CWE-1389: Incorrect Parsing of Numbers with Different Radices
Weakness ID: 1389
Vulnerability Mapping:ALLOWEDThis CWE ID may be used to map to real-world vulnerabilities Abstraction:
BaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.
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Description
The product parses numeric input assuming base 10 (decimal) values, but it does not account for inputs that use a different base number (radix).
Extended Description
Frequently, a numeric input that begins with "0" is treated as octal, or "0x" causes it to be treated as hexadecimal, e.g. by the inet_addr() function. For example, "023" (octal) is 35 decimal, or "0x31" is 49 decimal. Other bases may be used as well. If the developer assumes decimal-only inputs, the code could produce incorrect numbers when the inputs are parsed using a different base. This can result in unexpected and/or dangerous behavior. For example, a "0127.0.0.1" IP address is parsed as octal due to the leading "0", whose numeric value would be the same as 87.0.0.1 (decimal), where the developer likely expected to use 127.0.0.1.
The consequences vary depending on the surrounding code in which this weakness occurs, but they can include bypassing network-based access control using unexpected IP addresses or netmasks, or causing apparently-symbolic identifiers to be processed as if they are numbers. In web applications, this can enable bypassing of SSRF restrictions.
Common Consequences
This table specifies different individual consequences
associated with the weakness. The Scope identifies the application security area that is
violated, while the Impact describes the negative technical impact that arises if an
adversary succeeds in exploiting this weakness. The Likelihood provides information about
how likely the specific consequence is expected to be seen relative to the other
consequences in the list. For example, there may be high likelihood that a weakness will be
exploited to achieve a certain impact, but a low likelihood that it will be exploited to
achieve a different impact.
Scope
Impact
Likelihood
Confidentiality
Technical Impact: Read Application Data
An attacker may use an unexpected numerical base to access private application resources.
Unknown
Integrity
Technical Impact: Bypass Protection Mechanism; Alter Execution Logic
An attacker may use an unexpected numerical base to bypass or manipulate access control mechanisms.
Unknown
Potential Mitigations
Phase: Implementation
Strategy: Enforcement by Conversion
If only decimal-based values are expected in the application, conditional checks should be created in a way that prevent octal or hexadecimal strings from being checked. This can be achieved by converting any numerical string to an explicit base-10 integer prior to the conditional check, to prevent octal or hex values from ever being checked against the condition.
Phase: Implementation
Strategy: Input Validation
If various numerical bases do need to be supported, check for leading values indicating the non-decimal base you wish to support (such as 0x for hex) and convert the numeric strings to integers of the respective base. Reject any other alternative-base string that is not intentionally supported by the application.
Phase: Implementation
Strategy: Input Validation
If regular expressions are used to validate IP addresses, ensure that they are bounded using ^ and $ to prevent base-prepended IP addresses from being matched.
Relationships
This table shows the weaknesses and high level categories that are related to this
weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to
similar items that may exist at higher and lower levels of abstraction. In addition,
relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user
may want to explore.
Relevant to the view "Research Concepts" (CWE-1000)
Nature
Type
ID
Name
ChildOf
Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.
This table shows the weaknesses and high level categories that are related to this
weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to
similar items that may exist at higher and lower levels of abstraction. In addition,
relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user
may want to explore.
Relevant to the view "Software Development" (CWE-699)
Nature
Type
ID
Name
MemberOf
Category - a CWE entry that contains a set of other entries that share a common characteristic.
The different Modes of Introduction provide information
about how and when this
weakness may be introduced. The Phase identifies a point in the life cycle at which
introduction
may occur, while the Note provides a typical scenario related to introduction during the
given
phase.
Phase
Note
Implementation
Input validation used may assume decimal bases during conditional checks, when it may not always be the case.
Implementation
The application may rely on a service that supports different numerical bases.
Applicable Platforms
This listing shows possible areas for which the given
weakness could appear. These
may be for specific named Languages, Operating Systems, Architectures, Paradigms,
Technologies,
or a class of such platforms. The platform is listed along with how frequently the given
weakness appears for that instance.
Languages
Class: Not Language-Specific
(Undetermined Prevalence)
Technologies
Class: Not Technology-Specific
(Undetermined Prevalence)
Demonstrative Examples
Example 1
The below demonstrative example uses an IP validator that splits up an IP address by octet, tests to ensure each octet can be casted into an integer, and then returns the original IP address if no exceptions are raised. This validated IP address is then tested using the "ping" command.
