CVE-2019-12520: An issue was discovered in Squid through 4.7 and 5. When receiving a request, Squid checks its cache to see if it can serve up a response. It does thi...

Description

An issue was discovered in Squid through 4.7 and 5. When receiving a request, Squid checks its cache to see if it can serve up a response. It does this by making a MD5 hash of the absolute URL of the request. If found, it servers the request. The absolute URL can include the decoded UserInfo (username and password) for certain protocols. This decoded info is prepended to the domain. This allows an attacker to provide a username that has special characters to delimit the domain, and treat the rest of the URL as a path or query string. An attacker could first make a request to their domain using an encoded username, then when a request for the target domain comes in that decodes to the exact URL, it will serve the attacker's HTML instead of the real HTML. On Squid servers that also act as reverse proxies, this allows an attacker to gain access to features that only reverse proxies can use, such as ESI.

Metrics

CVSS 3.1

7.5/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

EPSS Probability

3.94%

89.1th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-20

Affected Software

Vendor	Product	Versions
Squid-Cache	Squid	<= 4.7
Canonical	Ubuntu Linux	16.04
Canonical	Ubuntu Linux	18.04
Debian	Debian Linux	9.0
Debian	Debian Linux	10.0

References

http://www.squid-cache.org/Versions/v4/Release Notes, Vendor Advisory
http://www.squid-cache.org/Versions/v4/changesets/Release Notes, Vendor Advisory
https://github.com/squid-cache/squid/commits/v4Patch, Third Party Advisory
https://gitlab.com/jeriko.one/security/-/blob/master/squid/CVEs/CVE-2019-12520.txtThird Party Advisory
https://lists.debian.org/debian-lts-announce/2020/07/msg00009.htmlMailing List, Third Party Advisory
https://security.netapp.com/advisory/ntap-20210205-0006/Third Party Advisory
https://usn.ubuntu.com/4446-1/Third Party Advisory
https://www.debian.org/security/2020/dsa-4682Third Party Advisory
http://www.squid-cache.org/Versions/v4/Release Notes, Vendor Advisory
http://www.squid-cache.org/Versions/v4/changesets/Release Notes, Vendor Advisory
https://github.com/squid-cache/squid/commits/v4Patch, Third Party Advisory
https://gitlab.com/jeriko.one/security/-/blob/master/squid/CVEs/CVE-2019-12520.txtThird Party Advisory
https://lists.debian.org/debian-lts-announce/2020/07/msg00009.htmlMailing List, Third Party Advisory
https://security.netapp.com/advisory/ntap-20210205-0006/Third Party Advisory
https://usn.ubuntu.com/4446-1/Third Party Advisory
https://www.debian.org/security/2020/dsa-4682Third Party Advisory

Timeline

Published: Apr 15, 2020
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2019-12520?

An issue was discovered in Squid through 4.7 and 5. When receiving a request, Squid checks its cache to see if it can serve up a response. It does this by making a MD5 hash of the absolute URL of the request. If found, it servers the request. The absolute URL can include the decoded UserInfo (username and password) for certain protocols. This decoded info is prepended to the domain. This allows an attacker to provide a username that has special characters to delimit the domain, and treat the rest of the URL as a path or query string. An attacker could first make a request to their domain using an encoded username, then when a request for the target domain comes in that decodes to the exact URL, it will serve the attacker's HTML instead of the real HTML. On Squid servers that also act as reverse proxies, this allows an attacker to gain access to features that only reverse proxies can use, such as ESI.

How severe is CVE-2019-12520?

CVE-2019-12520 has a CVSS score of 7.5/10 (HIGH severity). The EPSS model estimates a 3.94% probability of exploitation in the next 30 days.

How do I fix CVE-2019-12520?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.