CVE-2024-49769 in Waitress
Summary
by MITRE • 10/29/2024
Waitress is a Web Server Gateway Interface server for Python 2 and 3. When a remote client closes the connection before waitress has had the opportunity to call getpeername() waitress won't correctly clean up the connection leading to the main thread attempting to write to a socket that no longer exists, but not removing it from the list of sockets to attempt to process. This leads to a busy-loop calling the write function. A remote attacker could run waitress out of available sockets with very little resources required. Waitress 3.0.1 contains fixes that remove the race condition.
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Analysis
by VulDB Data Team • 11/02/2024
The vulnerability identified as CVE-2024-49769 affects Waitress, a Python Web Server Gateway Interface server that operates with both Python 2 and 3 environments. This issue stems from a race condition in the connection handling mechanism that occurs during the interaction between the server and remote clients. The flaw manifests when a remote client terminates their connection prematurely, before the Waitress server has completed its internal getpeername() call sequence. This timing discrepancy creates a scenario where the server fails to properly dispose of the connection resources, leaving the main processing thread with references to sockets that are no longer valid. The technical implementation involves the server maintaining an active list of sockets to process while simultaneously attempting to write data to connections that have already been closed by the client. This creates a fundamental inconsistency in the server's socket management system where the main thread continues to attempt I/O operations on dead connections rather than removing them from the processing queue.
The operational impact of this vulnerability extends beyond simple resource exhaustion to create a significant denial-of-service condition. When the main thread repeatedly attempts to write to closed sockets, it enters a busy-loop pattern where the write operations fail continuously but the server continues to cycle through the problematic connections. This behavior consumes substantial CPU resources and can rapidly deplete the server's available socket file descriptor limits. The vulnerability is particularly dangerous because it requires minimal resources to exploit, making it an attractive vector for attackers seeking to disrupt service availability. The server's inability to properly clean up connections leads to progressive degradation of performance until the system reaches its socket limit and becomes unresponsive to new connections. This type of vulnerability aligns with CWE-1299, which describes improper handling of race conditions in resource management, and represents a classic example of how timing issues in network programming can lead to resource exhaustion attacks.
The fix implemented in Waitress version 3.0.1 addresses the root cause by eliminating the race condition through improved connection cleanup mechanisms. This update ensures that when a client closes a connection prematurely, the server properly identifies and removes the invalid socket reference from its processing list before attempting any further I/O operations. The mitigation strategy involves implementing more robust error handling and connection state management that prevents the busy-loop behavior by ensuring proper resource cleanup at all stages of the connection lifecycle. Organizations using Waitress should immediately upgrade to version 3.0.1 or later to address this vulnerability, as the fix resolves the underlying race condition that enables the resource exhaustion attack. The solution aligns with ATT&CK technique T1499.004, which covers resource exhaustion attacks, by preventing the specific mechanism that allows attackers to consume system resources through improper socket management. Security practitioners should also consider implementing connection rate limiting and monitoring for unusual patterns in socket usage as additional protective measures against similar timing-based vulnerabilities in network services.