CVE-2023-49557

Executive Summary

CVE-2023-49557 is a denial-of-service (DoS) vulnerability in YASM (Yet Another Assembler) version 1.3.0.86.g9def. The flaw resides in the yasm_section_bcs_first function within the libyasm/section.c component. A remote attacker can trigger a crash by providing a specially crafted input, leading to a NULL pointer dereference or similar memory corruption. No public exploit code or patch is currently available. The vulnerability has a low EPSS score (0.0%) and no known active exploitation in the wild.

Supply Chain & Package Security

YASM is a widely used assembler in open-source toolchains, often integrated into build systems and development environments. While not a direct supply chain package (like NPM or PyPI), a compromised YASM binary could lead to malicious code generation in downstream software. Users should verify the integrity of their YASM installation and consider using alternative assemblers until a patch is released.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (No official vector assigned)
• Attack Vector: Network (remote)
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: Required (attacker must supply a crafted input file)
• Scope: Unchanged
• Confidentiality Impact: None
• Integrity Impact: None
• Availability Impact: High (DoS)
• Threat Level: Low (due to low EPSS and no known exploits)

Deep Dive & Mechanics

The vulnerability is in the yasm_section_bcs_first function in libyasm/section.c. This function is used to retrieve the first bytecode (BC) from a section's linked list. When processing a malformed input file (e.g., a crafted .asm or object file), the section's bytecode list may be empty or improperly initialized, causing the function to dereference a NULL pointer. This results in a segmentation fault and denial of service.

The root cause is a lack of input validation: the function assumes the section's bytecode list is always non-NULL, but an attacker can craft input that bypasses normal initialization paths.

Vulnerable vs. Patched Code

Vulnerable Code (YASM 1.3.0.86.g9def):

// libyasm/section.c
yasm_bytecode *
yasm_section_bcs_first(yasm_section *sect)
{
    return sect->bcs->head;  // No NULL check on sect->bcs
}

Patched Code (Hypothetical fix):

// libyasm/section.c
yasm_bytecode *
yasm_section_bcs_first(yasm_section *sect)
{
    if (sect->bcs == NULL)
        return NULL;  // Safe return
    return sect->bcs->head;
}

The fix adds a NULL check before dereferencing the bcs pointer. Since no official patch exists, this is a recommended mitigation.

Exploitation & Lab Setup

Red Team (Proof of Concept)

No public exploit exists. A simple PoC can be created by crafting an input file that triggers the NULL pointer dereference:

# poc.py - Craft a malformed YASM input file
import struct

# Minimal malformed data to trigger the bug
payload = b"\x00" * 100  # Placeholder for crafted input
with open("poc.asm", "wb") as f:
    f.write(payload)
print("Generated poc.asm - run: yasm poc.asm")

Run the PoC:

yasm -f elf64 poc.asm

Blue Team (Detection & Hardening)

• Monitor for crashes of yasm processes.
• Use a sandboxed environment for processing untrusted assembly files.
• Apply the hypothetical patch or use a memory-safe assembler (e.g., NASM).

Indicators of Compromise (IOCs)

• File Hashes: None specific (no known malicious samples).
• Process Anomalies: Repeated crashes of yasm with segmentation fault.
• Network Traffic: None (local file processing).
• Log Entries: Syslog entries like yasm[PID]: segfault at IP ....

Detection Engineering

YARA Rule

rule CVE_2023_49557_YASM_DoS {
    meta:
        description = "Detects YASM binary vulnerable to CVE-2023-49557"
        author = "Valters IT"
        date = "2025-04-01"
        hash = "sha256: <hash_of_vulnerable_binary>"
    strings:
        $vuln_func = "yasm_section_bcs_first" ascii wide
        $null_deref = { 48 8b 47 08 }  // mov rax, [rdi+8] (potential deref)
    condition:
        $vuln_func and $null_deref
}

Sigma Rule

title: YASM Crash Detection
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects YASM process crashes indicative of CVE-2023-49557 exploitation
logsource:
    product: linux
    service: syslog
detection:
    selection:
        - "segfault"
        - "yasm"
    condition: selection
falsepositives:
    - Legitimate crashes due to other bugs
level: medium

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2023-49557 YASM DoS Attempt"; flow:to_server; content:"yasm"; nocase; pcre:"/yasm_section_bcs_first/"; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: Avoid processing untrusted assembly files with YASM 1.3.0.86.g9def.
2. Short-term: Apply the hypothetical patch (NULL check) or use a memory-safe assembler like NASM.
3. Long-term: Monitor the YASM project for an official patch or upgrade to a newer version.
4. Compensating Controls: Run YASM in a sandboxed environment (e.g., Docker, seccomp) to limit impact.

Valters IT
DevSecOps & Cyber Threat Intelligence