Clear-text protocols such as ftp, telnet, or http lack encryption of transported data, as well as the capability to build an authenticated connection. It means that an attacker able to sniff traffic from the network can read, modify, or corrupt the transported content. These protocols are not secure as they expose applications to an extensive range of risks:

sensitive data exposure
traffic redirected to a malicious endpoint
malware-infected software update or installer
execution of client-side code
corruption of critical information

Even in the context of isolated networks like offline environments or segmented cloud environments, the insider threat exists. Thus, attacks involving communications being sniffed or tampered with can still happen.

For example, attackers could successfully compromise prior security layers by:

bypassing isolation mechanisms
compromising a component of the network
getting the credentials of an internal IAM account (either from a service account or an actual person)

In such cases, encrypting communications would decrease the chances of attackers to successfully leak data or steal credentials from other network components. By layering various security practices (segmentation and encryption, for example), the application will follow the defense-in-depth principle.

Note that using the http protocol is being deprecated by major web browsers.

In the past, it has led to the following vulnerabilities:

Ask Yourself Whether

Application data needs to be protected against falsifications or leaks when transiting over the network.
Application data transits over an untrusted network.
Compliance rules require the service to encrypt data in transit.
Your application renders web pages with a relaxed mixed content policy.
OS-level protections against clear-text traffic are deactivated.

There is a risk if you answered yes to any of those questions.

Recommended Secure Coding Practices

Make application data transit over a secure, authenticated and encrypted protocol like TLS or SSH. Here are a few alternatives to the most common clear-text protocols:
- Use ssh as an alternative to telnet.
- Use sftp, scp, or ftps instead of ftp.
- Use https instead of http.
- Use SMTP over SSL/TLS or SMTP with STARTTLS instead of clear-text SMTP.
Enable encryption of cloud components communications whenever it is possible.
Configure your application to block mixed content when rendering web pages.
If available, enforce OS-level deactivation of all clear-text traffic.

It is recommended to secure all transport channels, even on local networks, as it can take a single non-secure connection to compromise an entire application or system.

Sensitive Code Example

url = "http://example.com" # Sensitive
url = "ftp://anonymous@example.com" # Sensitive
url = "telnet://anonymous@example.com" # Sensitive

import telnetlib
cnx = telnetlib.Telnet("towel.blinkenlights.nl") # Sensitive

import ftplib
cnx = ftplib.FTP("ftp.example.com") # Sensitive

import smtplib
smtp = smtplib.SMTP("smtp.example.com", port=587) # Sensitive

For aws_cdk.aws_elasticloadbalancingv2.ApplicationLoadBalancer:

from aws_cdk import (
    aws_elasticloadbalancingv2 as elbv2,
)

lb = elbv2.ApplicationLoadBalancer(
    self,
    "LB",
    vpc=vpc,
    internet_facing=True
)

lb.add_listener(
    "Listener-default",
    port=80, # Sensitive
    open=True
)
lb.add_listener(
    "Listener-http-explicit",
    protocol=elbv2.ApplicationProtocol.HTTP, # Sensitive
    port=8080,
    open=True
)

For aws_cdk.aws_elasticloadbalancingv2.ApplicationListener:

from aws_cdk import (
    aws_elasticloadbalancingv2 as elbv2,
)

elbv2.ApplicationListener(
    self,
    "listener-http-explicit-const",
    load_balancer=lb,
    protocol=elbv2.ApplicationProtocol.HTTP, # Sensitive
    port=8081,
    open=True
)

For aws_cdk.aws_elasticloadbalancingv2.NetworkLoadBalancer:

from aws_cdk import (
    aws_elasticloadbalancingv2 as elbv2,
)
lb = elbv2.NetworkLoadBalancer(
    self,
    "LB",
    vpc=vpc,
    internet_facing=True
)

lb.add_listener( # Sensitive
    "Listener-default",
    port=1234
)
lb.add_listener(
    "Listener-TCP-explicit",
    protocol=elbv2.Protocol.TCP, # Sensitive
    port=1337
)

For aws_cdk.aws_elasticloadbalancingv2.NetworkListener:

from aws_cdk import (
    aws_elasticloadbalancingv2 as elbv2,
)

elbv2.NetworkListener(
    self,
    "Listener-TCP-explicit",
    protocol=elbv2.Protocol.TCP, # Sensitive
    port=1338,
    load_balancer=lb
)

