CVE-2021-3998: GNU Glibc Information Disclosure Flaw

CVE-2021-3998 Overview

A flaw was found in GNU glibc affecting the realpath() function. The vulnerability causes realpath() to mistakenly return an unexpected value, potentially leading to information leakage and disclosure of sensitive data. This issue affects systems relying on glibc's path resolution functionality, which is fundamental to many Unix-like operating systems and applications.

Critical Impact

The realpath() function may return unexpected values that can lead to information disclosure, potentially exposing sensitive file system paths and data to unauthorized parties.

Affected Products

• GNU glibc (affected versions)
• NetApp ONTAP Select Deploy Administration Utility
• NetApp H300S / H300S Firmware
• NetApp H500S / H500S Firmware
• NetApp H700S / H700S Firmware
• NetApp H410S / H410S Firmware
• NetApp H410C / H410C Firmware

Discovery Timeline

• 2022-08-24 - CVE-2021-3998 published to NVD
• 2025-06-09 - Last updated in NVD database

Technical Details for CVE-2021-3998

Vulnerability Analysis

This vulnerability is classified under CWE-125 (Out-of-Bounds Read) and CWE-252 (Unchecked Return Value). The flaw resides in the realpath() function within the GNU C Library (glibc), which is responsible for resolving symbolic links and returning the canonical absolute pathname of a given path.

The vulnerability occurs when realpath() encounters certain edge cases during path resolution, causing it to return an unexpected or incorrect value. This improper handling can result in the function returning data that should not be accessible, leading to information leakage. Applications that rely on realpath() for security-sensitive path validation may be vulnerable to exploitation.

The network-accessible nature of this vulnerability means that remote attackers may be able to trigger the flaw through applications that process user-supplied paths without additional authentication requirements.

Root Cause

The root cause stems from improper handling of return values and boundary conditions within the realpath() function. Specifically, the function may perform out-of-bounds read operations and fail to properly check return values during path canonicalization. This can occur when processing specially crafted paths that trigger edge cases in the path resolution logic.

The GNU glibc maintainers addressed this issue through commits that improve the boundary checking and return value handling within the affected code paths. The fixes ensure that realpath() properly validates memory access and return states before returning results to callers.

Attack Vector

The attack vector for CVE-2021-3998 is network-based, requiring no authentication and no user interaction to exploit. An attacker can potentially exploit this vulnerability by:

1. Providing specially crafted file paths to applications that use realpath() for path resolution
2. Triggering the edge cases that cause the function to return unexpected values
3. Harvesting the leaked information to gain knowledge about the file system structure or sensitive data locations

Since realpath() is a fundamental libc function used by countless applications and services, the potential attack surface is substantial. Any application that accepts user-supplied paths and processes them through realpath() could potentially be affected.

Detection Methods for CVE-2021-3998

Indicators of Compromise

• Unusual file access patterns or path resolution requests from network-facing services
• Application logs showing unexpected realpath() return values or path-related errors
• Memory access anomalies in processes making heavy use of path resolution functions
• Unexpected information disclosure in application outputs related to file system paths

Detection Strategies

• Monitor for applications processing unusually long or malformed file paths
• Implement anomaly detection for file system access patterns from network services
• Review application logs for path resolution failures or unexpected return values
• Use runtime application security monitoring to detect out-of-bounds memory access

Monitoring Recommendations

• Enable verbose logging for applications that heavily utilize path resolution functionality
• Monitor network traffic for requests containing potentially malicious path patterns
• Implement file integrity monitoring on sensitive directories that could be targeted
• Use SentinelOne's behavioral detection capabilities to identify suspicious path manipulation attempts

How to Mitigate CVE-2021-3998

Immediate Actions Required

• Update GNU glibc to a patched version that addresses CVE-2021-3998
• Apply vendor-specific patches from affected product vendors (NetApp, distribution maintainers)
• Audit applications that use realpath() with user-supplied input for additional input validation
• Implement application-level path validation as a defense-in-depth measure

Patch Information

The vulnerability has been addressed by the GNU glibc maintainers through security patches. Organizations should update to the latest patched version of glibc available for their distribution. Specific patches can be found in the following commits:

• Sourceware glibc Commit 84d2d0f
• Sourceware glibc Commit ee8d5e3

For distribution-specific guidance, consult the following security advisories:

• Red Hat CVE-2021-3998 Advisory
• Debian CVE-2021-3998 Tracker
• NetApp Security Advisory NTAP-20221020-0003

Workarounds

• Implement strict input validation on all user-supplied paths before passing them to realpath()
• Consider using alternative path canonicalization methods with additional validation layers
• Restrict network service access to minimize the attack surface for remote exploitation
• Deploy application sandboxing to limit the impact of potential information disclosure

# Check installed glibc version
ldd --version

# On Red Hat/CentOS, check for security updates
yum check-update glibc

# On Debian/Ubuntu, check for security updates
apt list --upgradable | grep glibc

# Apply available security updates
# Red Hat/CentOS:
yum update glibc

# Debian/Ubuntu:
apt update && apt upgrade libc6