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Bug 702488 (CVE-2019-18276) - app-shells/bash: when effective UID is not equal to its real UID the saved UID is not dropped (CVE-2019-18276)
Summary: app-shells/bash: when effective UID is not equal to its real UID the saved UI...
Status: IN_PROGRESS
Alias: CVE-2019-18276
Product: Gentoo Security
Classification: Unclassified
Component: Vulnerabilities (show other bugs)
Hardware: All Linux
: Normal normal (vote)
Assignee: Gentoo Security
URL:
Whiteboard: A3 [upstream/ebuild]
Keywords: PullRequest
Depends on:
Blocks:
 
Reported: 2019-12-10 18:52 UTC by Hank Leininger
Modified: 2019-12-10 20:20 UTC (History)
1 user (show)

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Package list:
Runtime testing required: ---


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Description Hank Leininger 2019-12-10 18:52:01 UTC
CVE-2019-18276 (https://nvd.nist.gov/vuln/detail/CVE-2019-18276):

  An issue was discovered in disable_priv_mode in shell.c in GNU Bash through
  5.0 patch 11. By default, if Bash is run with its effective UID not equal to
  its real UID, it will drop privileges by setting its effective UID to its real
  UID. However, it does so incorrectly. On Linux and other systems that support
  "saved UID" functionality, the saved UID is not dropped. An attacker with
  command execution in the shell can use "enable -f" for runtime loading of a
  new builtin, which can be a shared object that calls setuid() and therefore
  regains privileges. However, binaries running with an effective UID of 0 are
  unaffected.

Note I think actual exposure to this should be minor, because setuid bash is a bad idea anyway, and the normal way for a privileged process (sshd, /bin/login, etc.) to launch bash as a user is to fork, drop privs, and then exec.  If they do so correctly, bash won't have any privs to misuse; if they do not, that is a bug in them.

There is a patch sitting in bash's devel branch, mixed in with other changes:

https://github.com/bminor/bash/commit/951bdaad7a18cc0dc1036bba86b18b90874d39ff

The latest release version, 5.0_p11, does not include the fix (despite p11 being released well after that fix was added to devel).

I will submit a PR that includes a trimmed-down version of this patch to include only the relevant fixes.