What’s Vulnerable › Vulnerabilities

CVE-2021-21240: Regular Expression Denial of Service in httplib2

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 3.6% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2021-02-08 · updated 2026-06-17
• Weakness: CWE-400

Description

httplib2 is a comprehensive HTTP client library for Python. In httplib2 before version 0.19.0, a malicious server which responds with long series of "\xa0" characters in the "www-authenticate" header may cause Denial of Service (CPU burn while parsing header) of the httplib2 client accessing said server. This is fixed in version 0.19.0 which contains a new implementation of auth headers parsing using the pyparsing library.

Affected products

• httplib2: before 0.19.0 — fixed in 0.19.0

How to fix

Upgrade to a fixed version: httplib2 0.19.0. Check the vendor advisory for mitigations.

References

• https://github.com/httplib2/httplib2/commit/bd9ee252c8f099608019709e22c0d705e98d26bc
• https://github.com/httplib2/httplib2/pull/182
• https://github.com/httplib2/httplib2/security/advisories/GHSA-93xj-8mrv-444m
• https://pypi.org/project/httplib2
• https://github.com/httplib2/httplib2/commit/bd9ee252c8f099608019709e22c0d705e98d26bc
• https://github.com/httplib2/httplib2/pull/182
• https://github.com/httplib2/httplib2/security/advisories/GHSA-93xj-8mrv-444m
• https://pypi.org/project/httplib2

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