CVE-2021-41267Network attack vector

Web cache poisoning via unfiltered forwarded prefix

Published Nov 24, 2021 · Updated Aug 4, 2024

Web cache poisoning in Symfony HttpKernel 5.2.0 through 5.3.11 allows remote attackers to alter cached responses via a crafted header. SubRequestHandler omits X-Forwarded-Prefix from its trusted-header filter, leaving an untrusted prefix available to subrequests. A proxy or load balancer, a shared cache, and a reachable subrequest path are required; poisoned responses can alter content delivered to later users.

CVSS severity6.5
Medium
EPSS probability1.24%
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 Symfony HttpKernel 5.2.0 through 5.3.11 allows remote attackers to alter cached responses via a crafted header. SubRequestHandler omits X-Forwarded-Prefix from its trusted-header filter, leaving an untrusted prefix available to subrequests. A proxy or load balancer, a shared cache, and a reachable subrequest path are required; poisoned responses can alter content delivered to later users.

The record

CVE
CVE-2021-41267
Published
Nov 24, 2021
Updated
Aug 4, 2024
Vendor
Symfony
Product
Symfony
Classifications
CWE-444, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Nov 24, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 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=>= 5.2.0, < 5.3.12

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Symfony · Symfonyversion=>= 5.2.0, < 5.3.12

Published CVSS scores

6.5GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A: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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

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

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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