CVE-2022-39028: GNU Inetutils telnetd DoS Vulnerability

CVE-2022-39028 Overview

CVE-2022-39028 is a Null Pointer Dereference vulnerability affecting the telnetd daemon in GNU Inetutils through version 2.3, MIT krb5-appl through version 1.0.3, and derivative works including netkit-telnet. The vulnerability can be triggered remotely by sending specially crafted telnet escape sequences (0xff 0xf7 or 0xff 0xf8), causing the telnetd application to crash. While a single crash may not immediately disrupt service availability due to inetd's process management, repeated exploitation can result in a complete denial of service when inetd terminates the telnet service after detecting a looping failure condition.

Critical Impact

Remote attackers can cause persistent denial of service to telnet services by repeatedly triggering NULL pointer dereferences, potentially disrupting critical legacy network infrastructure.

Affected Products

• GNU Inetutils through version 2.3
• MIT krb5-appl through version 1.0.3
• Debian Linux 10.0
• netkit-telnet (all versions)

Discovery Timeline

• 2022-08-30 - CVE CVE-2022-39028 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2022-39028

Vulnerability Analysis

This vulnerability is classified as CWE-476: NULL Pointer Dereference. The flaw exists in the telnetd daemon's handling of certain telnet protocol escape sequences. When a remote attacker sends a malformed sequence containing 0xff 0xf7 (TELNET_ABORT) or 0xff 0xf8 (TELNET_AO) bytes, the daemon attempts to dereference a NULL pointer, causing the process to crash.

The attack is particularly impactful because of how the vulnerability interacts with the inetd super-server daemon. While inetd will restart the telnetd process after a crash, it implements a protective mechanism against runaway services. If telnetd crashes multiple times within a short interval, inetd will log a "telnet/tcp server failing (looping), service terminated" error message and permanently disable the telnet service until manual intervention restores it.

Root Cause

The root cause lies in insufficient input validation within the telnet protocol handling code. The telnetd daemon fails to properly validate or handle specific telnet escape sequences before attempting to process them. When the 0xff 0xf7 or 0xff 0xf8 byte sequences are received, the code path leads to a pointer dereference operation on an uninitialized or NULL pointer value, resulting in a segmentation fault that terminates the daemon process.

Attack Vector

The vulnerability is exploitable remotely over the network without authentication. An attacker only needs to establish a TCP connection to the telnet service (typically port 23) and send a minimal 2-byte payload. The simplicity of the attack makes it trivial to automate and weaponize for denial-of-service purposes. The attack does not require any user interaction and can be executed against any exposed telnetd service running vulnerable versions of the affected software.

The exploitation sequence involves:

1. Establishing a TCP connection to the target telnet port
2. Sending the 2-byte escape sequence (either 0xff 0xf7 or 0xff 0xf8)
3. Repeating the connection and payload multiple times to trigger inetd's service termination

Technical details and proof-of-concept information are available in the Pierre Kim Blog on 2-Byte DoS.

Detection Methods for CVE-2022-39028

Indicators of Compromise

• Repeated telnetd process crashes in system logs with segmentation fault signals
• inetd log entries containing "telnet/tcp server failing (looping), service terminated"
• Multiple short-lived TCP connections to port 23 from the same source IP
• Core dumps from telnetd processes showing NULL pointer dereference conditions

Detection Strategies

• Monitor system logs (/var/log/syslog, /var/log/daemon.log) for telnetd crash events and inetd service termination messages
• Deploy network intrusion detection signatures to identify telnet sessions containing the malicious 0xff 0xf7 or 0xff 0xf8 byte sequences
• Implement rate limiting on telnet connections to detect and block rapid connection attempts from single sources
• Use SentinelOne Singularity platform to detect anomalous process behavior and repeated daemon crashes

Monitoring Recommendations

• Configure alerting for inetd service termination events related to telnet
• Set up connection rate monitoring for telnet services (port 23) to identify potential DoS attacks
• Implement network flow analysis to detect high-frequency short-duration telnet connections
• Enable process crash monitoring for telnetd with automatic incident response workflows

How to Mitigate CVE-2022-39028

Immediate Actions Required

• Upgrade GNU Inetutils to a patched version beyond 2.3 where the vulnerability has been addressed
• Apply distribution-specific security patches (see Debian LTS Announcement for Debian systems)
• Consider disabling telnet services entirely and migrating to SSH for remote terminal access
• Implement network-level access controls to restrict telnet service exposure to trusted networks only

Patch Information

The fix for this vulnerability has been committed to the Debian inetutils package. The patch can be reviewed at the Hadrons Git Commit Update. Additionally, the original bug report and discussion can be found at the GNU Bug Report for Inetutils.

Note that MIT krb5-appl is no longer supported upstream and the affected telnetd code was removed from the supported MIT Kerberos 5 product at version 1.8. Organizations still using krb5-appl should migrate to supported alternatives.

Workarounds

• Restrict network access to telnet services using firewall rules to allow only trusted source IP addresses
• Replace telnet with SSH for all remote administration and terminal access requirements
• Configure inetd to restart services with delays or limits to mitigate the impact of crash loops
• Deploy a reverse proxy or connection filtering appliance in front of telnet services to inspect and block malicious payloads

# Example: Block telnet access except from trusted management network
iptables -A INPUT -p tcp --dport 23 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 23 -j DROP