CVE-2017-9789 in macOS
Summary
by MITRE
When under stress, closing many connections, the HTTP/2 handling code in Apache httpd 2.4.26 would sometimes access memory after it has been freed, resulting in potentially erratic behaviour.
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Analysis
by VulDB Data Team • 01/21/2021
The vulnerability identified as CVE-2017-9789 represents a critical memory management flaw within the HTTP/2 implementation of Apache httpd version 2.4.26. This issue manifests specifically under high load conditions when the server must handle numerous concurrent connection closures, creating a scenario where the application's memory management becomes compromised. The flaw falls under the category of memory corruption vulnerabilities, which are particularly dangerous as they can lead to unpredictable system behavior and potential exploitation opportunities.
The technical nature of this vulnerability stems from improper handling of memory allocation and deallocation within the HTTP/2 protocol processing code. When the server experiences stress from managing many simultaneous connection terminations, the code path responsible for cleaning up HTTP/2 connection resources contains a race condition or improper pointer management that allows the application to access memory that has already been freed. This type of error commonly maps to CWE-416, which describes the use of freed memory vulnerability, and may also relate to CWE-122, indicating heap-based buffer overflow conditions. The specific implementation issue occurs in the connection closure handling logic where the server fails to properly synchronize access to shared resources or maintain proper reference counting during the cleanup process.
The operational impact of CVE-2017-9789 extends beyond simple service disruption, as it can potentially lead to system instability and denial of service conditions. Under normal operating conditions, the server may continue to function despite the memory access violation, but under stress conditions with multiple concurrent connection closures, the behavior becomes unpredictable and erratic. This vulnerability could enable attackers to cause service interruptions or potentially leverage the memory corruption for more advanced exploitation techniques, particularly when combined with other vulnerabilities or when the server is under sustained attack conditions. The erratic behavior may manifest as application crashes, data corruption, or unexpected termination of the httpd process, which directly impacts the availability of web services hosted on the affected server.
Mitigation strategies for this vulnerability should focus on immediate patching with the updated Apache httpd version that resolves the memory management issue. Organizations should prioritize updating their httpd installations to versions that contain the fixed HTTP/2 handling code, as this represents the most effective solution to prevent exploitation. Additionally, implementing proper monitoring and alerting for unusual connection patterns or server instability can help detect potential exploitation attempts. Network-level defenses such as rate limiting and connection pooling controls can reduce the likelihood of triggering the vulnerability through excessive connection stress. Security teams should also consider implementing intrusion detection systems that can monitor for abnormal behavior patterns that might indicate exploitation attempts. The ATT&CK framework categorizes this type of vulnerability under the T1499 sub-technique for Network Denial of Service, as the erratic behavior can lead to service unavailability. Organizations should also review their incident response procedures to ensure they can effectively handle potential exploitation of memory corruption vulnerabilities, particularly those affecting core infrastructure components like web servers.