CVE-2021-21240: 
Python vulnerability analysis and mitigation

CVE-2021-21240 affects httplib2, a comprehensive HTTP client library for Python, in versions before 0.19.0. The vulnerability was discovered and disclosed on February 8, 2021. The issue affects the library's handling of WWW-Authenticate headers, specifically when parsing authentication information (NVD, Ubuntu).

The vulnerability stems from a regular expression parsing issue in the WWW-Authenticate header handling. When a malicious server responds with a long series of '\xa0' characters in the 'www-authenticate' header, it triggers a CPU-intensive parsing operation due to cubic complexity of whitespace matching in the WWW_AUTH_RELAXED (default) regexp. The vulnerability has been assigned a CVSS v3.1 base score of 7.5 (High), with the vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H (NVD, GitHub Advisory).

The vulnerability can lead to a Denial of Service condition through CPU resource exhaustion when parsing the malicious header. When exploited, it causes the httplib2 client to enter a CPU-intensive parsing operation, effectively creating a denial of service condition for the application using the library (GitHub Advisory).

The vulnerability can be exploited by a malicious server that responds with specifically crafted WWW-Authenticate headers containing long series of '\xa0' characters. The attack requires no special privileges or user interaction, making it relatively straightforward to exploit (GitHub Advisory).

The vulnerability was fixed in httplib2 version 0.19.0 with a new implementation of auth headers parsing using the pyparsing library. For users unable to upgrade immediately, a workaround is available by enabling strict parsing mode with the setting 'httplib2.USE_WWW_AUTH_STRICT_PARSING = True' (GitHub PR, GitHub Advisory).