CVE-2017-9801 in Commons Email
Summary
by MITRE
When a call-site passes a subject for an email that contains line-breaks in Apache Commons Email 1.0 through 1.4, the caller can add arbitrary SMTP headers.
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Analysis
by VulDB Data Team • 01/07/2021
The vulnerability identified as CVE-2017-9801 resides within Apache Commons Email library versions 1.0 through 1.4, representing a significant security flaw that enables unauthorized header injection in email communications. This issue stems from insufficient input validation and sanitization mechanisms within the email subject parameter processing, creating a pathway for malicious actors to manipulate the email transmission process. The vulnerability specifically manifests when email subjects contain line-break characters, which are typically used to separate lines in text formatting but become exploitable in this context due to improper handling of user-supplied data. The flaw allows attackers to inject additional SMTP headers into the email message, potentially altering the email's routing, content, or delivery behavior in ways that can be leveraged for various malicious purposes including phishing attacks, spam distribution, or email spoofing.
The technical implementation of this vulnerability aligns with CWE-115, which categorizes it as a "Misinterpretation of Input" where the system incorrectly interprets user input as part of the email structure rather than as content to be properly escaped or sanitized. This misinterpretation occurs during the email construction phase when the library processes the subject parameter without adequately separating the subject line from the SMTP header injection points. The vulnerability essentially allows an attacker to craft email subjects that contain embedded line-break characters followed by malicious SMTP header directives, effectively bypassing normal email security controls. When the email is processed by the underlying mail transport system, these injected headers are interpreted as legitimate components of the email message rather than malicious input, enabling the attacker to manipulate various aspects of the email's transmission and delivery behavior.
The operational impact of CVE-2017-9801 extends beyond simple email manipulation to encompass broader security implications that can compromise email infrastructure and user trust. Attackers can exploit this vulnerability to inject headers that redirect email traffic to malicious servers, add forged sender addresses to create spoofing scenarios, or even include additional recipients in the email distribution. The vulnerability can be particularly dangerous in environments where email serves as a primary communication channel for authentication tokens, sensitive information sharing, or business-critical operations. From an attacker's perspective, this flaw provides a relatively simple yet effective method for conducting email-based attacks without requiring complex exploitation techniques or elevated privileges. The impact is amplified by the widespread adoption of Apache Commons Email library across various applications and systems, meaning that a single vulnerable component can expose numerous interconnected services to potential compromise.
Mitigation strategies for CVE-2017-9801 must address both immediate remediation and long-term architectural improvements to prevent similar vulnerabilities from emerging. The most direct and effective solution involves upgrading to Apache Commons Email version 1.5 or later, where the library properly sanitizes input parameters and prevents line-break characters from being interpreted as header injection points. Organizations should implement comprehensive patch management processes to ensure all affected systems are updated promptly, as the vulnerability affects multiple versions of the library that remain in use across various enterprise environments. Additionally, developers should implement input validation at multiple layers, including application-level sanitization of email subject parameters before they are passed to the Commons Email library. The ATT&CK framework categorizes this vulnerability under T1566, specifically "Phishing" techniques, highlighting the potential for attackers to leverage this flaw to create more convincing phishing campaigns by manipulating email headers to appear more legitimate. Organizations should also consider implementing email header validation policies at network boundaries and monitoring for unusual header patterns that might indicate exploitation attempts, as this vulnerability can be used to bypass traditional email security controls that focus on content filtering rather than header manipulation.