CVE-2026-106582

A flaw was found in OpenSSH. When session compression is enabled, an attacker with access to a shared Secure Shell (SSH) connection can exploit the compression state shared across multiplexed channels. By analyzing changes in transmitted packet lengths caused by repeated data patterns, the attacker may deduce and recover sensitive plaintext information transmitted over concurrent channels in the same session.

A low severity information disclosure flaw was found in OpenSSH as included in Red Hat Hardened Images, where the openssh component is affected and a fix is being pursued through the product's delegated update process. When session compression is enabled, the LZ77 dictionary compression state is shared across all channels multiplexed over a single SSH connection. An attacker with the ability to send data over one channel of such a shared connection can perform a chosen-plaintext attack: because LZ77 replaces repeated strings with back-references into the shared search buffer, attacker-controlled input reflects measurably into the length of transmitted packets, allowing sensitive plaintext sent over a concurrent channel of the same session to be deduced and partially recovered. Exploitation requires session compression (a non-default option) to be enabled and trusted and untrusted traffic to be multiplexed over the same SSH connection, which limits practical exposure. Red Hat and upstream both assess this issue as low severity (CVSS 3.1 score 3.7).

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Session compression is not enabled by default in OpenSSH; keeping it disabled prevents this side-channel. Set 'Compression no' in sshd_config on servers and in ssh_config (or use 'ssh -o Compression=no') on clients where compression has been enabled. Avoid multiplexing trusted and untrusted traffic over a single shared SSH connection, for example via ssh connection multiplexing (ControlMaster). Where compression of transferred data is required, applying compression at the application level before data enters the SSH session is not affected by this type of attack. A fix is being pursued for the affected Red Hat Hardened Images product.