CVE-2021-28662Network attack vector

Proxy crash via Vary header lookup

Published May 27, 2021 · Updated Aug 3, 2024

Input validation failure in Squid 4.0.1 through 4.14 and 5.0.1 through 5.0.5 allows remote attackers to crash the proxy via an HTTP response. HeaderLookupTable_t::lookup in src/http/RegisteredHeaders.cc accepts the Vary pseudo-header value Other as a lookup ID instead of restricting lookups to BadHdr and registered IDs, triggering an assertion failure. Exploitation requires a Squid client to request content from an attacker-controlled or benign origin that returns the triggering header, and the crash interrupts proxy service for all clients.

CVSS severity6.5
Medium
EPSS probability71.76%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Input validation failure in Squid 4.0.1 through 4.14 and 5.0.1 through 5.0.5 allows remote attackers to crash the proxy via an HTTP response. HeaderLookupTable_t::lookup in src/http/RegisteredHeaders.cc accepts the Vary pseudo-header value Other as a lookup ID instead of restricting lookups to BadHdr and registered IDs, triggering an assertion failure. Exploitation requires a Squid client to request content from an attacker-controlled or benign origin that returns the triggering header, and the crash interrupts proxy service for all clients.

The record

CVE
CVE-2021-28662
Published
May 27, 2021
Updated
Aug 3, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 27, 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?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

6.5GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:R/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 exploitUnconfirmed

Weakness, pattern, technique

T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

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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