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Published: 15 April 2020

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.


fixed in Debian's 3.5.23-5+deb9u2


CVSS 3 base score: 7.5


Package Release Status
Launchpad, Ubuntu, Debian
bionic Does not exist

eoan Not vulnerable
focal Not vulnerable
groovy Not vulnerable
hirsute Not vulnerable
precise Does not exist

trusty Does not exist

Released (4.8)
xenial Does not exist

Launchpad, Ubuntu, Debian
Released (3.5.27-1ubuntu1.7)
eoan Does not exist

focal Does not exist

groovy Does not exist

hirsute Does not exist

precise Ignored
(end of ESM support, was needs-triage)
trusty Does not exist

upstream Needs triage

xenial Not vulnerable
(code not present)