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82 lines
2.7 KiB
Markdown
82 lines
2.7 KiB
Markdown
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# Linux - Evasion
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## Summary
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- [File names](#file-names)
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- [Command history](#command-history)
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- [Hiding text](#hiding-text)
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## File Names
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An Unicode zero-width space can be inserted into filenames which makes the names visually indistinguishable:
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```bash
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# A decoy file with no special characters
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touch 'index.php'
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# An imposter file with visually identical name
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touch $'index\u200D.php'
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```
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## Command History
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Most shells save their command history so a user can recall them again later. The command history can be viewed with the `history` command or by manually inspecting the contents of the file pointed to by `$HISTFILE` (e.g. `~/.bash_history`).
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This can be prevented in a number of ways.
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```bash
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# Prevent writing to the history file at all
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unset HISTFILE
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# Don't save this session's command history in memory
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export HISTSIZE=0
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```
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Individual commands that match a pattern in `HISTIGNORE` will be excluded from the command history, regardless of `HISTFILE` or `HISTSIZE` settings.
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By default, `HISTIGNORE` will ignore all commands that begin with whitespace:
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```bash
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# Note the leading space character:
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my-sneaky-command
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```
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If commands are accidentally added to the command history, individual command entries can be removed with `history -d`:
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```bash
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# Removes the most recently logged command.
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# Note that we actually have to delete two history entries at once,
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# otherwise the `history -d` command itself will be logged as well.
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history -d -2 && history -d -1
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```
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The entire command history can be purged as well, although this approach is much less subtle and very likely to be noticed:
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```bash
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# Clears the in-memory history and writes the empty history to disk.
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history -c && history -w
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```
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## Hiding Text
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ANSI escape sequences can be abused to hide text under certain circumstances.
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If the file's contents are printed to the terminal (e.g. `cat`, `head`, `tail`) then the text will be hidden.
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If the file is viewed with an editor (e.g. `vim`, `nano`, `emacs`), then the escape sequences will be visible.
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```bash
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echo "sneaky-payload-command" > script.sh
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echo "# $(clear)" >> script.sh
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echo "# Do not remove. Generated from /etc/issue.conf by configure." >> script.sh
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# When printed, the terminal will be cleared and only the last line will be visible:
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cat script.sh
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```
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## References
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- [ATT&CK - Impair Defenses: Impair Command History Logging](https://attack.mitre.org/techniques/T1562/003/)
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- [ATT&CK - Indicator Removal on Host: Clear Command History](https://attack.mitre.org/techniques/T1070/003/)
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- [ATT&CK - Masquerading: Match Legitimate Name or Location](https://attack.mitre.org/techniques/T1036/005/)
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- [Wikipedia - ANSI escape codes](https://en.wikipedia.org/wiki/ANSI_escape_code)
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