Medium [CVE-2026-54886] Erlang OTP ssh: Denial of Service via infinite loop in SFTP channel

Loop with Unreachable Exit Condition ('Infinite Loop') vulnerability in Erlang OTP ssh (ssh_sftpd module) allows an authenticated SFTP user to render an SFTP channel permanently unresponsive. The handle_data/4 function in ssh_sftpd contains a catch-all clause that accepts channel data of any type.

When channel data with a non-zero type code (SSH_MSG_CHANNEL_EXTENDED_DATA) arrives with an empty pending buffer and a payload at or below the SFTP packet size limit, the clause tail-calls itself with identical arguments, creating an infinite loop. The SFTP protocol operates exclusively on normal channel data (type 0).

Extended data (non-zero type) is meaningless for SFTP and is never sent by conforming clients.

However, the SSH protocol permits any channel participant to send extended data on an open channel, so an authenticated SFTP client can trigger the loop by sending SSH_MSG_CHANNEL_EXTENDED_DATA with any data_type_code and any non-empty payload at or below the size limit. The targeted ssh_sftpd process enters an infinite tail-recursive loop.

It never processes another message, its message queue grows without bound, and it can only be stopped by killing the process.

Affected products named by the advisory: Red Hat Hardened Images; Red Hat OpenStack Platform 16.2; Red Hat OpenStack Platform 17.1; Red Hat OpenStack Platform 18.0.