CVE-2021-3999: Gnu Glibc Buffer Overflow Vulnerability

CVE-2021-3999 Overview

A critical off-by-one buffer overflow and underflow vulnerability has been discovered in the GNU C Library (glibc), specifically within the getcwd() function. When the buffer size passed to getcwd() is exactly 1 byte, the function may trigger memory corruption due to improper boundary handling. This flaw can be exploited by a local attacker who can control the input buffer and size parameters passed to getcwd() within a setuid program, potentially leading to arbitrary code execution and privilege escalation on the affected system.

Critical Impact
Local attackers can exploit this vulnerability in setuid programs to achieve arbitrary code execution and escalate privileges to root-level access on vulnerable Linux systems.

Affected Products

GNU glibc (multiple versions)
Debian Linux 10.0 and 11.0
NetApp E-Series Performance Analyzer
NetApp NFS Plug-in for VMware VAAI
NetApp ONTAP Select Deploy Administration Utility
NetApp H300S, H500S, H700S, H410S, and H410C firmware and hardware

Discovery Timeline

2022-08-24 - CVE-2021-3999 published to NVD
2025-12-02 - Last updated in NVD database

Technical Details for CVE-2021-3999

Vulnerability Analysis

This vulnerability exists in the getcwd() function of glibc, which retrieves the current working directory of the calling process. The flaw manifests when the size parameter is set to exactly 1 byte, creating a boundary condition that the function fails to handle correctly.

The getcwd() function is designed to copy the absolute pathname of the current working directory into the supplied buffer. When a buffer size of 1 is provided, the function's internal logic incorrectly handles the boundary calculation, resulting in both an off-by-one overflow and underflow condition. This dual memory corruption scenario is particularly dangerous because it can corrupt adjacent memory regions in predictable ways.

The vulnerability becomes exploitable when an attacker can influence the parameters passed to getcwd() within a setuid program. Since setuid programs run with elevated privileges (typically root), successful exploitation allows the attacker to execute arbitrary code with those elevated privileges, effectively compromising the entire system.

Root Cause

The root cause is an off-by-one error (CWE-193) in the getcwd() implementation. When processing a buffer of size 1, the function's boundary checking logic fails to account for the null terminator requirement, leading to writes outside the allocated buffer boundaries. This implementation flaw results in both buffer overflow (writing past the end) and buffer underflow (writing before the start) conditions depending on the execution path.

Attack Vector

The attack requires local access to the system. An attacker must identify a setuid program that calls getcwd() with user-controllable buffer and size parameters. By carefully crafting the input to specify a buffer size of exactly 1, the attacker can trigger the memory corruption condition. Through techniques such as heap manipulation and careful memory layout control, the attacker can redirect program execution to achieve arbitrary code execution with the elevated privileges of the setuid program.

The vulnerability mechanism involves passing a carefully sized buffer to getcwd(). When the size is exactly 1 byte, the function's internal path resolution logic fails to properly validate buffer boundaries before writing. This creates a memory corruption primitive that can be leveraged for code execution. See the Sourceware Bug Report #28769 for additional technical details.

Detection Methods for CVE-2021-3999

Indicators of Compromise

Unexpected crashes or segmentation faults in setuid programs that utilize getcwd() function calls
Anomalous privilege escalation events from low-privileged user accounts to root
Memory corruption artifacts in process dumps showing irregular patterns near path buffer allocations
Unusual system call patterns involving getcwd() with abnormally small buffer sizes

Detection Strategies

Monitor system calls using tools like auditd to track getcwd() invocations with buffer sizes of 1 byte
Implement runtime application self-protection (RASP) to detect buffer overflow attempts in glibc functions
Deploy endpoint detection solutions capable of identifying exploitation attempts targeting setuid binaries
Use memory sanitizers (ASAN, MSAN) in development and testing environments to catch boundary violations

Monitoring Recommendations

Enable comprehensive logging for setuid program execution and track any anomalous behavior patterns
Configure system auditing to alert on privilege escalation events originating from unexpected processes
Implement file integrity monitoring on critical setuid binaries to detect tampering or exploitation artifacts
Review system logs regularly for signs of exploitation attempts or unusual getcwd() usage patterns

How to Mitigate CVE-2021-3999

Immediate Actions Required

Update glibc to the latest patched version available for your distribution immediately
Review and audit all setuid programs on the system for potential exposure to this vulnerability
Apply vendor-specific security patches from Debian, Red Hat, and NetApp as applicable to your environment
Consider temporarily removing setuid bits from non-essential programs until patching is complete

Patch Information

Security patches addressing this vulnerability are available from multiple vendors. The fix has been committed to the glibc source repository as documented in the glibc Git Commit Log. Distribution-specific patches are available through:

System administrators should prioritize updating glibc packages and reboot systems to ensure the new library is loaded by all processes.

Workarounds

Implement application-level input validation to prevent buffer sizes of 1 from being passed to getcwd()
Use SELinux or AppArmor policies to restrict the execution context of potentially vulnerable setuid programs
Deploy seccomp filters to monitor and restrict system calls in sensitive applications
Consider using static analysis tools to identify code paths that may expose the vulnerable function with small buffers

bash

# Check current glibc version
ldd --version

# List setuid programs that may need review
find / -perm -4000 -type f 2>/dev/null

# Update glibc on Debian/Ubuntu
sudo apt update && sudo apt upgrade libc6

# Update glibc on RHEL/CentOS
sudo yum update glibc