CentOS - Feb 2017 - Serious attack vector on pkcheck ignored by Red Hat

On Thu, 2017-02-02 at 13:40 -0800, Gordon Messmer wrote:
> Escalation *requires* attacking a program in a security context other 
> than your own.
Not necessarily. Suppose the adversary is aware of a root
exploit/privilege escalation in a random library. Then the heap spraying
allows this attacker to easily trigger this exploit because he is able
to initialize the entire contents of the heap to his liking and thus
call whatever function he likes, including the one that will cause the
root exploit.

So even though the heap spraying is not an attack in itself it is a
serious "crow bar" i.e. attack vector.

If you read the article carefully the author makes no claims that the
setuid on the binary is a necessity. He clearly states he is "giving
himself a break" by using a setuid binary.

Regards,
Leonard.

-- 
mount -t life -o ro /dev/dna /genetic/research

On 2/9/2017 1:03 PM, Leonard den Ottolander wrote:
> Not necessarily. Suppose the adversary is aware of a root
> exploit/privilege escalation in a random library. Then the heap spraying
> allows this attacker to easily trigger this exploit because he is able
> to initialize the entire contents of the heap to his liking and thus
> call whatever function he likes, including the one that will cause the
> root exploit.
if the adversary is aware of this exploit and has a login (required to 
invoke pkexec in the first place), they can simply execute a C program 
to invoke it, they don't need to mess about with what you're describing.




-- 
john r pierce, recycling bits in santa cruz

On Feb 9, 2017, at 2:03 PM, Leonard den Ottolander <leonard at
den.ottolander.nl> wrote:
> 
> On Thu, 2017-02-02 at 13:40 -0800, Gordon Messmer wrote:
>> Escalation *requires* attacking a program in a security context other 
>> than your own.
> 
> Not necessarily. Suppose the adversary is aware of a root
> exploit/privilege escalation in a random library.
There are two serious problems with this argument:

1.  Give me a scenario where this attacker can execute *only* pkcheck in order
to exploit this hypothetical library?s flaw, but where the attacker cannot
simply provide their own binary to do the same exploit.  Short of something
insane like exposing pkcheck via CGI over HTTP, I don?t see how a flaw in
pkcheck gives you something here that you don?t already have.

A vulnerable library is a vulnerable library.  Fix the library, don?t invent
reasons to fix all the other programs on the system because the library is
vulnerable.

2.  There?s no such thing as SUID libraries.  So, how is this hypothetical
library of yours going to gain privileges that the executable linked to it does
not have?  Point me at a CVE where a vulnerable library was used for privilege
escalation.

You can point at vulnerable libraries giving data exfiltration and such all day
long, but privilege escalation??

Hello Warren,

On Thu, 2017-02-09 at 14:22 -0700, Warren Young wrote:
> There are two serious problems with this argument:
> 
> 1.  Give me a scenario where this attacker can execute *only* pkcheck
> in order to exploit this hypothetical library?s flaw, but where the
> attacker cannot simply provide their own binary to do the same
> exploit.  Short of something insane like exposing pkcheck via CGI over
> HTTP, I don?t see how a flaw in pkcheck gives you something here that
> you don?t already have.
On many systems local users cannot execute their own uploaded binaries
(noexec mounts). This would also be true for an adversary entering a
system with a remote "unprivileged" exploit. In that situation pcheck
gives them a "crow bar" they did not have before.
> A vulnerable library is a vulnerable library.  Fix the library, don?t
> invent reasons to fix all the other programs on the system because the
> library is vulnerable.
I would say the modus operandi should be to eliminate all known attack
vectors, including such a powerful one as the described "heap
spraying".
> 2.  There?s no such thing as SUID libraries.
I never argued there are.
>   So, how is this hypothetical library of yours going to gain
> privileges that the executable linked to it does not have?  Point me
> at a CVE where a vulnerable library was used for privilege escalation.
Maybe the example using a library is wrong. But there are other ways to
gain a privilege escalation, kernel bugs for example. Those could be
triggered just as well.

Regards,
Leonard.


-- 
mount -t life -o ro /dev/dna /genetic/research

On 02/09/2017 01:03 PM, Leonard den Ottolander wrote:
>
> Not necessarily. Suppose the adversary is aware of a root
> exploit/privilege escalation in a random library.
There is no such thing as a root exploit in a library.  A "root
exploit"
is one that ends with the attacker executing code as root. That can only 
be done by attacking an existing process running as root, or by 
attacking a SUID binary.

There are certainly flaws in libraries that lead to code execution, but 
the exploit will be described in terms of the entire system that enables 
privilege escalation, which means the attack will be in the context of a 
daemon, or a SUID binary.
> If you read the article carefully the author makes no claims that the
> setuid on the binary is a necessity. He clearly states he is "giving
> himself a break" by using a setuid binary.
No, he doesn't.  He gave himself a break by attacking a 32-bit binary 
instead of the 64 bit one, because causing the heap and the stack to 
collide in a 64 bit address space is a *lot* more work.
> This would also be true for an adversary entering a
> system with a remote "unprivileged" exploit. In that situation
pcheck
> gives them a "crow bar" they did not have before.
pkcheck doesn't escalate privileges.  If they are able to launch a 
remote exploit which calls pkcheck, they can launch a remote exploit 
that just calls a shell directly instead.  Attacking pkcheck doesn't 
give them anything extra.

Does it not tell you anything that *everyone* is telling you that there 
is no security exploit in the ability to cause pkcheck to execute code?  
It's a bug, but it's not a security flaw.