nDPI Flow Risks¶
nDPI is designed not just to detect application protocols in traffic flows but also to evaluate potential security risks associated with the traffic. In nDPI parlance this is called a “flow risk”. Flows can have multiple risks detected hence nDPI reports them with a bitmap. Each risk detected corresponds to a bit in the flow risk bitmap. You can read more about ndpi_risk_enum for the list of all numeric risks currently supported.
Below you can find a description of each flow risk so that you can easily understand when a risk is triggered and its meaning. The flow risks are listed in numerical order as they are defined in ndpi_risk_enum.
HTTP only: this risk indicates a possible RCE (Remote Code Execution) attack.
HTTP only: this risk indicates that a binary application is downloaded/uploaded. Detected applications include Windows binaries, Linux executables, Unix scripts and Android apps.
This risk indicates a known protocol used on a non standard port. Example HTTP is supposed to use TCP/80, and in case it is detected on TCP/1234 this risk is detected.
TLS/QUIC only: this risk is triggered when a self-signed certificate is used.
Risk triggered when TLS version is older than 1.1.
Risk triggered when an unsafe TLS cipher is used. See this page for a list of insecure ciphers.
Risk triggered when a TLS certificate is expired, i.e. the current date falls outside of the certificate validity dates.
Risk triggered when a TLS certificate does not match the hostname we’re accessing. Example you do http://www.aaa.com and the TLS certificate returned is for www.bbb.com.
HTTP only: this risk is triggered whenever the user agent contains suspicious characters or its format is suspicious. Example: <?php something ?> is a typical suspicious user agent.
HTTP only: this risk is triggered whenever we’re accessing a host using its IP rather than its symbolic name. Example http://188.8.131.52.
HTTP only: this risk is triggered whenever the accessed URL is suspicious. Example: http://127.0.0.1/msadc/..%255c../..%255c../..%255c../winnt/system32/cmd.exe.
HTTP only: this risk is triggered whenever the HTTP peader contains suspicious entries such as Uuid, TLS_version, Osname that are unexpected on the HTTP header.
TLS only: this risk indicates that this TLS flow will not be used to transport HTTP content. Example VPNs use TLS to encrypt data rather to carry HTTP. This is useful to spot this type of cases.
A DGA is used to generate domain names often used by malwares. This risk indicates that this domain name can (but it’s not 100% sure) a DGA as its name is suspicious.
This risk is generated when a packet (e.g. a DNS packet) has an unexpected formt. This can indicate a protocol error or more often an attempt to jeopardize a valid protocol to carry other type of data.
This risk is generated whenever a SSH client uses an obsolete SSH protocol version or insecure ciphers.
This risk is generated whenever a SSH server uses an obsolete SSH protocol version or insecure ciphers.
SNI is a way to carry in TLS the host/domain name we’re accessing. ESNI means encrypted SNI and it is a way to mask SNI (carried in clear text in the TLS header) with encryption. While this practice is legal, it could be used for hiding data or for attacks such as a suspicious domain fronting.
This risk indicates that the protocol used is insecure and that a secure protocol should be used (e.g. Telnet vs SSH).
This risk is returned when DNS traffic returns an unexpected/obsolete record type.
TLS needs to carry the the SNI of the remote server we’re accessing. Unfortunately SNI is optional in TLS so it can be omitted. In this case this risk is triggered as this is a non-standard situation that indicates a potential security problem or a protocol using TLS for other purposes (or a protocol bug).
HTTP only: risk reported when HTTP carries content in expected format. Example the HTTP header indicates that the context is text/html but the real content is not readeable (i.e. it can transport binary data). In general this is an attempt to use a valid MIME type to carry data that does not match the type.
This is a placeholder for traffic exchanged with ASN that are considered risky. nDPI does not fill this risk that instead should be filled by aplications sitting on top of nDPI (e.g. ntopng).
This is a placeholder for traffic exchanged with domain names that are considered risky. nDPI does not fill this risk that instead should be filled by aplications sitting on top of nDPI (e.g. ntopng).
JA3 is a method to fingerprint TLS traffic. This risk indicates that the JA3 of the TLS connection is considered suspicious (i.e. it has been found in known malware JA3 blacklists). nDPI does not fill this risk that instead should be filled by aplications sitting on top of nDPI (e.g. ntopng).
TLS certificates are uniquely identified with a SHA1 hash value. If such hash is found on a blacklist, this risk can be used. As for other risks, this is a placeholder as nDPI does not fill this risk that instead should be filled by aplications sitting on top of nDPI (e.g. ntopng).
This risk is set when the flow carries desktop or file sharing sessions (e.g. TeamViewer or AnyDesk just to mention two).
This risk is set when the ALPN (it indicates the protocol carried into this TLS flow, for instance HTTP/1.1) is uncommon with respect to the list of expected values.
From 01/09/2020 TLS certificates lifespan is limited to 13 months. This risk is triggered for certificates not respecting this directive.
This risk is triggered when the domain name (SNI extension) is not printable and thus it is a problem. In TLS extensions can be dynamically specified by the client in the hello packet.
This risk is triggered when a TLS fatal alert is detected in the TLS flow. See this page for details.
This risk is used to detect suspicious data carried in ICMP packets whose entropy (used to measure how data is distributed, hence to indirectly guess the type of data carried on) is suspicious and thus that it can indicate a data leak.
Clear text protocols are not bad per-se, but they should be avoided when they carry credentials as they can be intercepted by malicious users. This risk is triggered whenever clear text protocols (e.g. FTP, HTTP, IMAP…) contain credentials in clear text (read it as nDPI does not trigger this risk for HTTP connections that do not carry credentials).
DNS packets over UDP should be limited to 512 bytes. DNS packets over this threshold indicate a potential security risk (e.g. use DNS to carry data) or a misconfiguration.
UDP DNS packets cannot be fragmented. If so, this indicates a potential security risk (e.g. use DNS to carry data) or a misconfiguration.
The risk is set whenever a dissected protocol contains characters not allowed in that protocol field. For example a DNS hostname must only contain a subset of all printable characters or else this risk is set. Additionally, some TLS protocol fields are checked for printable characters as well.
The risk is set whenever a possible exploit (e.g. Log4J/Log4Shell) is detected.
The risk is set whenever a TLS certificate is close to the expiration date.
The risk is set whenever a domain name is specified in IDN format as they are sometimes used in IDN homograph attacks.
The risk is set whenever an error code is detected in the underlying protocol (e.g. HTTP and DNS).
The risk is set whenever a crawler/bot/robot has been detected
The risk is set whenever the (source) ip address has been anonymized and it can’t be used to identify the subscriber. Example: the flow is generated by an iCloud-private-relay exit node.