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:

Providing specially crafted file paths to applications that use realpath() for path resolution
Triggering the edge cases that cause the function to return unexpected values
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:

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

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

bash

# 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