CVE-2021-23336Public exploitNetwork attack vector

Cache poisoning via semicolon parameter cloaking

Published Feb 15, 2021 · Updated Dec 17, 2025

Web cache poisoning in affected CPython releases allows remote attackers to alter cached responses through semicolon-delimited queries. urllib.parse.parse_qs and parse_qsl treat both semicolons and ampersands as query separators, while common proxies key only ampersand-delimited parameters. Affected releases precede 3.6.13, 3.7.10, 3.8.8, and 3.9.2; exploitation requires a consuming application behind a cache that omits the cloaked parameter from its key.

CVSS severity5.9
Medium
EPSS probability35.96%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Web cache poisoning in affected CPython releases allows remote attackers to alter cached responses through semicolon-delimited queries. urllib.parse.parse_qs and parse_qsl treat both semicolons and ampersands as query separators, while common proxies key only ampersand-delimited parameters. Affected releases precede 3.6.13, 3.7.10, 3.8.8, and 3.9.2; exploitation requires a consuming application behind a cache that omits the cloaked parameter from its key.

The record

CVE
CVE-2021-23336
Published
Feb 15, 2021
Updated
Dec 17, 2025
Vendor
Python Software Foundation
Product
CPython
Classifications
CWE-444, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 15, 2021CVE publishedPublication date reported by the CVE source.
  2. Dec 17, 2025Record 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 <unspecified
  2. Affected versionversion=3.7.0 <unspecified
  3. Affected versionversion=3.8.0 <unspecified
  4. Affected versionversion=3.9.0 <unspecified
  5. Affected versionversion=unspecified <3.6.13
  6. Affected versionversion=unspecified <3.7.10
  7. Affected versionversion=unspecified <3.8.8
  8. Affected versionversion=unspecified <3.9.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Python Software Foundation · CPythonversion=0 <unspecified; version=3.7.0 <unspecified; version=3.8.0 <unspecified; version=3.9.0 <unspecified; version=unspecified <3.6.13; version=unspecified <3.7.10; version=unspecified <3.8.8; version=unspecified <3.9.2

Published CVSS scores

5.9NVDCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:L/A:H
5.9SnykCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:L/A:H
4.0NVDAV:N/AC:H/Au:N/C:N/I:P/A:P

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.

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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-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
T1190Exploit Public-Facing Application

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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