CVE-2017-18190 in CUPS
Summary
by MITRE
A localhost.localdomain whitelist entry in valid_host() in scheduler/client.c in CUPS before 2.2.2 allows remote attackers to execute arbitrary IPP commands by sending POST requests to the CUPS daemon in conjunction with DNS rebinding. The localhost.localdomain name is often resolved via a DNS server (neither the OS nor the web browser is responsible for ensuring that localhost.localdomain is 127.0.0.1).
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Analysis
by VulDB Data Team • 02/08/2023
The vulnerability described in CVE-2017-18190 represents a critical security flaw in the Common Unix Printing System (CUPS) software that affects versions prior to 2.2.2. This issue stems from an overly permissive host validation mechanism within the CUPS scheduler component, specifically in the valid_host() function located in scheduler/client.c. The vulnerability creates a dangerous condition where remote attackers can bypass authentication mechanisms and execute arbitrary IPP (Internet Printing Protocol) commands against CUPS daemons, effectively allowing unauthorized access to printing services and potentially leading to complete system compromise.
The technical root cause of this vulnerability lies in the improper handling of hostname validation during IPP request processing. When CUPS receives a POST request containing IPP commands, it performs host validation to determine whether the request should be accepted or rejected. The flaw occurs because the system maintains a whitelist entry for localhost.localdomain, which is a common DNS name used to refer to the local machine. However, the implementation does not properly enforce strict IP address resolution checks, allowing DNS rebinding attacks to exploit this weakness. The localhost.localdomain name can be resolved through external DNS servers rather than being strictly bound to the loopback address 127.0.0.1, creating an avenue for attackers to redirect requests to local services through manipulated DNS responses.
This vulnerability operates under the principles of DNS rebinding attacks, which fall under the ATT&CK framework's technique T1212 (Exploitation for Credential Access) and T1071.1004 (Application Layer Protocol: DNS). The operational impact is severe as it allows remote attackers to execute arbitrary commands on systems running vulnerable CUPS versions without requiring any authentication credentials. An attacker can send specially crafted IPP requests that appear to originate from localhost but are actually processed by the target system, enabling them to manipulate print queues, access system resources, and potentially execute arbitrary code. The attack vector specifically targets the CUPS daemon's network interface, making it particularly dangerous in environments where printing services are exposed to untrusted networks or where local network access is not properly secured.
The security implications extend beyond simple command execution to encompass potential privilege escalation and information disclosure. Since CUPS typically runs with elevated privileges to manage printing services, successful exploitation could allow attackers to gain elevated system access. This vulnerability aligns with CWE-284 (Improper Access Control) and CWE-20 (Improper Input Validation), as the system fails to properly validate hostnames and fails to enforce strict access controls for IPP commands. The attack requires minimal prerequisites, as it only needs network access to the CUPS daemon and the ability to manipulate DNS responses, making it particularly dangerous in environments where network security controls are insufficient. Organizations running affected CUPS versions should immediately implement mitigations including upgrading to CUPS 2.2.2 or later, implementing strict firewall rules to restrict access to CUPS ports, and monitoring for suspicious IPP traffic patterns.