(bad code)
Example Language: Python
import subprocess
def validate_ip(ip: str):
split_ip = ip.split('.')
if len(split_ip) > 4 or len(split_ip) == 0:
raise ValueError("Invalid IP length")
for octet in split_ip:
try:
int(octet, 10)
except ValueError as e:
raise ValueError(f"Cannot convert IP octet to int - {e}")
# Returns original IP after ensuring no exceptions are raised
return ip
def run_ping(ip: str):
validated = validate_ip(ip) # The ping command treats zero-prepended IP addresses as octal
result = subprocess.call(["ping", validated])
print(result)
If run_ping() were to be called with one or more zero-prepended octets, validate_ip() will succeed as zero-prepended numerical strings can be interpreted as decimal by a cast ("012" would cast to 12). However, as the original IP with the prepended zeroes is returned rather than the casted IP, it will be used in the call to the ping command. Ping DOES check and support octal-based IP octets, so the IP reached via ping may be different than the IP assumed by the validator. For example, ping would considered "0127.0.0.1" the same as "87.0.0.1".
Example 2
This code uses a regular expression to validate an IP string prior to using it in a call to the "ping" command.
(bad code)
Example Language: Python
import subprocess
import re
def validate_ip_regex(ip: str):
ip_validator = re.compile(r"((25[0-5]|(2[0-4]|1\d|[1-9]|)\d)\.?\b){4}")
if ip_validator.match(ip):
return ip
else:
raise ValueError("IP address does not match valid pattern.")
def run_ping_regex(ip: str):
validated = validate_ip_regex(ip) # The ping command treats zero-prepended IP addresses as octal
result = subprocess.call(["ping", validated])
print(result)
Since the regular expression does not have anchors (CWE-777), i.e. is unbounded without ^ or $ characters, then prepending a 0 or 0x to the beginning of the IP address will still result in a matched regex pattern. Since the ping command supports octal and hex prepended IP addresses, it will use the unexpectedly valid IP address (CWE-1389). For example, "0x63.63.63.63" would be considered equivalent to "99.63.63.63". As a result, the attacker could potentially ping systems that the attacker cannot reach directly.
Example 3
Consider the following scenario, inspired by CWE team member Kelly Todd.
Kelly wants to set up monitoring systems for his two cats, who pose very different threats. One cat, Night, tweets embarrassing or critical comments about his owner in ways that could cause reputational damage, so Night's blog needs to be monitored regularly. The other cat, Taki, likes to distract Kelly and his coworkers during business meetings with cute meows, so Kelly monitors Taki's location using a different web site.
Suppose /etc/hosts provides the site info as follows:
The entry for night.example.com has a typo "010" in the first octet. When using ping to ensure the servers are up, the leading 0 causes the IP address to be converted using octal. So when Kelly's script attempts to access night.example.com, it inadvertently scans 8.1.0.8 instead of 10.1.0.8 (since "010" in octal is 8 in decimal), and Night is free to send new Tweets without being immediately detected.
Chain: incorrect validation of intended decimal-based IP address format (CWE-1286) enables parsing of octal or hexadecimal formats (CWE-1389), allowing bypass of an SSRF protection mechanism (CWE-918).
Mishandling of hex-valued usernames leads to unexpected decimal conversion and privilege escalation in the systemd Linux suite.
Memberships
This MemberOf Relationships table shows additional CWE Categories and Views that
reference this weakness as a member. This information is often useful in understanding where a
weakness fits within the context of external information sources.
Nature
Type
ID
Name
MemberOf
Category - a CWE entry that contains a set of other entries that share a common characteristic.
(this CWE ID may be used to map to real-world vulnerabilities)
Reason:
Acceptable-Use
Rationale:
This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Comments:
Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.
Description
This table specifies different individual consequences
associated with the weakness. The Scope identifies the application security area that is
violated, while the Impact describes the negative technical impact that arises if an
adversary succeeds in exploiting this weakness. The Likelihood provides information about
how likely the specific consequence is expected to be seen relative to the other
consequences in the list. For example, there may be high likelihood that a weakness will be
exploited to achieve a certain impact, but a low likelihood that it will be exploited to
achieve a different impact.
