Squid Proxy Under Threat: Critical Bug Enables Remote Code Execution

A critical vulnerability has been discovered in the Squid Web Proxy server, which could allow remote attackers to execute arbitrary code on affected systems. This vulnerability affects multiple versions and may impact many systems relying on Squid for caching and proxy functionality.

• CVE-ID: CVE-2025-54574
• CVSS Score: 9.3 (Critical) 
• EPSS Score: 0.93%
• Vulnerability: Heap Buffer Overflow vulnerability 
• Affected Versions: Squid 6.3 and earlier

• Attacker identifies a vulnerable Squid serverThe attacker scans the internet or internal networks to find Squid HTTP proxy servers running version 6.3 or earlier.
• Target is confirmed as accessible.The attacker confirms that the proxy server accepts incoming client requests (i.e., is publicly or internally reachable).
• Crafting a malicious URN requestA specially crafted URN (Uniform Resource Name) request is created. This request is designed to trigger the buffer management flaw in Squid’s URN handling logic.
• Sending the malicious requestThe attacker sends the malicious URN request to the Squid proxy.
• A heap buffer overflow is triggered.Squid processes the malformed URN without checking the proper bounds. Due to incorrect buffer management, a heap buffer overflow occurs in memory.
• Execution of arbitrary codeThe attacker’s payload delivered via the overflow may overwrite function pointers or control data in memory, allowing remote code execution (RCE) on the proxy server.

Successful exploitation of this vulnerability can lead to remote code execution on affected Squid proxy servers without authentication or user interaction. Additionally, it may allow attackers to exfiltrate up to 4KB of heap memory, potentially exposing sensitive information such as security credentials or other confidential data stored in memory.

Organizations are strongly advised to upgrade to Squid version 6.4, which completely fixes this vulnerability. If upgrading is not immediately possible, a temporary workaround can be applied by disabling URN access via configuration changes. This can be achieved by adding ACL rules to block URN protocol requests, preventing exploitation until a proper update is deployed.

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