[REVS] Second-Order Symlink Vulnerabilities

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Second-Order Symlink Vulnerabilities
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SUMMARY

Recently, Eric Romang of ZATAZ Audits reported several symlink issues that
are different than the usual symlink vulnerabilities [1] [2]. There are
probably a large number of applications that are safe with respect to
traditional symlink problems, but vulnerable to this particular variant.

Specifically, the problem arises when one program, "PARENT," invokes
another program, "CHILD," in which:
- The CHILD has a "standalone" design, i.e. its normal mode of operation
is to be run by an interactive user, or a script on behalf of the user

- The CHILD does not run with more privileges than the user that invokes
it, e.g. it is not setuid

- The CHILD program assumes that the user calling the program has control
over all files that are specified as arguments, i.e. the specified
filenames are trusted

- The CHILD program follows symlinks

- The PARENT uses filenames that are passed as arguments to the CHILD

- The filenames as used by the PARENT are:
- Controllable, or predictable, by the attacker, and
- In a directory that's writable by the attacker;

The attacker could then use a symlink attack on the filename arguments
that the PARENT passes to the CHILD.
This variant might be referred to as a "Second Order Symlink
Vulnerability".

DETAILS

Note that the four conditions for the CHILD, if treated alone, are not
normally regarded as a security vulnerability: there aren't any privilege
boundaries being crossed, and the filenames are under the control of the
user.

It's the interaction between the PARENT and the CHILD that becomes the
problem. There is a strong argument that it is the PARENT's responsibility
to protect against the second-order symlink vulnerability, since "fixing"
the child could significantly reduce
functionality in common standalone uses. However, there may be some cases
in which the PARENT does not have intrinsic knowledge of which CHILD will
be invoked at runtime.

Note that from the CHILD's perspective, the attack vectors do NOT involve
temporary files.

Current code scanning tools may not locate second-order symlink
vulnerabilities when scanning the PARENT, because the affected
fopen/create/etc. function call may not be in the PARENT at all, but in
the CHILD.

Examples:
Romang's reports for everybuddy and LutelWall both deal with the case in
which wget is the CHILD, and the PARENT specifies an output file to wget
using the "-O" argument.

It must be emphasized that the vulnerability is NOT in wget; as previously
discussed, it is in the interaction between wget and the process that
invokes it.

For LutelWall, we have:
echo `wget -C off -O $tmp-newfeat -q -t 1 -T 3 -w 3
http://firewall.lutel.pl/FEATURES-${new_ver}`

For everybuddy, we have:
258 g_snprintf(buf, 2048, "rm /tmp/.eb.%s.translator -f ; wget -O /tmp/.eb.%s.translator 'http://world.altavista.com/sites/gben/pos/babelfish/tr?
tt=urltext&lp=%s_%s&urltext=%s'",
259 getenv("USER"), getenv("USER"), from, to, string);
...
...
263 if(system(buf)!=0)

ADDITIONAL INFORMATION

References:
[1] "everybuddy <= 0.4.3 insecure temporary file creation" June 6, 2005
<http://www.zataz.net/adviso/everybuddy-06062005.txt>
http://www.zataz.net/adviso/everybuddy-06062005.txt
[2] "LutelWall <= 0.97 insecure temporary file creation", June 6, 2005
<http://www.zataz.net/adviso/lutelwall-05222005.txt>
http://www.zataz.net/adviso/lutelwall-05222005.txt
[3] "Second Order Code Injection Attacks" NGS Software
<http://www.nextgenss.com/papers/SecondOrderCodeInjection.pdf>
http://www.nextgenss.com/papers/SecondOrderCodeInjection.pdf

The information has been provided by <mailto:coley@mitre.org> coley.

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