CVE-2021-21240Public exploitNetwork attack vector

Client hang via authentication-header regex backtracking

Published Feb 8, 2021 · Updated Aug 3, 2024

Regular expression denial of service in httplib2 before 0.19.0 allows remote attackers to hang clients via crafted WWW-Authenticate headers. The relaxed parser's WWW_AUTH_RELAXED expression accepts nonbreaking spaces in three overlapping repetitions, causing cubic backtracking. The client must request a resource from an attacker-controlled server, and the resulting CPU burn stalls the calling process without affecting data.

CVSS severity7.5
High
EPSS probability3.63%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Regular expression denial of service in httplib2 before 0.19.0 allows remote attackers to hang clients via crafted WWW-Authenticate headers. The relaxed parser's WWW_AUTH_RELAXED expression accepts nonbreaking spaces in three overlapping repetitions, causing cubic backtracking. The client must request a resource from an attacker-controlled server, and the resulting CPU burn stalls the calling process without affecting data.

The record

CVE
CVE-2021-21240
Published
Feb 8, 2021
Updated
Aug 3, 2024
Vendor
httplib2
Product
httplib2
Classifications
CWE-400, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 8, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 0.19.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

httplib2 · httplib2version=< 0.19.0

Published CVSS scores

5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:N/A:P
7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
T1499Endpoint Denial of Service

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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