CVE-2021-29922 in Rust
Summary
by MITRE • 08/08/2021
library/std/src/net/parser.rs in Rust before 1.53.0 does not properly consider extraneous zero characters at the beginning of an IP address string, which (in some situations) allows attackers to bypass access control that is based on IP addresses, because of unexpected octal interpretation.
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Analysis
by VulDB Data Team • 08/10/2021
The vulnerability described in CVE-2021-29922 resides within the Rust standard library's network parsing functionality, specifically in the file library/std/src/net/parser.rs. This flaw represents a classic case of improper input validation that can lead to significant security implications when applications rely on IP address-based access controls. The issue manifests when parsing IP address strings that contain leading zero characters, which the parser incorrectly interprets as octal number representations rather than standard decimal notation. This misinterpretation occurs because the parser fails to properly validate the format of IP address strings before processing them, creating a potential bypass mechanism for access control systems.
The technical root cause of this vulnerability stems from the parser's inadequate handling of IP address string formats that contain leading zeros. In standard IP address notation, each octet should be interpreted as a decimal value ranging from 0 to 255. However, when leading zeros are present, the parser incorrectly treats these as octal representations, where leading zeros indicate octal number systems. For example, the string "010.010.010.010" would be interpreted as octal values rather than decimal values, potentially resulting in completely different IP address representations that could bypass access control lists. This behavior creates a path where attackers can craft IP address strings that appear legitimate but are actually interpreted differently by the system, leading to unauthorized access.
The operational impact of this vulnerability extends beyond simple access control bypasses, as it can compromise network security boundaries that depend on IP address filtering. When applications implement access control mechanisms based on IP addresses, such as firewall rules, authentication systems, or network segmentation policies, this vulnerability allows attackers to exploit the parsing inconsistency to gain unauthorized access to protected resources. The vulnerability is particularly concerning because it operates at the parsing layer of the standard library, meaning that any application using Rust's network functionality could be affected, regardless of the specific application logic. This widespread potential impact makes the vulnerability particularly dangerous in environments where IP-based access controls are commonly deployed.
The vulnerability aligns with CWE-192, which addresses improper handling of octal values in parsing operations, and demonstrates characteristics consistent with ATT&CK technique T1078.101, which involves valid accounts and access through legitimate network protocols. Organizations should implement immediate mitigation strategies including updating to Rust version 1.53.0 or later where the parsing logic has been corrected to properly validate IP address string formats. Additionally, defensive measures such as implementing additional input validation at application layers, monitoring for unusual IP address patterns, and ensuring proper access control enforcement mechanisms can help reduce the risk of exploitation. The fix in the Rust standard library ensures that IP address strings are properly validated to prevent octal interpretation, thereby maintaining the intended security properties of IP-based access controls.