telnet in GNU inetutils through 2.7 allows servers to read arbitrary environment variables from clients via NEW_ENVIRON SEND USERVAR.

telnetd in GNU inetutils through 2.7 allows an out-of-bounds write in the LINEMODE SLC (Set Local Characters) suboption handler because add_slc does not check whether the buffer is full.

telnetd in GNU inetutils through 2.7 allows privilege escalation that can be exploited by abusing systemd service credentials support added to the login(1) implementation of util-linux in release 2.40. This is related to client control over the CREDENTIALS_DIRECTORY environment variable, and requires an unprivileged local user to create a login.noauth file.

telnetd in GNU Inetutils through 2.7 allows remote authentication bypass via a "-f root" value for the USER environment variable.

GNU inetutils before 2.5 may allow privilege escalation because of unchecked return values of set*id() family functions in ftpd, rcp, rlogin, rsh, rshd, and uucpd. This is, for example, relevant if the setuid system call fails when a process is trying to drop privileges before letting an ordinary user control the activities of the process.

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.