CVE-2026-47729

A flaw was found in Squid. Due to improper input validation, an out-of-bounds read can occur in the FTP gateway. This issue allows an authenticated and trusted client to read memory from random transactions when accessing a misbehaving FTP server using the Squid gateway feature.

To exploit this issue, an attacker must have a valid account on the Squid proxy and must also control an FTP server reachable from the proxy on port 21. HTTPS traffic handled via CONNECT tunnels (the vast majority of modern web traffic) is opaque to the proxy because the underlying request data is encrypted and Squid does not have access to it. The impact is limited to information disclosure of cleartext HTTP request contents or traffic in TLS-terminating (SSL bump) proxy configurations where Squid decrypts and inspects traffic. FTP protocol usage has declined considerably in most environments since major browsers removed FTP support, further narrowing the practical attack surface. Due to these reasons, this vulnerability has been rated with a moderate severity.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

When FTP access is not required, block FTP traffic at the proxy by adding the following settings to the squid.conf configuration file above any custom 'http_access allow' rules:

acl FTP proto FTP
http_access deny FTP

When FTP access is required, configure Squid to only allow FTP access to specific and trusted destination domains. See the example below for restricting FTP access to the 'trusted.server.example.com' domain:

acl FTP proto FTP
acl ftp_allowlist dstdomain .trusted.server.example.com

http_access deny FTP !ftp_allowlist