CVE-2025-59089: 
Linux Debian vulnerability analysis and mitigation

CVE-2025-59089 is a vulnerability in python-kdcproxy that allows remote denial-of-service attacks through unbounded TCP upstream buffering. The vulnerability exists because kdcproxy does not enforce bounds on TCP response length, allowing an attacker to conduct a denial-of-service attack when connecting to an attacker-controlled KDC server (Debian Tracker).

The vulnerability stems from improper message length checks and redundant buffer exports in kdcproxy's TCP handling. While receiving the KDC's response, kdcproxy copies the entire buffered stream into a new buffer on each recv() call, even when the transfer is incomplete. Additionally, it accepts incoming response chunks even when individual chunks or the total buffer exceed the maximum length of a Kerberos message, as long as the received data length is not exactly equal to the length indicated in the response header (GitHub PR).

An attacker can exploit this vulnerability to send unbounded data until the connection timeout is reached (approximately 12 seconds), causing excessive memory allocation and CPU usage. Multiple concurrent requests can cause accept queue overflow, effectively denying service to legitimate clients (Debian Tracker).

The vulnerability has been patched with fixes that include interrupting message receiving when incoming messages exceed the maximum length of a Kerberos message or the length indicated in the message header. The fix also ensures the content of the input stream is exported to a buffer only once after the receiving process has ended (GitHub PR). Red Hat has released security updates for affected versions through multiple advisories (Red Hat Bugzilla).