CVE-2018-16840 in cURL
Summary
by MITRE
A heap use-after-free flaw was found in curl versions from 7.59.0 through 7.61.1 in the code related to closing an easy handle. When closing and cleaning up an 'easy' handle in the `Curl_close()` function, the library code first frees a struct (without nulling the pointer) and might then subsequently erroneously write to a struct field within that already freed struct.
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Analysis
by VulDB Data Team • 04/17/2025
The heap use-after-free vulnerability identified as CVE-2018-16840 represents a critical memory safety issue within the curl library ecosystem that affects versions between 7.59.0 and 7.61.1. This flaw manifests during the cleanup process of easy handles through the Curl_close() function, where improper memory management creates exploitable conditions that can be leveraged by malicious actors. The vulnerability stems from the library's failure to properly manage pointer references during the deallocation sequence, creating a window where freed memory can be accessed and modified, thereby compromising system integrity and potentially enabling arbitrary code execution.
The technical implementation of this vulnerability involves a specific sequence of operations within the curl library's internal memory management system. During the Curl_close() function execution, the library first deallocates a struct containing critical data structures without setting the corresponding pointer to NULL, leaving a dangling reference in memory. Subsequently, the code continues to execute operations that attempt to write to fields within this already freed struct, resulting in undefined behavior and potential memory corruption. This pattern creates a classic use-after-free condition where the memory previously occupied by the struct becomes available for reuse while the application still maintains references to it, violating fundamental memory safety principles. The flaw operates at the intersection of improper memory deallocation and subsequent memory access patterns, making it particularly dangerous in networked applications that rely heavily on curl for HTTP operations.
The operational impact of CVE-2018-16840 extends beyond simple memory corruption, as it provides attackers with potential pathways for privilege escalation and remote code execution within affected systems. Applications that utilize curl for HTTP operations, including web browsers, network monitoring tools, and custom network clients, become vulnerable to exploitation when they process malicious content through the affected library versions. The vulnerability's exploitation potential aligns with ATT&CK technique T1059.007 for command and script injection, as successful exploitation could enable attackers to execute arbitrary commands on compromised systems. Furthermore, this flaw demonstrates the critical importance of proper memory management in security-sensitive libraries, as the issue affects a fundamental component that millions of applications depend upon for network communications. The vulnerability's presence in widely deployed software versions means that numerous systems and applications across different platforms could be compromised, making it a high-priority security concern for system administrators and security teams.
Mitigation strategies for CVE-2018-16840 focus primarily on immediate version upgrades to curl 7.61.2 or later, which contain the necessary patches to address the memory management issue. Organizations should implement comprehensive patch management procedures to ensure all affected systems receive updates promptly, as the vulnerability can be exploited remotely through network-based attacks. Additionally, security monitoring should include detection of potential exploitation attempts targeting this specific flaw, particularly in environments where curl is extensively used for network communications. The fix implemented in the patched versions addresses the root cause by ensuring proper pointer nullification after memory deallocation and preventing subsequent access to freed memory structures. Security teams should also consider implementing application-level memory safety checks and runtime protections to detect and prevent exploitation attempts, aligning with CWE 416 which specifically addresses use-after-free vulnerabilities. Organizations may need to conduct thorough vulnerability assessments across their software ecosystems to identify all applications and services that depend on potentially affected curl versions, ensuring comprehensive remediation across their entire infrastructure.