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.


*From:*xml [] *On Behalf Of *Webb Scales
*Sent:* Tuesday, October 29, 2019 7:18 AM
*To:* Raphael de Carvalho Muniz <>
*Subject:* Re: [xml] Research about vulnerabilities


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.


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:

    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.

    e-Mail: <> / <>

    Fone: +55 84 98801 1218


    xml mailing list, project page  <>


Webb Scales
Principal Software Architect
603-673-2306 <> <>

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