Palo Alto's exploited Python code

watchTowr Labs has a nice blog post dissecting CVE-2024-3400. It’s very readable. Go check it out.

The awfulness of Palo Alto’s Python code in this snippet stood out to me:

def some_function():
    ...
    if source_ip_str is not None and source_ip_str != "": 
        curl_cmd = "/usr/bin/curl -v -H \"Content-Type: application/octet-stream\" -X PUT \"%s\" --data-binary @%s --capath %s --interface %s" \
                     %(signedUrl, fname, capath, source_ip_str)
    else:
        curl_cmd = "/usr/bin/curl -v -H \"Content-Type: application/octet-stream\" -X PUT \"%s\" --data-binary @%s --capath %s" \
                     %(signedUrl, fname, capath)
    if dbg:
        logger.info("S2: XFILE: send_file: curl cmd: '%s'" %curl_cmd)
    stat, rsp, err, pid = pansys(curl_cmd, shell=True, timeout=250)
    ...

def dosys(self, command, close_fds=True, shell=False, timeout=30, first_wait=None):
    """call shell-command and either return its output or kill it
       if it doesn't normally exit within timeout seconds"""

    # Define dosys specific constants here
    PANSYS_POST_SIGKILL_RETRY_COUNT = 5

    # how long to pause between poll-readline-readline cycles
    PANSYS_DOSYS_PAUSE = 0.1

    # Use first_wait if time to complete is lengthy and can be estimated 
    if first_wait == None:
        first_wait = PANSYS_DOSYS_PAUSE

    # restrict the maximum possible dosys timeout
    PANSYS_DOSYS_MAX_TIMEOUT = 23 * 60 * 60
    # Can support upto 2GB per stream
    out = StringIO()
    err = StringIO()

    try:
        if shell:
            cmd = command
        else:
            cmd = command.split()
    except AttributeError: cmd = command

    p = subprocess.Popen(cmd, stdout=subprocess.PIPE, bufsize=1, shell=shell,
             stderr=subprocess.PIPE, close_fds=close_fds, universal_newlines=True)
    timer = pansys_timer(timeout, PANSYS_DOSYS_MAX_TIMEOUT)

It uses string building to create a curl command line. Then it passes that command line down into a function that calls subprocess.Popen(cmd_line, shell=True). What? No! Don’t ever do that!

I fed that code into the open source bandit static analyzer. It flagged this code with a high severity, high confidence finding:

ᐅ bandit pan.py
[main]  INFO    profile include tests: None
[main]  INFO    profile exclude tests: None
[main]  INFO    cli include tests: None
[main]  INFO    cli exclude tests: None
[main]  INFO    running on Python 3.12.1
Run started:2024-04-16 17:14:52.240258

Test results:
>> Issue: [B604:any_other_function_with_shell_equals_true] Function call with shell=True parameter identified, possible security issue.
   Severity: Medium   Confidence: Low
   CWE: CWE-78 (https://cwe.mitre.org/data/definitions/78.html)
   More Info: https://bandit.readthedocs.io/en/1.7.8/plugins/b604_any_other_function_with_shell_equals_true.html
   Location: ./pan.py:14:26
13              logger.info("S2: XFILE: send_file: curl cmd: '%s'" % curl_cmd)
14          stat, rsp, err, pid = pansys(curl_cmd, shell=True, timeout=250)
15

--------------------------------------------------
>> Issue: [B602:subprocess_popen_with_shell_equals_true] subprocess call with shell=True identified, security issue.
   Severity: High   Confidence: High
   CWE: CWE-78 (https://cwe.mitre.org/data/definitions/78.html)
   More Info: https://bandit.readthedocs.io/en/1.7.8/plugins/b602_subprocess_popen_with_shell_equals_true.html
   Location: ./pan.py:49:8
48              bufsize=1,
49              shell=shell,
50              stderr=subprocess.PIPE,
51              close_fds=close_fds,
52              universal_newlines=True,
53          )
54          timer = pansys_timer(timeout, PANSYS_DOSYS_MAX_TIMEOUT)

--------------------------------------------------

Code scanned:
        Total lines of code: 41
        Total lines skipped (#nosec): 0

Run metrics:
        Total issues (by severity):
                Undefined: 0
                Low: 0
                Medium: 1
                High: 1
        Total issues (by confidence):
                Undefined: 0
                Low: 1
                Medium: 0
                High: 1
Files skipped (0):

From that we can infer that Palo Alto does not use effective static analysis on their Python code. If they did, this code would not have made it to production.

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