GnuPG 1.2.3, 1.3.3 external HKP interface format string issue

[S-Quadra Security Research <research@xxxxxxxxxxxx>]

           S-Quadra Advisory #2003-12-03

Topic: GnuPG 1.2.3, 1.3.3 external HKP interface format string issue
Severity: Low
Vendor URL: http://www.gnupg.org
Advisory URL: http://www.s-quadra.com/advisories/Adv-20031203.txt
Release date: 3 Dec 2003

1. DESCRIPTION

GnuPG is a complete and free replacement for PGP.
Because it does not use the patented IDEA algorithm, it can be used 
without any restrictions.
GnuPG is a RFC2440 (OpenPGP) compliant application.

GnuPG has external HKP inteface which is marked as experimental and not 
enabled by default in 1.2 stable branch and to use it you should compile 
GnuPG with '--enable-external-hkp' configuration option.
Also, on 1.3 devel branch external HKP interface is enabled by default 
and to disable you should compile GnuPG with '--disable-hkp' 
configuration option.

When the external HKP interface is enabled, GnuPG will make use of 
'gpgkeys_hkp' utility for keyserver accesses.

There exists a format string vulnerability in 'gpgkeys_hkp' utility 
which would allow a malicious
keyserver in the worst case to execute an arbitrary code on the user's 
machine.

2. DETAILS

The offending code can be found in keyserver/gpgkeys_hkp.c:

<snip>
int get_key(char *getkey)
{
 int rc,gotit=0;
 char search[29];
 char *request;
 struct http_context hd;

 ...

 if(verbose>2)
   fprintf(console,"gpgkeys: HTTP URL is \"%s\"\n",request);

 rc=http_open_document(&hd,request,http_flags);
 if(rc!=0)
   {
     fprintf(console,"gpgkeys: HKP fetch error: %s\n",
         rc==G10ERR_NETWORK?strerror(errno):g10_errstr(rc));
     fprintf(output,"KEY 0x%s FAILED\n",getkey);
   }
 else
   {
     unsigned int maxlen=1024,buflen;
     byte *line=NULL;

     while(iobuf_read_line(hd.fp_read,&line,&buflen,&maxlen))
   {
     maxlen=1024;

     if(gotit)
       {
         // S-Quadra: here is where format string bug lives
         fprintf(output,line);
         if(strcmp(line,"-----END PGP PUBLIC KEY BLOCK-----\n")==0)
       break;
       }
     else
       if(strcmp(line,"-----BEGIN PGP PUBLIC KEY BLOCK-----\n")==0)
         {
           // S-Quadra: here is where format string bug lives
       fprintf(output,line);
       gotit=1;
         }
   }
 ...
 return 0;
}

</snip>

3. FIX INFORMATION

S-Quadra alerted GnuPG development team to this issue on 27th November 2003.
For 1.2 branch fix available in CVS, latest devel version 1.3.4 also 
contains fix for the reported bug.

4. CREDITS

Evgeny Legerov <e.legerov@xxxxxxxxxxxx> is responsible for discovering 
this issue.

5. ABOUT

S-Quadra offers services in computer security, penetration testing and 
network assesment,
web application security, source code review and third party product 
vulnerability assesment,
forensic support and reverse engineering.

Security is an art and our goal is to bring responsible and high quality 
security
service to the IT market, customized to meet the unique needs of each 
individual client.

S-Quadra, (pronounced es quadra), is not an acronym.
It's unique, creative and innovative - just like the security services 
we bring to our clients.

           S-Quadra Advisory #2003-12-03