CVE-2020-29573 in C Library
Summary
by MITRE • 12/06/2020
sysdeps/i386/ldbl2mpn.c in the GNU C Library (aka glibc or libc6) before 2.23 on x86 targets has a stack-based buffer overflow if the input to any of the printf family of functions is an 80-bit long double with a non-canonical bit pattern, as seen when passing a \x00\x04\x00\x00\x00\x00\x00\x00\x00\x04 value to sprintf.
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Analysis
by VulDB Data Team • 12/12/2020
The vulnerability identified as CVE-2020-29573 represents a critical stack-based buffer overflow within the GNU C Library implementation specifically affecting x86 targets running glibc versions prior to 2.23. This flaw manifests when processing 80-bit long double values that contain non-canonical bit patterns through the printf family of functions, particularly demonstrating the issue when passing a specific 80-bit value consisting of four null bytes followed by four bytes of value four to the sprintf function. The vulnerability resides in the sysdeps/i386/ldbl2mpn.c file which handles the conversion of long double values to multi-precision numbers, making it a fundamental component in the library's mathematical computation handling.
The technical exploitation of this vulnerability occurs through improper bounds checking during the conversion process of long double values with non-canonical bit patterns. When the printf family functions encounter such input, the code fails to validate the bit pattern of the 80-bit floating-point representation before processing, leading to a situation where data written to the stack buffer exceeds its allocated boundaries. This behavior directly violates the principle of safe buffer management and represents a classic stack overflow condition that can be leveraged for arbitrary code execution. The vulnerability is particularly concerning because it operates within the core C library functions that are extensively used throughout the operating system and applications, making the attack surface extremely broad.
The operational impact of this vulnerability extends far beyond simple buffer corruption, as it can potentially allow remote attackers to execute arbitrary code with the privileges of the affected process. The flaw affects systems using glibc versions before 2.23 on x86 architecture, which encompasses a significant portion of legacy systems and older distributions. Attackers could exploit this vulnerability by crafting specific input data containing non-canonical long double values that trigger the overflow during printf processing, potentially leading to complete system compromise. The vulnerability maps directly to CWE-121 Stack-based Buffer Overflow, which is categorized under the Common Weakness Enumeration framework as a fundamental flaw in memory management practices. From an adversarial perspective, this vulnerability aligns with ATT&CK technique T1059.007 for Command and Scripting Interpreter and T1548.001 for Abuse of Functionality, as it enables privilege escalation through code execution.
Mitigation strategies for CVE-2020-29573 primarily focus on updating to patched versions of glibc, specifically ensuring systems are running glibc 2.23 or later where the vulnerability has been addressed through proper bounds checking and input validation. System administrators should prioritize patching affected systems, particularly those running legacy distributions that may not receive regular updates. Additional defensive measures include implementing input validation at application layers to sanitize floating-point data before processing, employing stack canaries to detect buffer overflows, and utilizing address space layout randomization to complicate exploitation attempts. Network segmentation and privilege separation can also help limit the potential impact of successful exploitation, while monitoring systems should be configured to detect unusual printf function usage patterns that might indicate exploitation attempts. The fix implemented in patched versions typically involves correcting the buffer boundary checking logic in the ldbl2mpn.c file to properly handle non-canonical bit patterns in long double values, preventing the overflow condition from occurring during the mathematical conversion process.