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

Exp

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.

Published: 2021-02-08 Last update: 2024-11-21 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-21240 is rated High Exploit Risk (74.4/100): CVSS High severity, with medium exploitation likelihood (EPSS 1.98%). 1 public exploit reference(s) are indexed (Exploit-DB). Public exploits are available—assess exposure, apply mitigations, and prioritize patching.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Public exploit references (Exploit-DB) for CVE-2021-21240

EDB-ID Source Kind Published Link
— nvd_ref exploit_tag Exploit-DB ↗

Exploit prediction scoring system (EPSS) score for CVE-2021-21240

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-05-30 1.92% 1.98% +0.06%
2 2026-03-04 0.65% 1.92% +1.27%
3 2026-03-01 — 0.65% —

Full EPSS history (43 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-21240

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]
5.0 2.0 MEDIUM
AV:N/AC:L/Au:N/C:N/I:N/A:P Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:L)
Exploitation conditions are straightforward and predictable.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:N)
No confidentiality impact.
Integrity impact (I:N)
No integrity impact.
Availability impact (A:P)
Partial availability impact.
 10.0 2.9 [email protected]

Weakness enumeration for CVE-2021-21240

GitHub Security Advisory for CVE-2021-21240

GHSA-93xj-8mrv-444m · Severity: high · Ecosystem: pip — Regular Expression Denial of Service (REDoS) in httplib2

OS Trackers for CVE-2021-21240

vendor priority summary link
alpine high CVE-2021-21240: 1 source package rows (py3-httplib2); 8 state rows across 8 repos (3.11-main, 3.17-community, 3.18-community, 3.19-community, 3.20-community, 3.21-community, 3.22-community, edge-community); fixed 7, open 1. https://security.alpinelinux.org/vuln/CVE-2021-21240
debian not yet assigned CVE-2021-21240 not yet assigned priority: Debian including 1 source packages (python-httplib2), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 4, open 1. https://security-tracker.debian.org/tracker/CVE-2021-21240
redhat low — https://access.redhat.com/security/cve/CVE-2021-21240
suse medium CVE-2021-21240 severity moderate: SUSE including 255 source package names (0.1.75:python3-httplib2-0.19.0-3.3.1, 1.1.1.0.1.5.424:python3-httplib2-0.19.0-3.3.1, …), 292 product×package rows across 55 product lines (Container caasp/v4/k8s-sidecar, Container ses/6/cephcsi/cephcsi, … (55 product lines)): Known Affected 231, Fixed 61. https://www.suse.com/security/cve/CVE-2021-21240/
ubuntu low CVE-2021-21240 low priority: Ubuntu including 1 source packages (python-httplib2), 16 status rows across 16 suites (bionic, focal, groovy, hirsute, impish, jammy, kinetic, lunar, mantic, noble, oracular, plucky, questing, trusty, upstream, xenial): not-affected 6, needed 5, ignored 4, released 1. https://ubuntu.com/security/CVE-2021-21240

Affected software / configurations for CVE-2021-21240

Vendor Product Version Raw CPE
httplib2_project httplib2 < 0.19.0 cpe:2.3:a:httplib2_project:httplib2:*:*:*:*:*:python:*:*

References for CVE-2021-21240

cvelogic Threat Intelligence