?1. Advisory Information Title: Heap overflow in freeswitch json parser < 1.6.2 & < 1.4.23 Submitter: Marcello Duarte (marcello@cybersightgroup.com) Product: freeswitch Product URL: http://freeswitch.org Affected Versions: freeswitch < 1.6.2 & < 1.4.23 Fixed Versions: 1.6.2 , 1.4.23 Link to source code diff: https://freeswitch.org/stash/projects/FS/repos/freeswitch/commits/cf892528a1a107ed6eb67fb98ed22533e27778fd CVE Status: CVE-2015-7392 2. Vulnerability Information Impact: Code execution Remotely Exploitable: Yes Locally Exploitable: No 3. Vulnerability Description Product Information: FreeSWITCH is a scalable open source cross-platform telephony platform designed to route and interconnect popular communication protocols using audio, video, text or any other form of media. It was created in 2006 to fill the void left by proprietary commercial solutions. FreeSWITCH also provides a stable telephony platform on which many applications can be developed using a wide range of free tools. Vulnerability: A carefully crafted json string supplied to cJSON_Parse will trigger a heap overflow with user controlled data. The underlying vulnerability occurs in the parse_string function. By passing a json string with \u at the end of the string will cause the parser to increment past the null at the end of string. This confuses the code responsible for copying the string. Since it doesn't detect the NULL in this situation, it will keep copying until it hits a null in memory. This leads to a heap overflow with user controlled data. Any modules or core code which allows user supplied json to enter the json parser will be vulnerable. Vulnerable Source Code: static const char *parse_string(cJSON *item, const char *str) { ... /* HACKLOG The length of string is determined here, it will stop counting when it hits a null */ while (*ptr != '\"' && *ptr && ++len) if (*ptr++ == '\\') ptr++; /* Skip escaped quotes. */ /* HACKLOG The buffer is alloced with the length obtained from the previous section */ out = (char *)cJSON_malloc( len + 1); /* This is how long we need for the string, roughly. */ if (!out) return 0; /* HACKLOG the following code will copy the string into the alloced buffer taking into account utf16 to utf8 conversion */ ptr = str + 1; ptr2 = out; /* 1 */ while (*ptr != '\"' && *ptr) { if (*ptr != '\\') *ptr2++ = *ptr++; else { ptr++; switch (*ptr) { case 'b': *ptr2++ = '\b'; break; case 'f': *ptr2++ = '\f'; break; case 'n': *ptr2++ = '\n'; break; case 'r': *ptr2++ = '\r'; break; case 't': *ptr2++ = '\t'; break; case 'u': /* transcode utf16 to utf8. */ if (sscanf(ptr + 1, "%4x", &uc) < 1) break; ptr += 4; /* get the unicode char. */ if ((uc >= 0xDC00 && uc <= 0xDFFF) || uc == 0) break; // check for invalid. if (uc >= 0xD800 && uc <= 0xDBFF) // UTF16 surrogate pairs. { if (ptr[1] != '\\' || ptr[2] != 'u') break; // missing second-half of surrogate. if (sscanf(ptr + 3, "%4x", &uc2) < 1) break; ptr += 6; if (uc2 < 0xDC00 || uc2 > 0xDFFF) break; // invalid second-half of surrogate. uc = 0x10000 | ((uc & 0x3FF) << 10) | (uc2 & 0x3FF); } len = 4; if (uc < 0x80) len = 1; else if (uc < 0x800) len = 2; else if (uc < 0x10000) len = 3; ptr2 += len; switch (len) { case 4: *--ptr2 = ((uc | 0x80) & 0xBF); uc >>= 6; case 3: *--ptr2 = ((uc | 0x80) & 0xBF); uc >>= 6; case 2: *--ptr2 = ((uc | 0x80) & 0xBF); uc >>= 6; case 1: *--ptr2 = (char)(uc | firstByteMark[len]); } ptr2 += len; break; default: *ptr2++ = *ptr; break; } /* HACKLOG INCREMENTS past null here, causing the while loop to not detect the end of the buffer so it keeps copying past the end of the alloced buffer */ ptr++; } 4. Vendor Information, Solutions Freeswitch has released versions 1.6.2 , 1.4.23 which fix the issue. 5. Credits This vulnerability was discovered and researched by Marcello Duarte ( marcello@cybersightgroup.com ) from CYBERSIGHT GROUP Vulnerability Research Labs. 6. Report Timeline 2015-09-02 - Vulnerability found 2015-09-13 - Freeswitch developers contacted 2015-09-14 - Freeswitch developers verified bug and patched in master 2015-09-25 - Freeswitch releases fixed packages. 2015-09-20 - CVE requested 2015-09-29 - CVE issued, Advisory released 7. About CYBERSIGHT GROUP CYBERSIGHT GROUP is an organization of security professionals specializing in several areas of offensive computer security research. We specialize in vulnerability research, exploit development, reverse engineering and cyber attack planning. http://cybersightgroup.com , contact@cybersightgroup.com 8. Disclaimer The information provided in the advisory is provided as is without any warranty. CYBERSIGHT GROUP and it's members are not liable in any case of damage, direct or indirect. Permission to redistribute the advisory in it's unmodified form is granted. -- Marcello Duarte Chief Research Officer CYBERSIGHT GROUP