curl: huge COLUMNS causes progress-bar to buffer overflow

ID H1:636013
Type hackerone
Reporter pendrek
Modified 2021-02-08T07:55:53



If an attacker can set environmental variables, curl will always crash with a buffer overflow when downloading a file – if the --progress-bar argument is set.

Steps To Reproduce:

Just run the following command on a 64-bit Linux system (verified on Ubuntu 19.04).


Of course you can set the COLUMNS variable in your .profile configuration file instead...

env COLUMNS="9223372032559808515" curl "" -o "./test.mpg" ```

Output 23,0%*** buffer overfow detected ***: curl terminated Aborted (core dumped)

Explanation of the bug The progress-bar feature parses the COLUMNS environment variable. The source code aims to guarantee this value to be above 20. However, on Linux systems this check fails due to a faulty integer cast in tool_cb_prg.c:

c colp = curlx_getenv("COLUMNS"); if(colp) { char *endptr; long num = strtol(colp, &endptr, 10); // Our value of 9223372032559808515 will be OK! if((endptr != colp) && (endptr == colp + strlen(colp)) && (num > 20)) // BUG! Back to int... 9223372032559808515 becomes 3. bar->width = (int)num;

Then on line 181 we have the buffer overflow:

c barwidth = bar->width - 7; // HERE we get 3-7 resulting in... num = (int) (((double)barwidth) * frac); if(num > MAX_BARLENGTH) num = MAX_BARLENGTH; memset(line, '#', num); // .... a crazy high value here!


If a server runs curl with the --progress-bar argument set and (intentionally or unintentionally) allows an attacker to set environmental variables, the server could easily become a victim of a DoS attack.