You are not wrong -- I just put this issue into the unlikely to happen 
category.  If it was higher level and easy to do I might have another opinion.  
This is like getting past the Dobermans :-)

I do have a funny story.  I had a customer with a simple firewall (basically IP 
rules) that cost a few hundred dollars.  They were never hacked in over a 
decade.  But, someone told them they had a theoretical security problem (e.g. 
in some insane case someone might hack them -- although they only sell T-Shirts 
so what is the point?).  

So they bought the latest and greatest 30k Cisco Firewall.  And were promptly 
and successfully hacked that week.  It seems that this new Cisco was a 
challenge that interested the hacker, whereas grinding through IP spoofing and 
passwords is just dull.

Luckily he was not malicious -- he just made the log in screens display "I was 
here" before executing the application and it was easy to remove.

Security is a funny issue.


-----Original Message-----
From: Aleksey Sanin [] 
Sent: Tuesday, October 29, 2019 3:41 PM
To: Eric Eberhard <>; 'Webb Scales' <>; 
'Raphael de Carvalho Muniz' <>
Subject: Re: [xml] Research about vulnerabilities

People do attack XML parsing (as well as any other input), the encryption on 
the wire doesn't stop a malicious client from crafting special input and 
sending it to the server.

I did a cursory look at the code and I believe Nick is correct that the 
function in question is never called with a user-input for the format string. 
Thus, I don't believe this particular issue is really a vulnerability.

LibXML2 was tested by Coverity (last test from 2018 build):

It might make sense to upload a fresh build and see what Coverity finds. Looks 
like in 2018 it found a few issues (though hard to say how real are those 
without deeper look).



On 10/29/19 3:05 PM, Eric Eberhard wrote:
> I agree.  I also don’t think people attack XML parsing.  The 
> sending/receiving can be done encrypted.  This seems a lot like a 
> theoretical problem, not a real-world problem.
> My feelings are that protecting against all possible attacks is not 
> possible.  Or stupid programming.
> Take the physical world.  Maybe you just lock your house.  Or maybe 
> you have an alarm system.  Or maybe locked gates.  Maybe patrolled by 
> Dobermans.  Maybe patrolled by armed guards with machine guns.
> My point is – people can pick locks.  Defeat alarms.  Scale gates.  
> Kill dogs.  Kill guards.  It can get silly.  Maybe one should make an 
> underground compound with fingerprint and retina scans as well.
> You have to stop it someplace.  If this was CIA or FBI or something 
> I’d go along with the complaint.  But most of us use this in a 
> business setting and we apply *reasonable *security measures, not 
> insanely overkill measures.
> Eric
> *From:*xml [] *On Behalf Of *Webb Scales
> *Sent:* Tuesday, October 29, 2019 7:18 AM
> *To:* Raphael de Carvalho Muniz <>
> *Cc:*
> *Subject:* Re: [xml] Research about vulnerabilities
> Raphael,
> First, the disclaimers:  I'm not an XML maintainer or even a 
> contributor; and, I've only given this a cursory glance.
> Here are my reactions.
> First, the routine in question is declared to be of module static 
> scope.  I believe that this means that any exploitation of it would 
> have to be indirect, coming from within LibXML2.
> Second, the code appears to properly protect against buffer overflow, 
> so the only exploitation that I see is the one that you illustrated, 
> leaking memory position information.  However, the function in 
> question does nothing that its caller could not do -- it calls 
> malloc() and (in the example you provided) returns the address of what 
> might be a stack location (although, really, it could be any random 
> junk, I think).  By definition, the caller already knows the stack 
> location, and the caller certainly has its own access to malloc().
> So, I for one, don't see any weakness exposed by the code which you 
> provided.  This doesn't mean that there is no weakness in LibXML2, 
> but, to find it if it's there, you'll have to interrogate the code 
> which calls this routine.
>                  Webb
> On 10/29/19 9:30 AM, Raphael de Carvalho Muniz wrote:
>       Dear libxml2 owners,
>     I am performing research about weaknesses in C open source programs.
>     As part of my research, I am studying weaknesses that may be
>     vulnerabilities in the Libxml2 project.
>     I found in the commit history of Libxml2 (commit 9acef28) the
>     presence of the following code snippet in the libxml.c file (Lines
>     1,597 - 1,612). I believe that this commit presents a weakness that,
>     If format strings can be influenced by an attacker, they can be
>     exploited. This weakness is characterized by CWE Project as CWE-134:
>     Use of Externally-Controlled Format String. When an attacker can
>     modify an externally-controlled format string, this can lead to
>     buffer overflows, denial of service, or data representation problems.
>     Moreover, I performed a software testing strategy with respect to
>     confirm the vulnerability. We provide as input to the char *msg the
>     value "%xtest" and the function libxml_buildMessage return the
>     value"fc0c748ex", exposing a memory position.
>     This is the GitHub link to the commit:
> 600773cf67
>     Code snippet:
>     static char *
>     libxml_buildMessage(const char *msg, va_list ap){
>        int chars;
>        char *str;
>        str = (char *) xmlMalloc(1000);
>        if (str == NULL)
>          return NULL;
>        chars = vsnprintf(str, 999, msg, ap);
>        if (chars >= 998)
>          str[999] = 0;
>        return str;
>     }
>     Looking at this code snippet, I am wondering if you could answer the
>     following brief questions:
>     We understand that this code has a weakness. Do you agree?
>     How do you detect weaknesses? Do you use any tool to detect them?
>     We would be very grateful if you say to us if you agree, and if you
>     are going to fix it.
>     --
>     Raphael de Carvalho Muniz, M.Sc.
>     Lattes:
>     e-Mail: <> /
> <>
>     Fone: +55 84 98801 1218
>     _______________________________________________
>     xml mailing list, project page
>  <>
> --
> Webb Scales
> Principal Software Architect
> 603-673-2306
> <> 
> <>
> _______________________________________________
> xml mailing list, project page 

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