CVE-2016-3957 in Web2py
Summary
by MITRE
The secure_load function in gluon/utils.py in web2py before 2.14.2 uses pickle.loads to deserialize session information stored in cookies, which might allow remote attackers to execute arbitrary code by leveraging knowledge of encryption_key.
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Analysis
by VulDB Data Team • 02/03/2023
The vulnerability described in CVE-2016-3957 represents a critical remote code execution flaw within the web2py web framework version 2.14.1 and earlier. This vulnerability exists in the secure_load function located within the gluon/utils.py file, which is responsible for deserializing session information stored in cookies. The flaw stems from the function's reliance on pickle.loads for deserialization operations, creating a dangerous attack vector that can be exploited by remote adversaries. The vulnerability is particularly concerning because it allows attackers to execute arbitrary code on the target system when they possess knowledge of the encryption_key used for session encryption. This creates a scenario where an attacker who can intercept or obtain the encryption_key can manipulate session cookies to inject malicious serialized data that will be executed when the application attempts to deserialize the session information.
The technical implementation of this vulnerability follows the standard pattern of pickle deserialization attacks that have been documented in numerous security advisories and threat intelligence reports. When web2py processes session cookies, it calls the secure_load function which uses pickle.loads to deserialize the session data. The pickle module in python is inherently dangerous when used with untrusted input because it can execute arbitrary code during the deserialization process. This behavior aligns with CWE-502 which specifically addresses the deserialization of untrusted data, and more broadly with CWE-134 which deals with format string vulnerabilities in the context of code execution. The attack requires knowledge of the encryption_key but once obtained, an attacker can craft malicious session cookies that, when processed by the vulnerable application, will result in arbitrary code execution on the server. This represents a classic privilege escalation scenario where an attacker can leverage session management to gain full control over the application server.
The operational impact of this vulnerability is severe and can lead to complete system compromise when exploited successfully. Remote attackers who can obtain the encryption_key can execute arbitrary commands on the target system, potentially leading to data exfiltration, system infiltration, or further attacks against the internal network. The vulnerability affects web2py applications that store session information in cookies, which is a common practice for stateless web applications. The attack surface is particularly wide because many web applications rely on session cookies for maintaining user state, making this vulnerability potentially exploitable across numerous web2py deployments. This vulnerability also aligns with several tactics in the MITRE ATT&CK framework, particularly those related to command and control, execution, and privilege escalation. The ability to execute arbitrary code remotely without requiring authentication makes this vulnerability particularly attractive to threat actors and significantly impacts the overall security posture of affected systems.
The primary mitigation strategy for this vulnerability involves upgrading to web2py version 2.14.2 or later, where the vulnerable secure_load function has been patched to address the pickle deserialization issue. Organizations should also implement proper key management practices to ensure that encryption_key values are not exposed or easily guessable, as the vulnerability requires knowledge of the encryption key to be exploitable. Additional protective measures include implementing proper input validation for session cookies, using alternative session storage mechanisms that do not rely on pickle serialization, and monitoring for suspicious session cookie activity. Security teams should also consider implementing network-level protections such as web application firewalls that can detect and block malicious session cookie payloads. The vulnerability serves as a reminder of the dangers associated with using pickle for deserialization in web applications and highlights the importance of following secure coding practices that avoid dangerous deserialization patterns. Organizations should also conduct thorough security assessments of their web2py applications to identify any other potential vulnerabilities related to session management and ensure that all components are properly updated to mitigate known security risks.