============================================================================= CA-95:04 CERT Advisory February 17, 1995 NCSA HTTP Daemon for UNIX Vulnerability ----------------------------------------------------------------------------- The CERT Coordination Center has received reports that there is a vulnerability in the NCSA HTTP Daemon V.1.3 for UNIX. Because of this vulnerability, the daemon can be tricked into executing shell commands. If you have any questions regarding this vulnerability, please send e-mail to Beth Frank at the NCSA, efrank@ncsa.uiuc.edu. ----------------------------------------------------------------------------- I. Description A vulnerability in the NCSA HTTP Daemon allows it to be tricked into executing shell commands. II. Impact Remote users may gain unauthorized access to the account (uid) under which the httpd process is running. III. Solution The following solution was provided by the HTTPD Team at SDG at NCSA. Step 1: In the file httpd.h, change the string length definitions from: /* The default string lengths */ #define MAX_STRING_LEN 256 #define HUGE_STRING_LEN 8192 to: /* The default string lengths */ #define HUGE_STRING_LEN 8192 #define MAX_STRING_LEN HUGE_STRING_LEN Step 2: Install the following patch, which performs the functionality of strsubfirst (i.e., copy src followed by dest[start] into dest) without the use of a temporary buffer. ----------------------------------cut here------------------------------------- *** util.c.bak Sat May 7 21:47:15 1994 --- util.c Thu Feb 16 04:17:07 1995 *************** *** 158,168 **** void strsubfirst(int start,char *dest, char *src) { ! char tmp[MAX_STRING_LEN]; ! strcpy(tmp,&dest[start]); ! strcpy(dest,src); ! strcpy(&dest[strlen(src)],tmp); } /* --- 158,174 ---- void strsubfirst(int start,char *dest, char *src) { ! int src_len, dest_len, i; ! if ((src_len=strlen(src))<start){ /** src "fits" in dest **/ ! for (i=0;dest[i]=src[i];i++); ! for (i=src_len;dest[i]=dest[i-src_len+start];i++); ! } ! else { /** src doesn't fit in dest **/ ! for (dest_len=strlen(dest),i=dest_len+src_len-start;i>=src_len;i--) ! dest[i] = dest[i-src_len+start]; ! for (i=0;i<src_len;i++) dest[i]=src[i]; ! } } /* ---------------------------------cut here------------------------------------- After you apply this patch, recompile httpd, kill the current running process, and restart the new httpd. --------------------------------------------------------------------------- The CERT Coordination Center thanks Steve Weeber, Carlos Varela, and Beth Frank for their support in responding to this problem. --------------------------------------------------------------------------- If you believe that your system has been compromised, contact the CERT Coordination Center or your representative in Forum of Incident Response and Security Teams (FIRST). If you wish to send sensitive incident or vulnerability information to CERT staff by electronic mail, we strongly advise that the e-mail be encrypted. The CERT Coordination Center can support a shared DES key, PGP (public key available via anonymous FTP on info.cert.org), or PEM (contact CERT staff for details). Internet E-mail: cert@cert.org Telephone: +1 412-268-7090 (24-hour hotline) CERT personnel answer 8:30 a.m.-5:00 p.m. EST(GMT-5)/EDT(GMT-4), and are on call for emergencies during other hours. Fax: +1 412-268-6989 Postal address: CERT Coordination Center Software Engineering Institute Carnegie Mellon University Pittsburgh, PA 15213-3890 USA CERT advisories and bulletins are posted on the USENET newsgroup comp.security.announce. If you would like to have future advisories and bulletins mailed to you or to a mail exploder at your site, please send mail to cert-advisory-request@cert.org. Past advisories, CERT bulletins, information about FIRST representatives, and other information related to computer security are available for anonymous FTP from info.cert.org. CERT is a service mark of Carnegie Mellon University.