CVE-2016-0772 in CPython
Summary
by MITRE
The smtplib library in CPython (aka Python) before 2.7.12, 3.x before 3.4.5, and 3.5.x before 3.5.2 does not return an error when StartTLS fails, which might allow man-in-the-middle attackers to bypass the TLS protections by leveraging a network position between the client and the registry to block the StartTLS command, aka a "StartTLS stripping attack."
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Analysis
by VulDB Data Team • 01/18/2025
The vulnerability identified as CVE-2016-0772 resides within the smtplib library of CPython, affecting multiple Python versions prior to specific security patches. This flaw represents a critical weakness in the email transmission security framework where the library fails to properly validate the successful establishment of TLS encryption during email communications. The issue specifically manifests when the StartTLS command, which is designed to upgrade a plain text connection to a secure encrypted one, encounters failures that are not properly detected or reported by the library. This creates a dangerous scenario where attackers can exploit the lack of proper error handling to perform man-in-the-middle attacks by intercepting network traffic and preventing the successful negotiation of secure connections.
The technical implementation of this vulnerability stems from the smtplib library's inadequate error checking mechanisms during the StartTLS handshake process. When a client attempts to establish a secure connection with an email server using StartTLS, the library should verify that the TLS negotiation completed successfully before proceeding with encrypted communication. However, in affected versions, the library continues to operate as if the connection is secure even when the StartTLS command fails, typically due to network interference that blocks or modifies the command. This behavior violates fundamental security principles and creates a false sense of security for applications using the library to send emails over potentially insecure networks.
The operational impact of this vulnerability extends beyond simple email transmission failures to represent a significant threat to email security and data integrity. Attackers can exploit this weakness by positioning themselves between the email client and the mail server to intercept and modify network traffic, specifically targeting the StartTLS command to prevent its successful execution. When the client fails to detect this failure, it proceeds with communication using plaintext, exposing sensitive email content to interception and modification. This vulnerability directly maps to the attack pattern described in the ATT&CK framework under credential access and defense evasion techniques, where adversaries manipulate encryption protocols to gain unauthorized access to communications.
The security implications of CVE-2016-0772 are particularly severe for applications that rely on email authentication and secure communication channels. Organizations using affected Python versions for email processing, automated notification systems, or any application that sends sensitive information via email are at risk of having their communications intercepted and potentially modified by malicious actors. The vulnerability affects not just individual email clients but also enterprise applications, web services, and automated systems that depend on the smtplib library for secure email transmission. This weakness creates a persistent security gap that can be exploited across various network environments where attackers have the capability to manipulate network traffic between the client and server.
Mitigation strategies for this vulnerability require immediate patching of affected Python installations to versions that properly handle StartTLS failures. System administrators should ensure that all Python environments using smtplib are updated to Python 2.7.12, 3.4.5, or 3.5.2 and later versions where this vulnerability has been addressed. Additionally, network administrators should implement proper monitoring to detect unusual traffic patterns that might indicate StartTLS stripping attacks. The fix implemented in the patched versions ensures that when StartTLS fails, the library properly raises exceptions and terminates the connection rather than proceeding with insecure communication. This aligns with the CWE classification for improper error handling in cryptographic protocols, specifically CWE-310, which addresses weaknesses in cryptographic implementations related to improper error handling that can lead to security vulnerabilities. Organizations should also consider implementing additional security measures such as certificate pinning and strict TLS validation policies to further protect against similar attacks that might exploit other aspects of the communication stack.