CVE-2021-37845 in Citadel
Summary
by MITRE • 05/29/2023
An issue was discovered in Citadel through webcit-932. A meddler-in-the-middle attacker can fixate their own session during the cleartext phase before a STARTTLS command (a violation of "The STARTTLS command is only valid in non-authenticated state." in RFC2595). This potentially allows an attacker to cause a victim's e-mail messages to be stored into an attacker's IMAP mailbox, but depends on details of the victim's client behavior.
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Analysis
by VulDB Data Team • 06/21/2023
The vulnerability identified as CVE-2021-37845 represents a significant security flaw in the Citadel email server software, specifically affecting versions through webcit-932. This issue stems from improper handling of the STARTTLS encryption negotiation process during email client connections, creating a window of opportunity for man-in-the-middle attacks. The vulnerability exploits the fundamental security principle that the STARTTLS command should only be accepted when the client is in a non-authenticated state, as mandated by RFC2595 standards. When an attacker intercepts communication between a victim client and the Citadel server, they can manipulate the session establishment process to force the use of cleartext communication instead of proper encryption, effectively undermining the security of the entire email transmission chain.
The technical implementation of this vulnerability involves the attacker positioning themselves between the email client and the Citadel server during the initial connection phase. During this cleartext communication period, the attacker can inject malicious commands that manipulate the session state, essentially forcing the server to accept their session credentials before the proper STARTTLS encryption handshake occurs. This flaw violates the established protocol flow where authentication should occur only after secure communication has been established. The vulnerability specifically targets the gap between when a client connects to the server and when the STARTTLS command is properly processed, creating a temporal window where the server's authentication state can be manipulated by an external attacker.
The operational impact of this vulnerability extends beyond simple data interception, potentially allowing attackers to redirect email messages to their own IMAP mailboxes through the manipulated session state. This means that victims' emails could be automatically copied or moved to attacker-controlled accounts, effectively enabling long-term surveillance and data theft without the victim's knowledge. The severity of this impact depends heavily on the specific behavior of the victim's email client software, as different clients may handle session state changes differently during the transition period. This client dependency creates a complex attack scenario where the success of the exploitation can vary significantly across different email platforms and configurations, making the vulnerability particularly challenging to detect and defend against.
The mitigation strategies for CVE-2021-37845 should focus on implementing proper protocol enforcement mechanisms within the Citadel server software, ensuring that the STARTTLS command is only accepted in appropriate authentication states as defined by RFC2595. Organizations should also consider implementing mandatory encryption policies that prevent cleartext communication from being accepted, and deploy network monitoring tools to detect anomalous session establishment patterns that might indicate such attacks. This vulnerability aligns with CWE-310 (Cryptographic Issues) and represents a specific implementation failure in the security protocol handling that could be addressed through proper input validation and state management. From an ATT&CK framework perspective, this vulnerability maps to T1566 (Phishing) and T1041 (Data Compressed) as it enables attackers to gain unauthorized access to email communications and potentially exfiltrate sensitive information through manipulated session states.