For aws_cdk.aws_elasticloadbalancingv2.CfnListener:

from aws_cdk import (
    aws_elasticloadbalancingv2 as elbv2,
)

elbv2.CfnListener(
    self,
    "listener-http",
    default_actions=[application_default_action],
    load_balancer_arn=lb.load_balancer_arn,
    protocol="HTTP", # Sensitive
    port=80
)

elbv2.CfnListener(
    self,
    "listener-tcp",
    default_actions=[network_default_action],
    load_balancer_arn=lb.load_balancer_arn,
    protocol="TCP", # Sensitive
    port=1000
)

For aws_cdk.aws_elasticloadbalancing.LoadBalancerListener:

from aws_cdk import (
    aws_elasticloadbalancing as elb,
)

elb.LoadBalancerListener(
    external_port=10000,
    external_protocol=elb.LoadBalancingProtocol.TCP, # Sensitive
    internal_port=10000
)

elb.LoadBalancerListener(
    external_port=10080,
    external_protocol=elb.LoadBalancingProtocol.HTTP, # Sensitive
    internal_port=10080
)

For aws_cdk.aws_elasticloadbalancing.CfnLoadBalancer:

from aws_cdk import (
    aws_elasticloadbalancing as elb
)

elb.CfnLoadBalancer(
    self,
    "elb-tcp",
    listeners=[
        elb.CfnLoadBalancer.ListenersProperty(
            instance_port="10000",
            load_balancer_port="10000",
            protocol="tcp" # Sensitive
        )
    ],
    subnets=vpc.select_subnets().subnet_ids
)

elb.CfnLoadBalancer(
    self,
    "elb-http-dict",
    listeners=[
        {
            "instancePort":"10000",
            "loadBalancerPort":"10000",
            "protocol":"http" # Sensitive
        }
    ],
    subnets=vpc.select_subnets().subnet_ids
)

For aws_cdk.aws_elasticloadbalancing.LoadBalancer:

from aws_cdk import (
    aws_elasticloadbalancing as elb,
)

elb.LoadBalancer(
    self,
    "elb-tcp-dict",
    vpc=vpc,
    listeners=[
        {
            "externalPort":10000,
            "externalProtocol":elb.LoadBalancingProtocol.TCP, # Sensitive
            "internalPort":10000
        }
    ]
)

loadBalancer.add_listener(
    external_port=10081,
    external_protocol=elb.LoadBalancingProtocol.HTTP, # Sensitive
    internal_port=10081
)
loadBalancer.add_listener(
    external_port=10001,
    external_protocol=elb.LoadBalancingProtocol.TCP, # Sensitive
    internal_port=10001
)

For aws_cdk.aws_elasticache.CfnReplicationGroup:

from aws_cdk import (
    aws_elasticache as elasticache
)

elasticache.CfnReplicationGroup(
    self,
    "unencrypted-explicit",
    replication_group_description="a replication group",
    automatic_failover_enabled=False,
    transit_encryption_enabled=False, # Sensitive
    cache_subnet_group_name="test",
    engine="redis",
    engine_version="3.2.6",
    num_cache_clusters=1,
    cache_node_type="cache.t2.micro"
)

elasticache.CfnReplicationGroup( # Sensitive, encryption is disabled by default
    self,
    "unencrypted-implicit",
    replication_group_description="a test replication group",
    automatic_failover_enabled=False,
    cache_subnet_group_name="test",
    engine="redis",
    engine_version="3.2.6",
    num_cache_clusters=1,
    cache_node_type="cache.t2.micro"
)

For aws_cdk.aws_kinesis.CfnStream:

from aws_cdk import (
    aws_kinesis as kinesis,
)

kinesis.CfnStream( # Sensitive, encryption is disabled by default for CfnStreams
    self,
    "cfnstream-implicit-unencrytped",
    shard_count=1
)

kinesis.CfnStream(self,
    "cfnstream-explicit-unencrytped",
    shard_count=1,
    stream_encryption=None # Sensitive
)

For aws_cdk.aws_kinesis.Stream:

from aws_cdk import (
    aws_kinesis as kinesis,
)

stream = kinesis.Stream(self,
    "stream-explicit-unencrypted",
    shard_count=1,
    encryption=kinesis.StreamEncryption.UNENCRYPTED # Sensitive
)

Compliant Solution

url = "https://example.com"
url = "sftp://anonymous@example.com"
url = "ssh://anonymous@example.com"

import ftplib
cnx = ftplib.FTP_TLS("ftp.example.com")

import smtplib
smtp = smtplib.SMTP("smtp.example.com", port=587)
smtp.starttls(context=context)

smtp_ssl = smtplib.SMTP_SSL("smtp.gmail.com", port=465)