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This issue affects Screen 5.0.0 when it runs with setuid-root privileges. The function logfile_reopen() does not drop privileges while operating on a user supplied path. This allows unprivileged users to create files in arbitrary locations with root ownership, the invoking user’s (real) group ownership and file mode 0644. All data written to the Screen PTY will be logged into this file. Also already existing files can be abused for logging in this manner: the data will be appended to the file in question, but the file mode and ownership will be left unchanged. |
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Screen correctly drops privileges when it initially opens the logfile. The privilege escalation becomes possible as soon as Screen believes it is necessary to reopen the logfile. Screen checks this by calling stolen_logfile() before writing to the file. The call to logfile_reopen() happens when the link count of the originally opened logfile drops to zero, or if it unexpectedly changes in size. This condition can be triggered at will on the end of the unprivileged user. |
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