CVE-2021-38189 in lettre Crate
Summary
by MITRE • 08/08/2021
An issue was discovered in the lettre crate before 0.9.6 for Rust. In an e-mail message body, an attacker can place a . character after two sequences and then inject arbitrary SMTP commands.
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Analysis
by VulDB Data Team • 08/14/2021
The vulnerability identified as CVE-2021-38189 affects the lettre crate version 0.9.5 and earlier in the rust programming language ecosystem. This security flaw resides within the email message processing functionality where the crate handles email body content and SMTP command injection. The issue specifically manifests when an attacker crafts malicious email content that exploits improper input validation mechanisms within the lettre library's email parsing and transmission processes.
The technical flaw stems from inadequate sanitization of email message bodies where the presence of a period character following two sequential characters creates a condition that allows arbitrary SMTP command injection. This vulnerability operates at the protocol level where the lettre crate fails to properly escape or validate special characters that have significance in SMTP communication protocols. The period character in SMTP contexts is used to terminate message bodies, making it a critical control character that requires careful handling. When the crate processes email content containing maliciously placed periods, it does not properly sanitize these inputs, allowing attackers to inject commands that the underlying SMTP server might execute.
The operational impact of this vulnerability extends beyond simple email processing as it represents a potential pathway for remote command execution or data exfiltration through compromised email servers. An attacker could leverage this flaw to inject malicious SMTP commands such as DATA, RSET, or VRFY commands that could manipulate the email server's behavior, potentially leading to unauthorized access to server resources, relay attacks, or even complete server compromise. The vulnerability affects any system using the affected lettre crate version for email processing, making it particularly dangerous in environments where email servers handle sensitive communications or where the email infrastructure serves as a gateway to internal networks.
Mitigation strategies should focus on immediate version upgrades to lettre 0.9.6 or later where the vulnerability has been patched. Organizations should also implement additional input validation layers and consider implementing network segmentation to limit the potential impact of successful exploitation. The vulnerability aligns with CWE-77 and CWE-78 categories related to command injection and improper input sanitization, and it maps to ATT&CK techniques such as T1190 for exploit public-facing applications and T1071.004 for application layer protocol usage. Security teams should monitor email server logs for unusual command patterns and implement proper email content filtering to detect and prevent exploitation attempts that might involve SMTP command injection vectors.