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posted by Fnord666 on Monday July 03 2017, @10:12PM   Printer-friendly
from the it's-a-feature dept.

Submitted via IRC for TheMightyBuzzard

A bug in Linux's systemd init system causes root permissions to be given to services associated with invalid usernames, and while this could pose a security risk, exploitation is not an easy task.

A developer who uses the online moniker "mapleray" last week discovered a problem related to systemd unit files, the configuration files used to describe resources and their behavior. Mapleray noticed that a systemd unit file containing an invalid username – one that starts with a digit (e.g. "0day") – will initiate the targeted process with root privileges instead of regular user privileges.

Systemd is designed not to allow usernames that start with a numeric character, but Red Hat, CentOS and other Linux distributions do allow such usernames.

"It's systemd's parsing of the User= parameter that determines the naming doesn't follow a set of conventions, and decides to fall back to its default value, root," explained developer Mattias Geniar.

While this sounds like it could be leveraged to obtain root privileges on any Linux installation using systemd, exploiting the bug in an attack is not an easy task. Geniar pointed out that the attacker needs root privileges in the first place to edit the systemd unit file and use it.

[...] Systemd developers have classified this issue as "not-a-bug" and they apparently don't plan on fixing it. Linux users are divided on the matter – some believe this is a vulnerability that could pose a serious security risk, while others agree that a fix is not necessary.

See, this is why we can't have nice init systems.

Source: http://www.securityweek.com/linux-systemd-gives-root-privileges-invalid-usernames


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  • (Score: 2) by frojack on Tuesday July 04 2017, @01:21AM (12 children)

    by frojack (1554) Subscriber Badge on Tuesday July 04 2017, @01:21AM (#534628) Journal

    Using a normal, valid username in one of the systemd config files, in a field that takes a username, will cause that username to effectively get root.

    No, that's not what happens here.
    Using an invalid username causes that unit to run as root. That unit can do anything.
    Using a valid user name causes that unit to run as that (valid) user. The unit can't do anything the user couldn't do.

    Only root can instantiate a new unit.

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  • (Score: 3, Disagree) by Arik on Tuesday July 04 2017, @01:32AM (9 children)

    by Arik (4543) on Tuesday July 04 2017, @01:32AM (#534631) Journal
    Systemd may define it as not a valid username, however systemd is not the judge of this and no one gives a fsck what Poettering thinks, it's simply not his call. There's nothing 'invalid' about a *nix username starting with a null and many, many have done so.

    --
    If laughter is the best medicine, who are the best doctors?
    • (Score: 2) by isostatic on Tuesday July 04 2017, @02:20PM (3 children)

      by isostatic (365) on Tuesday July 04 2017, @02:20PM (#534796) Journal

      no one gives a fsck what Poettering thinks, it's simply not his call

      Sadly that's not true. I don't know why, but for some reason what he decides goes. Time and time again he causes a horrendous mess, and time and time again we follow him. He has an obscene amount of power - on the order of that of Linus. What he says goes, that's the new reality.

      • (Score: 1) by Arik on Tuesday July 04 2017, @02:51PM (2 children)

        by Arik (4543) on Tuesday July 04 2017, @02:51PM (#534804) Journal
        I get your point, but I think you missed mine.

        Not even the distro he works for actually considers these filenames invalid.
        --
        If laughter is the best medicine, who are the best doctors?
        • (Score: 0) by Anonymous Coward on Tuesday July 04 2017, @02:53PM

          by Anonymous Coward on Tuesday July 04 2017, @02:53PM (#534805)
          Meant usernames not files obviously.
        • (Score: 2) by isostatic on Tuesday July 04 2017, @06:51PM

          by isostatic (365) on Tuesday July 04 2017, @06:51PM (#534883) Journal

          Sure, but that's effectively changed now. As systemd is so ingrained into linux, and pottering is in charge, if he decides that "usernames beginging with a number aren't valid", that makes them defacto invalid.

          Technically you're correct, but in practice mroe and more of systemd will refuse to work with usernames begining with numbers, and it will become the defacto standard.

    • (Score: 2) by butthurt on Tuesday July 04 2017, @08:23PM (4 children)

      by butthurt (6141) on Tuesday July 04 2017, @08:23PM (#534913) Journal

      > There's nothing 'invalid' about a *nix username starting with a null and many, many have done so.

      I disagree. A commenter on Stack Exchange wrote that the allowable characters (according to IEEE Std 1003.1-2001) are:

      The set of characters from which portable filenames are constructed.

      A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
      a b c d e f g h i j k l m n o p q r s t u v w x y z
      0 1 2 3 4 5 6 7 8 9 . _ -

      -- https://www.linuxquestions.org/questions/linux-server-73/valid-user-names-573768/ [linuxquestions.org]

      The summary mentions "usernames that start with a numeric character." A null character is not a numeric character. It's a control character. Perhaps you're confusing it with the zero symbol, "0"?

      https://en.wikipedia.org/wiki/Null_character [wikipedia.org]

      • (Score: 1) by Arik on Wednesday July 05 2017, @12:27AM (3 children)

        by Arik (4543) on Wednesday July 05 2017, @12:27AM (#534990) Journal
        C defines things differently and that's fine, but the normal meaning of the word 'null' is precisely the same as 'zero.'
        --
        If laughter is the best medicine, who are the best doctors?
        • (Score: 2) by FatPhil on Wednesday July 05 2017, @08:06AM (1 child)

          by FatPhil (863) <{pc-soylent} {at} {asdf.fi}> on Wednesday July 05 2017, @08:06AM (#535098) Homepage
          Not just C, Unix - you stepped on both toes.
          --
          Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
          • (Score: 1) by Arik on Wednesday July 05 2017, @05:23PM

            by Arik (4543) on Wednesday July 05 2017, @05:23PM (#535281) Journal
            You got me, my native tongue is English, and I don't worship Kernighan and Ritchie.
            --
            If laughter is the best medicine, who are the best doctors?
        • (Score: 2) by butthurt on Thursday July 06 2017, @12:37AM

          by butthurt (6141) on Thursday July 06 2017, @12:37AM (#535473) Journal

          In the context of character sets in computing, they have distinct meanings. In the character set used for POSIX usernames, a printable "0" is different from the non-printable NUL.

          https://en.wikipedia.org/wiki/File:US-ASCII_code_chart.png [wikipedia.org]

          If you refer to "0" as a null, you're just going to cause confusion.

  • (Score: 2, Informative) by Anonymous Coward on Tuesday July 04 2017, @01:38AM

    by Anonymous Coward on Tuesday July 04 2017, @01:38AM (#534632)

    Using a valid user name causes that unit to run as that (valid) user. The unit can't do anything the user couldn't do.

    This is false. If the username is "1strike" it is a valid Linux username but not to systemd so it gets root privileges. If the username is invalid, or starts with a number (which systemd says is bad), it runs as root.

  • (Score: 0) by Anonymous Coward on Tuesday July 04 2017, @01:41AM

    by Anonymous Coward on Tuesday July 04 2017, @01:41AM (#534633)

    Using a valid user name causes that unit to run as that (valid) user.

    You are wrong. From the article:

    Systemd is designed not to allow usernames that start with a numeric character, but Red Hat, CentOS and other Linux distributions do allow such usernames.

    Perfectly *valid* usernames ("0day") are *invalid* in crappyd. Yet another case of Poopering and his cronies getting things wrong and failing to support perfectly legal semantics. The rest of the system doesn't care if your username starts with a digit but somehow systemd does care.