CVE-2015-8777 in C Library
Summary
by MITRE
The process_envvars function in elf/rtld.c in the GNU C Library (aka glibc or libc6) before 2.23 allows local users to bypass a pointer-guarding protection mechanism via a zero value of the LD_POINTER_GUARD environment variable.
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Analysis
by VulDB Data Team • 07/05/2022
The vulnerability identified as CVE-2015-8777 resides within the GNU C Library implementation of the process_envvars function in the elf/rtld.c file. This flaw affects glibc versions prior to 2.23 and represents a significant security weakness that undermines pointer guarding mechanisms designed to protect against memory corruption attacks. The vulnerability specifically targets the LD_POINTER_GUARD environment variable which serves as a critical security feature in modern glibc implementations. When this environment variable is set to zero, the pointer guarding protection mechanism fails to function properly, creating a bypass opportunity for local attackers.
The technical nature of this vulnerability stems from improper validation of the LD_POINTER_GUARD environment variable within the runtime linker's processing logic. According to CWE-284, this represents an inadequate privilege management issue where the system fails to properly enforce access controls. The flaw allows attackers to manipulate the runtime linker's behavior by setting LD_POINTER_GUARD to zero, effectively disabling a crucial security feature that was designed to prevent pointer overwrites and memory corruption attacks. This mechanism typically works by adding guard pointers to prevent certain types of buffer overflows and arbitrary code execution attempts.
From an operational perspective, this vulnerability creates a significant risk for systems running affected glibc versions. Local users can exploit this weakness to bypass memory protection mechanisms that are fundamental to modern exploit mitigation strategies. The attack vector is particularly concerning because it requires minimal privileges and can be executed by any local user on the system. The impact extends beyond simple privilege escalation as it undermines the entire memory safety framework that glibc provides, making systems more susceptible to more sophisticated attacks that might otherwise be prevented by the pointer guarding mechanism.
The vulnerability aligns with several ATT&CK techniques including privilege escalation through environment manipulation and defense evasion by disabling security features. Attackers can leverage this flaw to establish persistent access or escalate privileges within the system. The mitigation strategy involves updating to glibc version 2.23 or later where the issue has been resolved through proper validation of the LD_POINTER_GUARD environment variable. Additionally, system administrators should implement strict environment variable controls and monitor for unauthorized modifications to security-relevant environment variables. The fix implemented in newer glibc versions ensures that when LD_POINTER_GUARD is explicitly set to zero, the system maintains its security posture rather than disabling critical protections. Organizations should also consider implementing runtime monitoring to detect suspicious environment variable modifications and ensure that security mechanisms remain active even when users attempt to bypass them through environmental manipulation.