var-201704-1519
Vulnerability from variot

Use-after-free vulnerability in the mg_http_multipart_wait_for_boundary function in mongoose.c in Cesanta Mongoose Embedded Web Server Library 6.7 and earlier and Mongoose OS 1.2 and earlier allows remote attackers to cause a denial of service (crash) via a multipart/form-data POST request without a MIME boundary string. Cesanta Mongoose Embedded Web Server Library and Mongoose OS are both products of Cesanta, USA. The former is a network library for embedded Web servers; the latter is an open source operating system for the Internet of Things. Mongoose OS is prone to a denial-of-service vulnerability. An attacker can exploit this issue to crash the server, resulting in a denial-of-service condition. #############################################################

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

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    "data": "Use-after-free vulnerability in the mg_http_multipart_wait_for_boundary function in mongoose.c in Cesanta Mongoose Embedded Web Server Library 6.7 and earlier and Mongoose OS 1.2 and earlier allows remote attackers to cause a denial of service (crash) via a multipart/form-data POST request without a MIME boundary string. Cesanta Mongoose Embedded Web Server Library and Mongoose OS are both products of Cesanta, USA. The former is a network library for embedded Web servers; the latter is an open source operating system for the Internet of Things. Mongoose OS is prone to a denial-of-service vulnerability. \nAn attacker can exploit this issue to crash the server, resulting in a denial-of-service condition. #############################################################\n#\n# COMPASS SECURITY ADVISORY\n# https://www.compass-security.com/en/research/advisories/\n#\n#############################################################\n#\n# Product: Mongoose OS\n# Vendor: Cesanta\n# CVE ID: CVE-2017-7185\n# CSNC ID: CSNC-2017-003\n# Subject: Use-after-free / Denial of Service\n# Risk: Medium\n# Effect: Remotely exploitable\n# Authors:\n# Philipp Promeuschel \u003cphilipp.promeuschel@compass-security.com\u003e\n# Carel van Rooyen \u003ccarel.vanrooyen@compass-security.com\u003e\n# Stephan Sekula \u003cstephan.sekula@compass-security.com\u003e\n# Date: 2017-04-03\n#\n#############################################################\n \nIntroduction:\n-------------\nCesanta\u0027s Mongoose OS [1] - an open source operating system for the Internet of Things. Supported micro controllers:\n* ESP32\n* ESP8266\n* STM32\n* TI CC3200\n \nAdditionally, 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). \n \nAffected versions:\n---------\nVulnerable:\n * \u003c= Release 1.2\nNot vulnerable:\n * Patched in current dev / master branch\nNot tested:\n * N/A\n \nTechnical Description\n---------------------\nThe handling of HTTP-Multipart boundary [3] headers does not properly close connections when malformed requests are sent to the Mongoose server. \nThis 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. \n \n \nThe 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:\n---------------\u003e8---------------\nvoid mg_http_handler(struct mg_connection *nc, int ev, void *ev_data) {\n[CUT BY COMPASS]\n #if MG_ENABLE_HTTP_STREAMING_MULTIPART\n     if (req_len \u003e 0 \u0026\u0026 (s = mg_get_http_header(hm, \"Content-Type\")) != NULL \u0026\u0026\n         s-\u003elen \u003e= 9 \u0026\u0026 strncmp(s-\u003ep, \"multipart\", 9) == 0) {\n      mg_http_multipart_begin(nc, hm, req_len); // properly checks for empty boundary\n      // however, the socket is not closed, and mg_http_multipart_continue() is executed\n      mg_http_multipart_continue(nc);\n      return;\n}\n---------------8\u003c---------------\nIn the mg_http_multipart_begin function, the boundary is correctly verified:\n---------------\u003e8---------------\n  boundary_len =\n      mg_http_parse_header(ct, \"boundary\", boundary, sizeof(boundary));\n \n  if (boundary_len == 0) {\n    /*\n     * Content type is multipart, but there is no boundary,\n     * probably malformed request\n     */\n    nc-\u003eflags = MG_F_CLOSE_IMMEDIATELY;\n    DBG((\"invalid request\"));\n    goto exit_mp;\n  }\n---------------8\u003c---------------\nHowever, the socket is not closed (even though the flag \"MG_F_CLOSE_IMMEDIATELY\" has been set), and mg_http_multipart_continue is executed. \nIn mg_http_multipart_continue(), the method mg_http_multipart_wait_for_boundary() is executed:\n---------------8\u003c---------------\nstatic void mg_http_multipart_continue(struct mg_connection *c) {\n  struct mg_http_proto_data *pd = mg_http_get_proto_data(c);\n  while (1) {\n    switch (pd-\u003emp_stream.state) {\n      case MPS_BEGIN: {\n        pd-\u003emp_stream.state = MPS_WAITING_FOR_BOUNDARY;\n        break;\n      }\n      case MPS_WAITING_FOR_BOUNDARY: {\n        if (mg_http_multipart_wait_for_boundary(c) == 0) {\n          return;\n        }\n        break;\n      }\n---------------\u003e8---------------\nThen, 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. \n---------------8\u003c---------------\nstatic int mg_http_multipart_wait_for_boundary(struct mg_connection *c) {\n  const char *boundary;\n  struct mbuf *io = \u0026c-\u003erecv_mbuf;\n  struct mg_http_proto_data *pd = mg_http_get_proto_data(c);\n \n  if ((int) io-\u003elen \u003c pd-\u003emp_stream.boundary_len + 2) {\n    return 0;\n  }\n \n  boundary = c_strnstr(io-\u003ebuf, pd-\u003emp_stream.boundary, io-\u003elen);\n  if (boundary != NULL) {\n[CUT BY COMPASS]\n---------------\u003e8---------------\n \n \nSteps to reproduce\n-----------------\nRequest to HTTP server (code running on hardware device):\n---------------8\u003c---------------\nPOST / HTTP/1.1\nConnection: keep-alive\nContent-Type: multipart/form-data;\nContent-Length: 1\n1\n---------------\u003e8---------------\nThe above request results in a stack trace on the mongoose console:\n---------------8\u003c---------------\nGuru Meditation Error of type LoadProhibited occurred on core  0. Exception was unhandled. \nRegister dump:\nPC      : 0x400014fd  PS      : 0x00060330  A0      : 0x801114b4  A1      : 0x3ffbfcf0 \nA2      : 0x00000000  A3      : 0xfffffffc  A4      : 0x000000ff  A5      : 0x0000ff00 \nA6      : 0x00ff0000  A7      : 0xff000000  A8      : 0x00000000  A9      : 0x00000085 \nA10     : 0xcccccccc  A11     : 0x0ccccccc  A12     : 0x00000001  A13     : 0x00000000 \nA14     : 0x00000037  A15     : 0x3ffbb3cc  SAR     : 0x0000000f  EXCCAUSE: 0x0000001c \nEXCVADDR: 0x00000000  LBEG    : 0x400014fd  LEND    : 0x4000150d  LCOUNT  : 0xffffffff \n \nBacktrace: 0x400014fd:0x3ffbfcf0 0x401114b4:0x3ffbfd00 0x401136cc:0x3ffbfd30 0x401149ac:0x3ffbfe30 0x40114b71:0x3ffbff00 0x40112b80:0x3ffc00a0 0x40112dc6:0x3ffc00d0 0x40113295:0x3ffc0100 0x4011361a:0x3ffc0170 0x40111716:0x3ffc01d0 0x40103b8f:0x3ffc01f0 0x40105099:0x3ffc0210\n---------------\u003e8---------------\n \n \nFurther debugging shows that an uninitialized string has indeed been passed to c_strnstr:\n---------------8\u003c---------------\n(gdb) info symbol 0x401114b4\nc_strnstr + 12 in section .flash.text\n(gdb) list *0x401114b4\n0x401114b4 is in c_strnstr (/mongoose-os/mongoose/mongoose.c:1720). \nwarning: Source file is more recent than executable. \n1715    }\n1716    #endif /* _WIN32 */\n1717   \n1718    /* The simplest O(mn) algorithm. Better implementation are GPLed */\n1719    const char *c_strnstr(const char *s, const char *find, size_t slen) WEAK;\n1720    const char *c_strnstr(const char *s, const char *find, size_t slen) {\n1721      size_t find_length = strlen(find);\n1722      size_t i;\n1723   \n1724      for (i = 0; i \u003c slen; i++) {\n(gdb) list *0x401136cc\n0x401136cc is in mg_http_multipart_continue (/mongoose-os/mongoose/mongoose.c:5893). \n5888      mg_http_free_proto_data_mp_stream(\u0026pd-\u003emp_stream);\n5889      pd-\u003emp_stream.state = MPS_FINISHED;\n5890   \n5891      return 1;\n5892    }\n5893   \n5894    static int mg_http_multipart_wait_for_boundary(struct mg_connection *c) {\n5895      const char *boundary;\n5896      struct mbuf *io = \u0026c-\u003erecv_mbuf;\n5897      struct mg_http_proto_data *pd = mg_http_get_proto_data(c);\n(gdb)\n---------------\u003e8---------------\n \nWorkaround / Fix:\n-----------------\nApply the following (tested and confirmed) patch:\n---------------8\u003c---------------\n$ diff --git a/mongoose/mongoose.c b/mongoose/mongoose.c\nindex 91dc8b9..063f8c6 100644\n--- a/mongoose/mongoose.c\n+++ b/mongoose/mongoose.c\n@@ -5889,6 +5889,12 @@ static int mg_http_multipart_wait_for_boundary(struct mg_connection *c) {\n     return 0;\n   }\n  \n+  if(pd-\u003emp_stream.boundary == NULL){\n+      pd-\u003emp_stream.state = MPS_FINALIZE;\n+      LOG(LL_INFO, (\"invalid request: boundary not initialized\"));\n+      return 0;\n+  }\n+\n   boundary = c_strnstr(io-\u003ebuf, pd-\u003emp_stream.boundary, io-\u003elen);\n   if (boundary != NULL) {\n     const char *boundary_end = (boundary + pd-\u003emp_stream.boundary_len);\n---------------\u003e8---------------\nThe patch has been merged into Mongoose OS on github.com on 2017-04-03 [4]\n \nTimeline:\n---------\n2017-04-03: Coordinated public disclosure date\n2017-04-03: Release of patch\n2017-03-20: Initial vendor response, code usage sign-off\n2017-03-19: Initial vendor notification\n2017-03-19: Assigned CVE-2017-7185\n2017-03-11: Confirmation and patching Philipp Promeuschel, Carel van Rooyen\n2017-03-08: Initial inspection Philipp Promeuschel, Carel van Rooyen\n2017-03-08: Discovery by Philipp Promeuschel\n \nReferences:\n-----------\n[1] https://www.cesanta.com/\n[2] https://github.com/cesanta/mongoose/blob/66a96410d4336c312de32b1cf5db954aab9ee2ec/mongoose.c#L7760\n[3] http://www.ietf.org/rfc/rfc2046.txt\n[4] https://github.com/cesanta/mongoose-os/commit/042eb437973a202d00589b13d628181c6de5cf5b\n",
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