OpenSSH Security Advisory: x11fwd.adv This document may be found at: http://www.openssh.com/txt/x11fwd.adv 1. Affected configurations All versions of OpenSSH prior to 7.2p2 with X11Forwarding enabled. 2. Vulnerability Missing sanitisation of untrusted input allows an authenticated user who is able to request X11 forwarding to inject commands to xauth(1). Injection of xauth commands grants the ability to read arbitrary files under the authenticated user's privilege, Other xauth commands allow limited information leakage, file overwrite, port probing and generally expose xauth(1), which was not written with a hostile user in mind, as an attack surface. xauth(1) is run under the user's privilege, so this vulnerability offers no additional access to unrestricted accounts, but could circumvent key or account restrictions such as sshd_config ForceCommand, authorized_keys command="..." or restricted shells. 3. Mitigation Set X11Forwarding=no in sshd_config. This is the default. For authorized_keys that specify a "command" restriction, also set the "restrict" (available in OpenSSH >=7.2) or "no-x11-forwarding" restrictions. 4. Details As part of establishing an X11 forwarding session, sshd(8) accepts an X11 authentication credential from the client. This credential is supplied to the xauth(1) utility to establish it for X11 applications that the user subsequently runs. The contents of the credential's components (authentication scheme and credential data) were not sanitised to exclude meta-characters such as newlines. An attacker could therefore supply a credential that injected commands to xauth(1). The attacker could then use a number of xauth commands to read or overwrite arbitrary files subject to file permissions, connect to local ports or perform attacks on xauth(1) itself. OpenSSH 7.2p2 implements a whitelist of characters that are permitted to appear in X11 authentication credentials. 5. Credit This issue was identified by github.com/tintinweb and communicated to the OpenSSH developers on March 3rd, 2016. 6. Fix Portable OpenSSH 7.2p2 contains a fix for this vulnerability. Patches for supported OpenBSD releases (5.7, 5.8 and 5.9) have been committed to the -STABLE branches and are available on the errata pages: http://www.openbsd.org/errata57.html http://www.openbsd.org/errata58.html http://www.openbsd.org/errata59.html _______________________________________________ openssh-unix-dev mailing list openssh-unix-dev@xxxxxxxxxxx https://lists.mindrot.org/mailman/listinfo/openssh-unix-dev
