CVE-2026-105326 in Red Hat
Summary
by MITRE • 10/05/2026
An argument injection flaw was found in CUPS. When email notification is configured, the CUPS scheduler accepts printer subscription requests that supply a mailto notify-recipient-uri. The mailto notifier passes the recipient address to the configured sendmail program without ensuring it cannot be interpreted as command-line options. A remote attacker who can reach the CUPS service could supply a crafted recipient value starting with "-" to influence sendmail behavior. Successful exploitation depends on the installed mail transfer agent and CUPS network exposure, and may lead to execution of attacker-controlled commands with the privileges of the CUPS service user.
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Analysis
by VulDB Data Team • 10/05/2026
The vulnerability identified in the Common Unix Printing System (CUPS) represents a critical argument injection flaw within its email notification subsystem. This security issue arises from improper input validation when handling printer subscription requests that utilize mailto Uniform Resource Identifiers as notify-recipient-uri values. When an administrator configures CUPS to send email notifications for print job events, the system relies on external mail transfer agents such as Sendmail or Postfix to deliver these messages. The core technical flaw lies in how the CUPS scheduler passes the recipient address provided by the user directly to the underlying command-line interface of the configured mail program without sanitizing special characters that hold specific meaning for shell execution contexts.
Specifically, if a remote attacker can reach the exposed CUPS service and submits a crafted subscription request where the email recipient field begins with a hyphen character followed by valid flags or options recognized by the local Mail Transfer Agent, this input is passed directly to the executable without adequate escaping or validation. In many Unix-like environments, command-line arguments starting with dashes are interpreted as switches rather than data payloads. Consequently, an attacker can manipulate the execution flow of the mail utility by injecting additional parameters that alter its behavior beyond simple message delivery. This type of flaw is classically categorized under CWE-78 Improper Neutralization of Special Elements used in an OS Command and aligns with ATT&CK technique T1059 Command and Scripting Interpreter, as it allows for arbitrary command execution through the manipulation of system utilities.
The operational impact of this vulnerability is severe due to its potential for remote code execution. Since CUPS typically runs with elevated privileges or specific service account permissions depending on the distribution configuration, successful exploitation could allow an unauthenticated attacker to execute commands under the context of the CUPS daemon user. This privilege level often grants access to sensitive system files, network configurations, and potentially other services running on the same host. The exploitability depends heavily on several factors including whether the CUPS service is exposed over a network without authentication, which version of the mail transfer agent is installed, and how that specific MTA interprets malformed recipient arguments. In some configurations, this could lead to full system compromise if the attacker can chain this argument injection with other local vulnerabilities or misconfigurations.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. The most effective solution is to apply vendor-provided patches that update CUPS to versions where input validation for mailto URIs has been strengthened, ensuring that recipient addresses are properly escaped before being passed to external commands. Administrators should also enforce strict access controls on the CUPS web interface and IPP service by requiring authentication for all administrative actions including subscription management. Additionally, reviewing the configuration of the underlying Mail Transfer Agent can help mitigate risk; some modern MTAs have stricter parsing rules that reject ambiguous arguments or require explicit end-of-options markers such as -- to distinguish between options and data. Network-level protections like firewalls should restrict access to CUPS ports from untrusted networks to reduce the attack surface available for remote exploitation attempts.