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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: 1) by pTamok on Tuesday July 04 2017, @11:15AM (4 children)

    by pTamok (3042) on Tuesday July 04 2017, @11:15AM (#534762)

    Part of the issue here is the differences in opinion of what constitutes a valid username. Are leading digits allowed or not?
    Similarly, should the code in question check if the username is actually present on the system it is being executed on, as a username can be valid, but non-existent on the system.
    As I understand it, in either of the above cases, the code defaults to running as root.

    Now, I am not a Linux programmer, so I don't know if there is a standard routine available for checking the validity of a username, or even if a generally agreed upon standard exists. Either way, I don't think is is correct for code that does not have a major function of username processing to implement its own idiosyncratic standard for username validity. At the very least, I would expect the code to check if the username requested actually exists on the system (I understand some might see this as a security breach*) - because in such a case, if the username exists, you can probably fairly safely assume that it is regarded as valid by the rest of the system.

    *The obvious approach is to call a function, offering a username and returning a flag to say of that username is present in /etc/passwd (for added fun, also check that the given username is in a group allowed to run the desired script). The obviously obvious brain dead approach of allowing read-only access to the entire /etc/passwd and searching it yourself, while tempting for neophytes, would be silly. By checking usernames one at a time against the oracle, rather than just reading the file, it makes exploiting the function to get a list of usernames time-consuming.

  • (Score: 2) by ledow on Tuesday July 04 2017, @11:40AM (2 children)

    by ledow (5567) on Tuesday July 04 2017, @11:40AM (#534767) Homepage

    The problem is also one of username mapping. Just because you're fredbloggs on one machine doesn't mean you're allowed access to anything of fredbloggs on another that happens to have the same username. Do you go by username, or by user ID (which can be mapped differently but it was is encoded on the filesystem for permissions).

    At minimum, checking the username is valid using a standardised piece of library code is a no-brainer, rather than making up your own rules.

    But just because a specified username exists or not, and acting in a poor way after that point, is just terrible. Why would you do that? Error and fail, or carry on if you're happy as the specified user.

    It reeks of "coding for convenience", where if you move a systemd install that doesn't end up on a machine with the right packages/user, rather than error, it just ignores it and carries on as root.

    This reinvention-and-redefinition of the wheel is the main reason that something huge, monolithic, and with so many fingers in so many pies, like systemd does, is stupendously bad.

    • (Score: 2) by Justin Case on Tuesday July 04 2017, @02:59PM (1 child)

      by Justin Case (4239) on Tuesday July 04 2017, @02:59PM (#534806) Journal

      (Grandparent)

      what constitutes a valid username

      (Parent)

      checking the username is valid using a standardised piece of library code is a no-brainer

      If the user ID exists it is evidently valid. That decision has been made by the creating-user-ID-thingy. Why would you expect to re-implement that logic elsewhere, to gain nothing?

      If the user ID doesn't exist you probably don't want to go spinning up processes under that ID.

      So all that is needed is to check if the supplied string is an existing user.

      • (Score: 2) by PocketSizeSUn on Tuesday July 04 2017, @08:42PM

        by PocketSizeSUn (5340) on Tuesday July 04 2017, @08:42PM (#534923)

        man 3 getpwnam_r

        CONFORMING TO
                      POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD. The pw_gecos field is not specified in POSIX, but is present on
                      most implementations.

        NOTE
                      The user password database mostly refers to /etc/passwd. However, with recent systems it also refers to net‐
                      work wide databases using NIS, LDAP and other local files as configured in /etc/nsswitch.conf.

  • (Score: 2) by rleigh on Thursday July 06 2017, @07:28AM

    by rleigh (4887) on Thursday July 06 2017, @07:28AM (#535614) Homepage

    The only thing a programmer needs to do is call getpwnam: http://pubs.opengroup.org/onlinepubs/9699919799/functions/getpwnam.html [opengroup.org] an if it returns a value, it's valid.

    systemd's problem is also trying to set policy. Plus some awful noddy bugs. Naming policy could come from the local admin or your sidewide LDAP/AD. It's way out of scope for systemd.