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Incorrect Parsing of Numbers with Different Radices

Incomplete Base
Structure: Simple
Description

This vulnerability occurs when software processes numeric input expecting standard decimal numbers (base 10), but fails to handle inputs formatted in other bases like octal or hexadecimal. This mismatch leads to the system interpreting the same digits as a completely different numeric value.

Extended Description

The core issue often arises from functions that automatically interpret numeric prefixes. A leading '0' can trigger octal parsing, while '0x' indicates hexadecimal. For instance, the IP address '0127.0.0.1' is read as octal, becoming equivalent to 87.0.0.1 in decimal—a stark and dangerous difference from the intended 127.0.0.1 (localhost). Common C functions like `inet_addr()` exhibit this behavior, but the problem can appear in any parsing logic that doesn't explicitly define or validate the input's radix. In practice, this parsing flaw can have severe security consequences. An attacker can exploit it to bypass security controls, such as network allow/deny lists or SSRF (Server-Side Request Forgery) filters, by supplying an IP address or identifier that looks correct to a developer but resolves to a different, permitted address. It can also corrupt data flows when identifiers that resemble numbers with leading zeros are processed incorrectly, leading to logical errors and unexpected system behavior.
Common Consequences 2
Scope: Confidentiality

Impact: Read Application Data

An attacker may use an unexpected numerical base to access private application resources.

Scope: Integrity

Impact: Bypass Protection MechanismAlter Execution Logic

An attacker may use an unexpected numerical base to bypass or manipulate access control mechanisms.
Potential Mitigations 3
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.
Demonstrative Examples 3
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.

Code Example:

Bad
Python
python

Returns original IP after ensuring no exceptions are raised*

python
python
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".

ID : DX-154

This code uses a regular expression to validate an IP string prior to using it in a call to the "ping" command.

Code Example:

Bad
Python
python

The ping command treats zero-prepended IP addresses as octal*

python
Since the regular expression does not have anchors (Regular Expression without Anchors), 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 (Incorrect Parsing of Numbers with Different Radices). 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.
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:

Code Example:

Bad
Other
other
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.
Observed Examples 7
CVE-2021-29662Chain: Use of zero-prepended IP addresses in Perl-based IP validation module can lead to an access control bypass.
CVE-2021-28918Chain: Use of zero-prepended IP addresses in a product that manages IP blocks can lead to an SSRF.
CVE-2021-29921Chain: Use of zero-prepended IP addresses in a Python standard library package can lead to an SSRF.
CVE-2021-29923Chain: Use of zero-prepended IP addresses in the net Golang library can lead to an access control bypass.
CVE-2021-29424Chain: Use of zero-prepended IP addresses in Perl netmask module allows bypass of IP-based access control.
CVE-2016-4029Chain: incorrect validation of intended decimal-based IP address format (Improper Validation of Syntactic Correctness of Input) enables parsing of octal or hexadecimal formats (Incorrect Parsing of Numbers with Different Radices), allowing bypass of an SSRF protection mechanism (Server-Side Request Forgery (SSRF)).
CVE-2020-13776Mishandling of hex-valued usernames leads to unexpected decimal conversion and privilege escalation in the systemd Linux suite.
References 1
Universal "netmask" npm package, used by 270,000+ projects, vulnerable to octal input data
Sick Codes
28-03-2021
https://sick.codes/universal-netmask-npm-package-used-by-270000-projects-vulnerable-to-octal-input-data-server-side-request-forgery-remote-file-inclusion-local-file-inclusion-and-more-cve-2021-28918/
ID: REF-1284
Applicable Platforms
Languages:
Not Language-Specific : Undetermined
Technologies:
Not Technology-Specific : Undetermined
Modes of Introduction
Implementation
Implementation
Related Weaknesses
ChildOf: Incorrect Type Conversion or Cast (CWE-704)