CVE-2020-15917 in Claws Mail
Summary
by MITRE
common/session.c in Claws Mail before 3.17.6 has a protocol violation because suffix data after STARTTLS is mishandled.
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Analysis
by VulDB Data Team • 07/24/2020
The vulnerability identified as CVE-2020-15917 resides within the Claws Mail email client's session handling component, specifically in the common/session.c file. This issue represents a protocol violation that occurs during the Secure Sockets Layer/TLS negotiation process, particularly when handling the STARTTLS command sequence. The flaw manifests when the application receives suffix data following the STARTTLS command execution, indicating improper state management and data processing within the email client's network communication stack. Such protocol violations can compromise the integrity of secure communications between the client and mail servers, potentially undermining the encryption assurances that users expect during email transmission.
The technical nature of this vulnerability stems from inadequate handling of data sequences after the STARTTLS command is processed. When Claws Mail establishes a secure connection with an email server, it must properly manage the transition from plaintext to encrypted communication. The mishandling of suffix data suggests that the application fails to correctly parse or discard additional data that may arrive after the TLS negotiation completes, creating a potential attack surface where malicious actors could manipulate the communication flow. This type of vulnerability falls under the category of protocol implementation flaws that can lead to various security consequences including man-in-the-middle attacks or data interception. The issue demonstrates a classic failure in proper state machine implementation where the application does not adequately validate or process the sequence of events following a security protocol handshake.
The operational impact of this vulnerability extends beyond simple protocol violations to potentially compromise user privacy and data integrity. Email clients like Claws Mail serve as critical gateways for personal and professional communications, making them attractive targets for adversaries seeking to intercept sensitive information. When suffix data after STARTTLS is mishandled, it creates opportunities for attackers to inject malicious content or manipulate the communication channel, potentially leading to unauthorized access to email accounts, exposure of confidential messages, or disruption of secure communication channels. The vulnerability affects users who rely on Claws Mail for secure email communications, particularly in environments where email security is paramount such as corporate networks, government agencies, or organizations handling sensitive data. This flaw could be exploited in conjunction with other vulnerabilities to create more sophisticated attack vectors targeting the email infrastructure.
The security implications of CVE-2020-15917 align with CWE-347, which addresses improper certificate validation, and relates to ATT&CK technique T1566 for credential access through phishing and social engineering. Organizations using Claws Mail should prioritize immediate remediation by updating to version 3.17.6 or later, which contains the necessary patches to properly handle suffix data after STARTTLS commands. System administrators should also implement network monitoring to detect unusual communication patterns that might indicate exploitation attempts. The vulnerability underscores the importance of proper protocol implementation in security-critical applications and demonstrates how seemingly minor implementation flaws can have significant security consequences. Additionally, this issue highlights the necessity for thorough testing of security protocols, particularly in email clients where the integrity of communications is fundamental to user trust and organizational security posture. Organizations should also consider implementing additional security controls such as email encryption protocols beyond TLS, network segmentation, and regular security assessments to mitigate potential exploitation of similar vulnerabilities in their email infrastructure.