CVE-2023-46218 in cURL
Summary
by MITRE • 12/07/2023
This flaw allows a malicious HTTP server to set "super cookies" in curl that are then passed back to more origins than what is otherwise allowed or possible. This allows a site to set cookies that then would get sent to different and unrelated sites and domains. It could do this by exploiting a mixed case flaw in curl's function that verifies a given cookie domain against the Public Suffix List (PSL). For example a cookie could be set with `domain=co.UK` when the URL used a lower case hostname `curl.co.uk`, even though `co.uk` is listed as a PSL domain.
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Analysis
by VulDB Data Team • 02/14/2025
The vulnerability identified as CVE-2023-46218 represents a critical security flaw in the curl HTTP client library that undermines fundamental web security mechanisms. This issue stems from a specific implementation weakness in curl's cookie handling system where the software fails to properly validate cookie domain attributes against the Public Suffix List. The flaw specifically manifests when curl processes cookies with mixed-case domain attributes, creating a bypass mechanism that allows malicious servers to establish persistent tracking mechanisms across unrelated domains. This vulnerability directly impacts the core principle of same-origin policy enforcement that browsers and HTTP clients rely upon for user privacy protection.
The technical exploitation of this vulnerability occurs through a subtle but significant mismatch in case-sensitive string comparison logic within curl's cookie domain validation routine. When a malicious HTTP server sets a cookie with a domain attribute containing uppercase letters such as `domain=co.UK` while the originating URL uses lowercase `curl.co.uk`, the flawed validation logic fails to properly recognize that `co.uk` constitutes a public suffix domain. This oversight enables the cookie to be accepted and stored in a manner that permits it to be transmitted to completely different domains that should normally be excluded from such cookie transmission. The vulnerability essentially creates a mechanism for what security researchers term "super cookies" that can persist across multiple unrelated websites and tracking domains, effectively bypassing traditional cross-site tracking protections.
The operational impact of this vulnerability extends beyond simple privacy concerns to encompass significant security implications for user data protection and web application integrity. Attackers can leverage this flaw to establish persistent tracking mechanisms that span across different website domains, potentially enabling sophisticated cross-site tracking campaigns that would otherwise be prevented by standard cookie security policies. This capability directly aligns with attack patterns described in the attack technique matrix under ATT&CK framework category T1531 for "Account Access Removal" and T1580 for "Weaken Encryption". The vulnerability creates a persistent surveillance mechanism that can track user behavior across multiple unrelated sites, undermining the security assumptions that web applications and browsers make regarding cookie isolation and user privacy. Organizations relying on curl for web scraping, API communications, or automated client interactions face elevated risk of unauthorized tracking and data leakage.
The root cause of this vulnerability maps directly to CWE-284, which addresses "Improper Access Control" in software implementations, and CWE-310, which covers "Cryptographic Issues" related to improper validation. This flaw demonstrates how seemingly minor implementation details in security-critical code can create substantial vulnerabilities that compromise core web security principles. The mixed-case comparison issue specifically relates to CWE-242, "Use of Inherently Dangerous Function", and CWE-129, "Improper Validation". Organizations should immediately implement mitigation strategies including updating to patched versions of curl, implementing additional cookie validation layers, and monitoring for suspicious cookie patterns. The vulnerability serves as a reminder of the critical importance of proper case-sensitive string handling in security-sensitive contexts and the need for comprehensive testing of edge cases in cookie validation logic. Security teams should also consider implementing network-level controls to detect and prevent suspicious cookie behaviors that may indicate exploitation attempts.