How severe is CVE-2021-21240?

CVE-2021-21240 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.

Is CVE-2021-21240 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 4% (89th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-21240?

CVE-2021-21240 affects Httplib2 Project Httplib2. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-21240?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

When was CVE-2021-21240 published?

CVE-2021-21240 was published on 2021-02-08 and last updated on 2026-06-17.

CVE-2021-21240

CVE-2021-21240 is a high-severity vulnerability in Httplib2 Project Httplib2 with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-400.

Key facts

Severity: High (CVSS 3.x base score 7.5)
CVSS v2: 5.0
EPSS exploit prediction: 4% (89th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-400
Affected product: Httplib2 Project Httplib2
Published: 2021-02-08
Last modified: 2026-06-17

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.

Frequently asked questions

What is CVE-2021-21240?: 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.
How severe is CVE-2021-21240?: CVE-2021-21240 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2021-21240 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 4% (89th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-21240?: CVE-2021-21240 affects Httplib2 Project Httplib2. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-21240?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2021-21240 published?: CVE-2021-21240 was published on 2021-02-08 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:httplib2_project:httplib2:*:*:*:*:*:python:*:*

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