CVE-2023-1972 - Heap-Based Buffer Overflow in _bfd_elf_slurp_version

In April 2023, a security flaw tracked as CVE-2023-1972 was discovered in the GNU Binary File Descriptor (BFD) library, a part of the GNU Binutils project. This vulnerability, located in the function _bfd_elf_slurp_version_tables() in bfd/elf.c, is a heap-based buffer overflow that attackers could potentially exploit to crash applications using the library—leading to a denial of service (DoS).

In this long-read post, we’ll break down the vulnerability, show a simplified code snippet of the issue, walk through how exploitation might happen, and share references for further reading.

## What Is bfd/elf.c and What Went Wrong?

The BFD library allows GNU tools (like objdump, readelf, or ld) to handle many different binary object formats. The file bfd/elf.c contains code that processes ELF (Executable and Linkable Format) binaries.

_bfd_elf_slurp_version_tables() is a function responsible for reading ("slurping") version table data from an ELF file into memory.

A heap-based buffer overflow happens when a program writes more data to a heap-allocated buffer than it has space for. This can corrupt other data, crash the program, or even allow arbitrary code execution in some cases.

Vulnerable Code Example

Below is a simplified version of how a heap buffer overflow can occur in code similar to _bfd_elf_slurp_version_tables(). It's not the exact source, but illustrates the root cause for educational purposes:

unsigned int num_entries = get_version_table_count(section);
Elf_Internal_Versym *versyms = (Elf_Internal_Versym *) malloc(sizeof(Elf_Internal_Versym) * num_entries);

if (!versyms)
    return FALSE;

// Assuming 'data' is a pointer to the input from the ELF file
for (unsigned int i = ; i < num_entries; i++) {
    // Vulnerable: No length check if the input data has at least num_entries items!
    versyms[i] = parse_versym(data);
}

free(versyms);

The problem: If the incoming num_entries value is *larger* than the actual data buffer’s size points to, this loop overwrites past the end of allocated memory. This heap overflow could overwrite important structures in memory belonging to the program.

Exploitation: What Attackers Can Do

In practice, exploiting this flaw requires crafting a malicious ELF file (think: a weird object file or executable) that advertises more version entries in its headers than it really contains. When the vulnerable version of BFD tries to load this file, it attempts to allocate and read past the end—leading to a crash.

Run a vulnerable tool

Run a tool like objdump -x <malicious.elf> on a system with a vulnerable Binutils/BFD library.

Crash (Denial of Service)

The tool crashes, possibly taking down a service or system that relies on parsing ELF files automatically.

Note: This vulnerability is as far as we know only usable for crashing software, not for gaining code execution, since only a heap overflow with no direct ability to write controlled data is present.

Proof-of-Concept: Malicious ELF File

A real exploit would require precise ELF crafting. Here’s a pseudocode outline showing the logic of creating such a file:

# Not real code! Just a conceptual snippet.
elf_data = create_minimal_elf_header()

# Add version table section with a declared count larger than the actual buffer size
fake_versym_section = create_versym_section(num_entries=x100, actual_entries=1)

elf_data += fake_versym_section
write_file("crashme.elf", elf_data)

When the vulnerable BFD parser loads this file, it allocates a buffer and tries to read x100 entries, overrunning the buffer and causing a crash.

Fixed Version

Upstream Binutils project fixed the bug in May 2023. The fix simply adds checks to ensure that the read size matches allocated size, avoiding reading/writing past memory.

Patch example (commit):

 if (num_entries > max_possible_entries)
     return FALSE;

Should You Worry?

Most people will never be exposed to this flaw unless they manually analyze untrusted or attacker-provided ELF binaries (e.g., malware researchers, sysadmins handling unknown files, anyone using Binutils on files uploaded by others). Servers which automatically process object files, firmware, or executables could be vulnerable to a remote DoS.

References

- CVE-2023-1972 at NVD
- Official Binutils Patch
- Debian Security Tracker
- Red Hat Security Advisory

Summary

CVE-2023-1972 showcases how unchecked memory handling, even in well-established open-source projects, can lead to dangerous crashes. If you’re a developer or sysadmin working with ELF files and using GNU Binutils, make sure to keep your tools updated for safe, reliable operation.

Timeline

Published on: 05/17/2023 22:15:00 UTC
Last modified on: 06/02/2023 17:40:00 UTC