CVE-2019-9948 in Python
Summary
by MITRE
urllib in Python 2.x through 2.7.16 supports the local_file: scheme, which makes it easier for remote attackers to bypass protection mechanisms that blacklist file: URIs, as demonstrated by triggering a urllib.urlopen('local_file:///etc/passwd') call.
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Analysis
by VulDB Data Team • 08/04/2023
The vulnerability identified as CVE-2019-9948 represents a significant security flaw in Python's urllib library that affects versions 2.x through 2.7.16. This issue stems from the library's support for the local_file: URI scheme, which creates an unexpected bypass mechanism for file access restrictions that security systems rely upon. The flaw enables remote attackers to circumvent protection mechanisms that are specifically designed to block file: URIs, thereby undermining the intended security controls in place to prevent unauthorized file system access.
The technical implementation of this vulnerability exploits the fact that urllib accepts multiple URI schemes including local_file:, which allows direct file system access through HTTP-like URI syntax. When an attacker constructs a malicious URI such as local_file:///etc/passwd and passes it to urllib.urlopen(), the library processes this request as if it were a legitimate file access operation. This behavior occurs because the local_file: scheme is not properly filtered out by existing security mechanisms that only target the standard file: URI scheme, creating a gap in the protection layer. The vulnerability is particularly concerning because it allows attackers to bypass security controls that would normally prevent file system access through network-based protocols, effectively providing a backdoor mechanism for reading local files.
The operational impact of CVE-2019-9948 extends beyond simple file access, as it can enable more sophisticated attacks including information disclosure, privilege escalation, and potential system compromise. Attackers can leverage this vulnerability to access sensitive system files, configuration data, and potentially sensitive information stored in standard system locations. The vulnerability is particularly dangerous in web applications that process user-supplied URLs or in systems that use urllib for handling external resource requests, as it allows attackers to bypass security controls that are intended to protect against file system access. This vulnerability can be exploited in various contexts including web applications, automated security tools, and systems that rely on urllib for handling external resource requests, making it a critical concern for organizations using affected Python versions.
Organizations should immediately upgrade to Python 2.7.17 or later versions where this vulnerability has been addressed through proper URI scheme filtering and validation. The mitigation strategy involves implementing comprehensive URI validation that explicitly blocks not only the file: scheme but also the local_file: scheme and other potentially dangerous URI schemes. Security controls should be enhanced to include proper input sanitization and validation of all URI schemes before processing, with specific attention to ensuring that local_file: and similar schemes are rejected or properly handled. Additionally, organizations should conduct thorough security reviews of all applications that utilize urllib to identify potential attack vectors and implement proper access controls to prevent unauthorized file system access. This vulnerability aligns with CWE-22, which describes improper limitation of a pathname to a restricted directory, and represents a specific instance of path traversal vulnerabilities that can be exploited through URI manipulation. The attack pattern follows the MITRE ATT&CK framework's technique for privilege escalation through exploitation of application vulnerabilities, making it a critical target for defensive measures.