CVE-2021-32838Public exploitNetwork attack vector

API worker hang via email regex backtracking

Published Sep 20, 2021 · Updated Aug 3, 2024

Regular expression denial of service in Flask-RESTX before 0.5.1 allows remote attackers to hang API workers via crafted email input. The email_regex pattern in flask_restx/inputs.py lets its server character class consume dots also matched by a repeated dot-separated group, causing catastrophic backtracking on a nonmatching suffix. Reachability requires a consuming Flask application to apply email_regex to attacker-controlled text; CPU exhaustion can interrupt its worker service.

CVSS severity8.7
High
EPSS probability1.86%
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 Flask-RESTX before 0.5.1 allows remote attackers to hang API workers via crafted email input. The email_regex pattern in flask_restx/inputs.py lets its server character class consume dots also matched by a repeated dot-separated group, causing catastrophic backtracking on a nonmatching suffix. Reachability requires a consuming Flask application to apply email_regex to attacker-controlled text; CPU exhaustion can interrupt its worker service.

The record

CVE
CVE-2021-32838
Published
Sep 20, 2021
Updated
Aug 3, 2024
Vendor
python-restx
Product
Flask-RESTX
Classifications
CWE-1333, CWE-400, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 20, 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=< 0.5.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

python-restx · Flask-RESTXversion=< 0.5.1

Published CVSS scores

8.7GitHubCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

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-1333Inefficient Regular Expression Complexity
CWE-400Uncontrolled Resource Consumption
T1499.004Endpoint Denial of Service: Application or System Exploitation

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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