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Date: Fri, 16 Dec 2016 19:01:54 +0100
From: Florent Rougon <f.rougon@...e.fr>
To: oss-security@...ts.openwall.com
Subject: Re: CVE Request: FlightGear: Allows the route manager to overwrite arbitrary files

[ This is in reply to Salvatore Bonaccorso's mail from Wed, 14 Dec 2016
  16:57:11 +0100, i.e.
  <http://www.openwall.com/lists/oss-security/2016/12/14/11>,
  unfortunately I don't have its Message-ID to reply in the same thread
  (just subscribed a few hours ago). ]

Hello,

As already written in private to Salvatore and maintainers of a few
distributions, it is quite unclear to me how to achieve arbitrary code
execution using this vulnerability. The reason I'm saying this is that
the bug allows an attacker to choose which user-writable files he wants
to overwrite, but *not their contents*, at least not freely at all. The
actual writing is not done by Nasal code but by the Route manager's C++
code, which doesn't seem to give much freedom as to the contents being
written (Nasal code can only *trigger* the flightplan writing).

Here is how the flightplan is saved
(flightgear/src/Autopilot/route_mgr.cxx, code from FlightGear's 'next'
branch):

  bool FGRouteMgr::saveRoute(const SGPath& p)
  {
    if (!_plan) {
      return false;
    }

    return _plan->save(p);
  }

calling (flightgear/src/Navaids/FlightPlan.cxx):

  bool FlightPlan::save(const SGPath& path)
  {
    SG_LOG(SG_NAVAID, SG_INFO, "Saving route to " << path);
    try {
      SGPropertyNode_ptr d(new SGPropertyNode);
      d->setIntValue("version", 2);

      if (_departure) {
        d->setStringValue("departure/airport", _departure->ident());
        if (_sid) {
          d->setStringValue("departure/sid", _sid->ident());
        }

        if (_departureRunway) {
          d->setStringValue("departure/runway", _departureRunway->ident());
        }
      }

      if (_destination) {
        d->setStringValue("destination/airport", _destination->ident());
        if (_star) {
          d->setStringValue("destination/star", _star->ident());
        }

        if (_approach) {
          d->setStringValue("destination/approach", _approach->ident());
        }

        //d->setStringValue("destination/transition", destination->getStringValue("transition"));

        if (_destinationRunway) {
          d->setStringValue("destination/runway", _destinationRunway->ident());
        }
      }

      // route nodes
      SGPropertyNode* routeNode = d->getChild("route", 0, true);
      for (unsigned int i=0; i<_legs.size(); ++i) {
        Waypt* wpt = _legs[i]->waypoint();
        wpt->saveAsNode(routeNode->getChild("wp", i, true));
      } // of waypoint iteration
      writeProperties(path, d, true /* write-all */);
      return true;
    } catch (sg_exception& e) {
      SG_LOG(SG_NAVAID, SG_ALERT, "Failed to save flight-plan '" << path << "'. " << e.getMessage());
      return false;
  }

calling [1] and [2] with:

[1] (flightgear/src/Navaids/route.cxx):

  void Waypt::saveAsNode(SGPropertyNode* n) const
  {
    n->setStringValue("type", type());
    writeToProperties(n);
  }

calling (flightgear/src/Navaids/route.cxx):

  void Waypt::writeToProperties(SGPropertyNode_ptr aProp) const
  {
    if (flag(WPT_OVERFLIGHT)) {
      aProp->setBoolValue("overflight", true);
    }

    if (flag(WPT_DEPARTURE)) {
      aProp->setBoolValue("departure", true);
    }

    if (flag(WPT_ARRIVAL)) {
      aProp->setBoolValue("arrival", true);
    }

    if (flag(WPT_APPROACH)) {
      aProp->setBoolValue("approach", true);
    }

    if (flag(WPT_MISS)) {
      aProp->setBoolValue("miss", true);
    }

    if (flag(WPT_GENERATED)) {
      aProp->setBoolValue("generated", true);
    }

    if (_altRestrict != RESTRICT_NONE) {
      aProp->setStringValue("alt-restrict", restrictionToString(_altRestrict));
      aProp->setDoubleValue("altitude-ft", _altitudeFt);
    }

    if (_speedRestrict != RESTRICT_NONE) {
      aProp->setStringValue("speed-restrict", restrictionToString(_speedRestrict));
      aProp->setDoubleValue("speed", _speed);
    }
  }

(not very interesting IMHO), the actual writing to file being done
above in FlightPlan::save() by

[2] (simgear/props/props_io.cxx):

  void
  writeProperties (const SGPath &path, const SGPropertyNode * start_node,
                   bool write_all, SGPropertyNode::Attribute archive_flag)
  {
    SGPath dpath(path);
    dpath.create_dir(0755);

    ofstream output(path.local8BitStr().c_str());
    if (output.good()) {
      writeProperties(output, start_node, write_all, archive_flag);
    } else {
      throw sg_io_exception("Cannot open file", sg_location(path.utf8Str()));
    }
  }

which relies on (same file):

  void
  writeProperties (ostream &output, const SGPropertyNode * start_node,
                   bool write_all, SGPropertyNode::Attribute archive_flag)
  {
    int nChildren = start_node->nChildren();

    output << "<?xml version=\"1.0\"?>" << endl << endl;
    output << "<PropertyList>" << endl;

    for (int i = 0; i < nChildren; i++) {
      writeNode(output, start_node->getChild(i), write_all, INDENT_STEP, archive_flag);
    }

    output << "</PropertyList>" << endl;
  }

...

All this to say that the *contents* written to an arbitrary file is
rather constrained, unless I missed something, and that an attacker has
very little control over it. This contents is a flightplan in
FlightGear's XML PropertyList format
(<http://wiki.flightgear.org/PropertyList_XML_files>) and the attacker
basically only gets to choose the departure, arrival and intermediate
waypoints... which gives something as the LFPO-EDDF.xml file I am
attaching to this mail.

So, from my POV, an attacker can:
  - destroy any user-writable file he wants (not remove it, but
    overwrite it with a flightplan in XML format)
    -> data loss
  - because of this, cause software malfunctions
    -> “DoS”

But AFAICT, the attacker has *very little control* over the kinds of
malfunctions he can cause, thus it is unclear to me how to go from the
vulnerability to arbitrary code execution.

Of course, I might have missed something and am ready to be educated if
this happens to be the case; though, as Salvatore said in private mail
inviting me to post this here, the mention of arbitrary code execution
in his post was rather the result of a misunderstanding (no worries).

Regards

-- 
Florent

Download attachment "LFPO-EDDF.xml" of type "application/xml" (2176 bytes)

Download attachment "signature.asc" of type "application/pgp-signature" (833 bytes)

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