CVE-2021-29923: Golang Go Auth Bypass Vulnerability

CVE-2021-29923 Overview

CVE-2021-29923 is an input validation vulnerability in the Go programming language affecting versions prior to 1.17. The vulnerability exists in how Go's net.ParseIP and net.ParseCIDR functions handle IP address octets with leading zero characters. When parsing IP addresses, Go does not properly consider extraneous zero characters at the beginning of an octet, leading to unexpected octal interpretation. This behavior can allow attackers to bypass IP-based access control mechanisms by crafting specially formatted IP addresses that parse differently than intended.

Critical Impact

Attackers can bypass IP-based access control lists (ACLs) and security restrictions by exploiting inconsistent IP address parsing, potentially gaining unauthorized access to protected resources.

Affected Products

• Golang Go (versions before 1.17)
• Oracle TimesTen In-Memory Database
• Fedora Project Fedora 36

Discovery Timeline

• 2021-08-07 - CVE CVE-2021-29923 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-29923

Vulnerability Analysis

The core issue lies in how Go's standard library interprets IP address octets containing leading zeros. In traditional IP address notation, leading zeros can indicate octal (base-8) representation rather than decimal (base-10). However, most users and security applications expect decimal interpretation. When an attacker provides an IP address like 0127.0.0.1 or 010.0.0.1, the Go net package may interpret these octets differently than the security controls intended.

This parsing inconsistency creates a security gap where applications performing IP-based allowlist/denylist checks could be bypassed. For example, if an application blocks access from 127.0.0.1, an attacker could potentially use 0177.0.0.1 (octal for 127) to circumvent the restriction, as the parsed IP differs from the blocked value despite representing the same address in some contexts.

Root Cause

The root cause is insufficient input validation in the net.ParseIP and net.ParseCIDR functions within Go's standard library. These functions do not properly reject or normalize IP address strings containing leading zeros in octets, creating ambiguity between octal and decimal interpretation. The lack of strict parsing allows attackers to craft IP addresses that evaluate to different values depending on the interpretation context.

Attack Vector

This vulnerability is exploitable over the network without authentication or user interaction. An attacker can exploit this vulnerability by:

1. Identifying applications that use Go's net.ParseIP or net.ParseCIDR for IP-based access control decisions
2. Crafting IP address strings with leading zero characters that exploit octal interpretation differences
3. Submitting requests with these specially crafted IP addresses to bypass security restrictions
4. Gaining unauthorized access to resources that should be protected by IP-based ACLs

The attack vector description demonstrates how parsing differences can lead to access control bypass. For detailed technical analysis, refer to the Sick Codes Security Advisory and the GitHub Issue #30999.

Detection Methods for CVE-2021-29923

Indicators of Compromise

• HTTP requests or API calls containing IP addresses with unusual leading zeros (e.g., 010.0.0.1, 0127.0.0.1)
• Access log entries showing requests from IP addresses with octal-style formatting attempting to access restricted endpoints
• Authentication or authorization bypass attempts correlating with non-standard IP address representations

Detection Strategies

• Implement log analysis rules to detect IP addresses containing leading zeros in any octet
• Deploy web application firewall (WAF) rules that flag requests with malformed or non-standard IP address formats
• Create SIEM correlation rules to identify access control bypass attempts where IP validation may be circumvented
• Review application code for usage of net.ParseIP or net.ParseCIDR in security-critical contexts

Monitoring Recommendations

• Monitor application logs for IP parsing errors or warnings related to malformed IP addresses
• Track access patterns to restricted resources, particularly from IP addresses that don't match standard decimal notation
• Enable verbose logging for IP-based access control components to capture parsing behavior

How to Mitigate CVE-2021-29923

Immediate Actions Required

• Upgrade Go to version 1.17 or later where this vulnerability has been addressed
• Review and audit all code using net.ParseIP and net.ParseCIDR for security-sensitive operations
• Implement additional input validation to reject IP addresses containing leading zeros before passing to Go's parsing functions
• Consider using alternative IP parsing libraries that enforce strict decimal interpretation

Patch Information

The vulnerability has been addressed in Go version 1.17 and later. Oracle has released fixes as part of the Oracle January 2022 Critical Patch Update for affected Oracle products. The official Go code change can be reviewed in the Go Code Review Update. Fedora users should apply available updates per the Fedora Package Announcement. Gentoo users should reference Gentoo GLSA 2022-08-02.

Workarounds

• Implement pre-validation logic that rejects IP addresses with leading zeros before passing them to Go's net package
• Use regular expressions to normalize IP addresses by removing leading zeros before parsing
• Deploy network-layer IP filtering that operates on normalized IP representations rather than string-based matching

# Example: Update Go to patched version
go version  # Check current version
# If below 1.17, upgrade Go installation
# Download from https://golang.org/dl/ and install Go 1.17+