-2023-24329 and is rated with a CVSS score of 7.5. Security researcher Yebo Cao identified and reported this issue in August 2022, prompting a response from developers in subsequent Python releases, including versions 3.12 and several updates to earlier series.
The urllib.parse library is a popular tool for deconstructing URLs into their separate components or reconstructing them into a complete string. This flaw is primarily due to inadequate input validation, allowing adversaries to exploit URLs that begin with whitespace characters, circumventing standard filtering practices. An example of a problematic URL could be ” https://youtube[.]com”.
Cao emphasized the implications of this vulnerability, stating, “Although blocklist is considered an inferior choice, there are many scenarios where blocklist is still needed. This vulnerability would enable an attacker to bypass the protections set by developers for scheme and host, potentially aiding in server-side request forgery (SSRF) and remote code execution (RCE) attacks.”
The release of this advisory coincides with recent findings from research revealing that Python security patches often emerge through “silent” code commits that lack associated Common Vulnerabilities and Exposures (CVE) identifiers. This practice creates opportunities for malicious actors to exploit undisclosed vulnerabilities in systems that remain unpatched, underlining the urgent need for vigilance in implementing updates.
