CVE-2019-12521Public exploitNetwork attack vector

Proxy crash via off-by-one ESI stack write

Published Apr 15, 2020 · Updated Aug 4, 2024

Denial of service in Squid 3.x through 3.5.28, 4.x through 4.10, and 5.0.1 allows remote attackers to crash the proxy via ESI responses. ESIContext::addStackElement accepts an eleventh ESIElement because its depth check permits index 10 in the ten-entry parserState.stack array, overwriting the adjacent stackdepth member. Exploitation requires ESI support and control of an upstream server or its transmission channel; successful processing terminates the proxy and interrupts service for its clients.

CVSS severity5.9
Medium
EPSS probability5.91%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in Squid 3.x through 3.5.28, 4.x through 4.10, and 5.0.1 allows remote attackers to crash the proxy via ESI responses. ESIContext::addStackElement accepts an eleventh ESIElement because its depth check permits index 10 in the ten-entry parserState.stack array, overwriting the adjacent stackdepth member. Exploitation requires ESI support and control of an upstream server or its transmission channel; successful processing terminates the proxy and interrupts service for its clients.

The record

CVE
CVE-2019-12521
Published
Apr 15, 2020
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-787, CWE-193, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 15, 2020CVE 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?

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

4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:N/A:P
5.9NIST NVDCVSS:3.1/AV:N/AC:H/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-787Out-of-bounds Write
CWE-193Off-by-one Error
T1499.004Application 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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