CVE-2022-39028 in Inetutils
Summary
by MITRE • 08/30/2022
telnetd in GNU Inetutils through 2.3, MIT krb5-appl through 1.0.3, and derivative works has a NULL pointer dereference via 0xff 0xf7 or 0xff 0xf8. In a typical installation, the telnetd application would crash but the telnet service would remain available through inetd. However, if the telnetd application has many crashes within a short time interval, the telnet service would become unavailable after inetd logs a "telnet/tcp server failing (looping), service terminated" error. NOTE: MIT krb5-appl is not supported upstream but is shipped by a few Linux distributions. The affected code was removed from the supported MIT Kerberos 5 (aka krb5) product many years ago, at version 1.8.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 06/25/2026
The vulnerability CVE-2022-39028 represents a critical NULL pointer dereference flaw in telnetd implementations within GNU Inetutils and MIT krb5-appl software packages. This vulnerability specifically manifests when the telnet daemon receives malformed telnet protocol sequences containing the byte sequences 0xff 0xf7 or 0xff 0xf8, which are recognized as telnet protocol negotiation commands. The flaw stems from inadequate input validation and error handling within the telnet protocol processing code, where these specific byte sequences trigger a condition that leads to a NULL pointer dereference operation. According to CWE-476, this vulnerability maps directly to a NULL pointer dereference weakness, where the application attempts to access memory through a null pointer reference, resulting in an immediate crash.
The technical exploitation of this vulnerability occurs during normal telnet protocol communication when a remote attacker sends specially crafted telnet negotiation sequences to the vulnerable telnet daemon. The affected code paths within the telnetd implementation fail to properly validate incoming telnet protocol commands before attempting to process them, leading to a scenario where a NULL pointer is dereferenced during protocol sequence handling. This results in an immediate application crash that can be repeated multiple times to create a denial of service condition. The vulnerability affects versions through GNU Inetutils 2.3 and MIT krb5-appl 1.0.3, with the latter being a derivative work that has been deprecated upstream but still ships in some Linux distributions. The ATT&CK framework categorizes this as a denial of service attack technique under the T1499.004 sub-technique for "Network Denial of Service" where an adversary leverages protocol-level vulnerabilities to disrupt service availability.
The operational impact of this vulnerability extends beyond simple application crashes, particularly in environments where telnet services are managed through inetd or similar super-server daemons. While individual crashes typically do not completely terminate the telnet service due to the daemon's restart capabilities, repeated exploitation within short time intervals can overwhelm the inetd process management system. When multiple telnetd crashes occur in rapid succession, the inetd daemon logs a specific error message indicating "telnet/tcp server failing (looping), service terminated" and subsequently terminates the telnet service entirely. This creates a cascading failure scenario where legitimate users cannot establish telnet connections, effectively rendering the telnet service unavailable for legitimate administrative purposes. The vulnerability is particularly concerning in environments where telnet is used for remote system administration, as it can be exploited to disrupt critical network operations and system management functions.
Mitigation strategies for CVE-2022-39028 should prioritize immediate patching of affected software versions, particularly for GNU Inetutils and MIT krb5-appl installations. Organizations should ensure that all telnet services are either migrated to more secure alternatives such as SSH or properly patched to address the NULL pointer dereference condition. The recommended approach involves updating to versions that have removed the vulnerable code paths, with GNU Inetutils 2.4 and later versions providing the necessary fixes. Additionally, network-level mitigations should be implemented including firewall rules that restrict telnet access to trusted networks, disabling telnet services where possible, and implementing intrusion detection systems to monitor for exploitation attempts. Security monitoring should specifically track for repeated telnet daemon crashes and potential exploitation attempts, as the vulnerability can be used in conjunction with other attack vectors to create more sophisticated denial of service scenarios. The vulnerability serves as a reminder of the importance of proper input validation and error handling in network services, particularly those implementing legacy protocols that may contain inherent security weaknesses.