CVE-2018-20852 in Communications Operations Monitor
Summary
by MITRE
http.cookiejar.DefaultPolicy.domain_return_ok in Lib/http/cookiejar.py in Python before 3.7.3 does not correctly validate the domain: it can be tricked into sending existing cookies to the wrong server. An attacker may abuse this flaw by using a server with a hostname that has another valid hostname as a suffix (e.g., pythonicexample.com to steal cookies for example.com). When a program uses http.cookiejar.DefaultPolicy and tries to do an HTTP connection to an attacker-controlled server, existing cookies can be leaked to the attacker. This affects 2.x through 2.7.16, 3.x before 3.4.10, 3.5.x before 3.5.7, 3.6.x before 3.6.9, and 3.7.x before 3.7.3.
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Analysis
by VulDB Data Team • 05/20/2024
The vulnerability described in CVE-2018-20852 represents a critical cookie validation flaw within Python's http.cookiejar module that has persisted across multiple Python versions. This issue specifically affects the DefaultPolicy implementation in Lib/http/cookiejar.py and stems from inadequate domain validation logic that allows for cookie leakage through domain suffix manipulation. The flaw enables attackers to exploit the cookie handling mechanism by crafting hostnames that contain valid domain names as suffixes, creating a scenario where cookies intended for one domain are inadvertently sent to a malicious server controlled by the attacker. The vulnerability impacts Python versions ranging from 2.x through 2.7.16, 3.x versions before 3.4.10, 3.5.x before 3.5.7, 3.6.x before 3.6.9, and 3.7.x before 3.7.3, demonstrating the widespread nature of this security gap that has remained unaddressed for an extended period.
The technical implementation of this vulnerability lies in the domain_return_ok method within the cookiejar module, which fails to properly validate domain matching rules according to established web standards. When a client attempts to establish an HTTP connection to an attacker-controlled server, the flawed validation logic permits cookies to be sent to domains that should not receive them based on standard cookie security policies. The attack vector relies on the fact that Python's cookie handling mechanism does not sufficiently enforce the domain restriction rules that browsers and web servers typically implement, allowing for a scenario where a hostname like "pythonicexample.com" could be used to intercept cookies meant for "example.com". This bypass occurs because the validation does not properly check whether the requesting domain is a subdomain of the cookie's domain or if it's a legitimate parent domain, creating a potential for cross-domain cookie leakage.
The operational impact of this vulnerability extends beyond simple information disclosure, as it provides attackers with the capability to hijack user sessions and access sensitive data that should remain protected. When applications utilize http.cookiejar.DefaultPolicy for handling HTTP cookies, they become vulnerable to this attack, potentially compromising user authentication tokens, session identifiers, and other sensitive cookies that contain personal or confidential information. The attack scenario becomes particularly dangerous when applications make HTTP requests to external services or when users interact with web applications that rely on Python's built-in HTTP client functionality. This vulnerability directly violates the fundamental security principle of cookie isolation, which is designed to prevent unauthorized access to user sessions across different domains and applications. The impact is amplified in environments where Python applications handle authentication flows, user management, or any scenario where cookie-based session management is employed.
Mitigation strategies for CVE-2018-20852 require immediate action to upgrade affected Python installations to patched versions, specifically targeting Python 3.7.3 or later, and Python 3.6.9, 3.5.7, and 3.4.10 or higher. Organizations should also consider implementing additional defensive measures such as explicit cookie validation in their applications, avoiding the use of DefaultPolicy in security-sensitive contexts, and implementing custom cookie handling logic that enforces stricter domain validation rules. The vulnerability aligns with CWE-295, which addresses improper certificate validation, and relates to ATT&CK technique T1566.001 for credential access through the exploitation of cookie manipulation. Security teams should also implement network monitoring to detect unusual cookie behavior patterns and consider implementing additional security layers such as secure cookie flags, SameSite attributes, and proper HTTP headers to minimize the impact of such vulnerabilities in environments where immediate patching is not immediately feasible.