how to gain privilege

This is a discussion on how to gain privilege within the Linux Programming forums, part of the Platform Specific Boards category; File /etc/shadow belongs to root, and access permission is set to 400 or "-r--------" (unless chmod). So it can't be ...

  1. #1
    still a n00b Jaguar's Avatar
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    how to gain privilege

    File /etc/shadow belongs to root, and access permission is set to 400 or "-r--------" (unless chmod).
    So it can't be modified by even root.
    But when I login as a normal user (UID 500 up) and I change password with the command /usr/bin/passwd, file /etc/shadow is consequently changed.
    I think such binary can gain privilege over even root.
    I question how to make such bin with gcc/g++?
    slackware 10.0; kernel 2.6.7
    gcc 3.4.0; glibc 2.3.2; vim editor
    migrating to freebsd 5.4

  2. #2
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    Waaaaaaaait a second... there is no restriction to what root can do!

    I just tried chmod'ing a file to 400... root could still edit it fine.

    Sounds like there's another issue with your system, but I'm far from a linux expert.

  3. #3
    End Of Line Hammer's Avatar
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    /etc/shadow is supposed to be accessed by privilaged users only. You should grant access to it lightly.

    >>I don't know why that file is only readable by root
    Because it is supposed to hold the users passwords (encrypted), its "safer" than storing them in /etc/passwd which must be globally accessable. shadow
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  4. #4
    End Of Line Hammer's Avatar
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    >>Yeah, I know what Shadow passwords are,
    I thought you would I was just picking up on:
    >I don't know why that file is only readable by root
    and
    >Debian machine and it's rw root, r other

    >>but why should the file not be writeable by root
    Don't know, that's a strange one. Maybe it's one extra level of "security" to frighten a newbie haxor... yes it's lame (and pointless), but it's all I can think of!
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  5. #5
    End Of Line Hammer's Avatar
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    Makes sense now
    http://www.debian.org/doc/manuals/se...to/ch3.en.html
    /etc/shadow
    Only the root user and the group shadow have read access to this file,
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  6. #6
    still a n00b Jaguar's Avatar
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    I did not change access permission, -r-------- is default one.
    Anyway thank for all hints.

    But I still wnder when I login as a normal user, I can use /usr/bin/passwd to change /etc/shadow, which is not writeable for normal users.
    slackware 10.0; kernel 2.6.7
    gcc 3.4.0; glibc 2.3.2; vim editor
    migrating to freebsd 5.4

  7. #7
    End Of Line Hammer's Avatar
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    Originally posted by Jaguar
    But I still wnder when I login as a normal user, I can use /usr/bin/passwd to change /etc/shadow, which is not writeable for normal users.
    Look at the permissions on /usr/bin/passwd. What are they set to?
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  8. #8
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    I could be wrong, but I do not belive suid allows a program to 'set' it's UID to 0, it's rather given the UID of 0. A program does not have to be aware if it is suid or not, you can suid any program and it will be given super users priveleges.

    On a side note, besides the given suid apps (passwd being one of them), sudo is a great tool to give specific users the ability to run suid apps.

    (Just thought I'd add my 2 cents)

  9. #9
    Comment your source code! Lynux-Penguin's Avatar
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    I run a few linux boxes some redhat etc. And all of them
    have

    -r-------- root root ***** /etc/shadow

    note: no matter what, root can change anything ANYTHING. Like it was the kernel or in control of the kernel. If it can't change something then it's not Unix.
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  10. #10
    End Of Line Hammer's Avatar
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    Do you really think they're still looking nearly 3 months later...?
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