CVE-2023-6779: GNU Glibc Buffer Overflow Vulnerability

CVE-2023-6779 Overview

CVE-2023-6779 is an off-by-one heap-based buffer overflow vulnerability discovered in the __vsyslog_internal function of the GNU C Library (glibc). This critical function is invoked by the widely-used syslog and vsyslog functions, which are fundamental to system logging across Linux distributions. The vulnerability is triggered when these logging functions are called with a message larger than INT_MAX bytes, causing an incorrect calculation of the buffer size required to store the message. This miscalculation results in a heap-based buffer overflow that leads to application crashes and potential denial of service conditions.

Critical Impact

Applications using glibc's syslog functionality can be crashed remotely by sending oversized log messages, potentially causing service disruption across Linux-based infrastructure.

Affected Products

• GNU glibc 2.37 and newer versions
• Fedora 38
• Fedora 39

Discovery Timeline

• 2024-01-31 - CVE CVE-2023-6779 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-6779

Vulnerability Analysis

This vulnerability exists within the __vsyslog_internal function, a core component of glibc's syslog implementation. The flaw stems from an off-by-one error in the buffer size calculation logic when processing exceptionally large log messages. When the syslog() or vsyslog() functions receive a message exceeding INT_MAX bytes (2,147,483,647 bytes), the internal buffer allocation arithmetic produces an incorrect size value. This leads to a heap-based buffer overflow condition classified under CWE-122 (Heap-based Buffer Overflow) and CWE-787 (Out-of-bounds Write).

The vulnerability affects glibc version 2.37 and all subsequent releases, representing a significant attack surface given glibc's ubiquitous presence in Linux systems. While exploitation requires the ability to send extremely large messages to syslog-enabled applications, the network-accessible nature of many logging services increases the practical risk of denial of service attacks.

Root Cause

The root cause lies in integer overflow behavior during buffer size calculation in the __vsyslog_internal function. When computing the required buffer size for messages approaching or exceeding INT_MAX, the arithmetic operation wraps around or produces an incorrect value due to the off-by-one error. This causes the function to allocate a buffer that is too small to hold the incoming message data, resulting in heap memory corruption when the oversized message is written to the undersized buffer.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by sending crafted, oversized log messages to any application that uses glibc's syslog() or vsyslog() functions. The attack is particularly concerning for network-exposed services that accept external input and log it through these functions.

The vulnerability manifests when extremely large messages (exceeding INT_MAX bytes) are processed by the syslog functions. The off-by-one error in the __vsyslog_internal function causes an incorrect buffer allocation, leading to heap corruption when the message is copied. For detailed technical analysis, refer to the Qualys CVE-2023-6246 Analysis and the OpenWall OSS-Security Post.

Detection Methods for CVE-2023-6779

Indicators of Compromise

• Unexpected application crashes in services utilizing syslog functionality
• Heap corruption errors or segmentation faults in system logs
• Abnormally large log messages (approaching or exceeding 2GB) in network traffic
• Memory allocation failures followed by service termination

Detection Strategies

• Monitor for anomalous log message sizes in network traffic destined for syslog ports (UDP/TCP 514)
• Implement heap corruption detection using tools like AddressSanitizer in development environments
• Deploy runtime application self-protection (RASP) solutions to detect buffer overflow attempts
• Use SentinelOne's behavioral AI to identify abnormal process crashes and memory access patterns

Monitoring Recommendations

• Enable verbose logging for syslog daemon processes to capture pre-crash states
• Monitor system stability metrics for services heavily dependent on syslog
• Track glibc library versions across infrastructure to identify vulnerable systems
• Implement network-level monitoring for oversized syslog protocol messages

How to Mitigate CVE-2023-6779

Immediate Actions Required

• Update glibc to the latest patched version provided by your Linux distribution
• Apply distribution-specific security updates from Fedora, Red Hat, Gentoo, or your vendor
• Consider implementing input validation on applications that pass external data to syslog functions
• Review network exposure of syslog-enabled services and restrict access where possible

Patch Information

Security patches have been released by major Linux distributions to address this vulnerability. Fedora has released updates for both Fedora 38 and Fedora 39 as detailed in their package announcements. Red Hat has published guidance in their CVE-2023-6779 Advisory, and Gentoo users should refer to GLSA 202402-01. NetApp customers should consult the NetApp Security Advisory ntap-20240223-0006.

Workarounds

• Implement network-level filtering to drop syslog messages exceeding reasonable size thresholds
• Use application-level input validation to prevent oversized messages from reaching syslog functions
• Deploy intrusion prevention systems (IPS) with signatures for oversized syslog traffic
• Consider using alternative logging mechanisms that do not rely on glibc's syslog implementation until patches are applied

# Check current glibc version
ldd --version

# Example: Update glibc on Fedora systems
sudo dnf update glibc

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