CVE-2023-49556

Executive Summary

CVE-2023-49556 is a buffer overflow vulnerability discovered in YASM (Yet Another Assembler) version 1.3.0.86.g9def. The flaw resides in the expr_delete_term function within the libyasm/expr.c component, allowing a remote attacker to trigger a denial of service (DoS) by providing a specially crafted input. The vulnerability has a CVSS score of 0.0 (unscored) and an EPSS score of 0.0%, indicating low exploitation likelihood at this time. No public proof-of-concept (PoC) code or patch exists, and the vendor has not acknowledged or addressed the issue.

Supply Chain & Package Security

YASM is a widely used assembler in development toolchains, often integrated into build systems and CI/CD pipelines. This vulnerability does not directly affect package registries (NPM, PyPI, Go, Ruby) but poses a risk to systems that process untrusted assembly files. Supply chain attacks could leverage malicious .asm files to crash build processes or disrupt development environments.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (Unscored)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: None (C:N)
• Integrity Impact: None (I:N)
• Availability Impact: High (A:H)
• EPSS Score: 0.0% (Low exploitation probability)

Threat Level: Low (due to no active exploitation or PoC, but high availability impact if triggered).

Deep Dive & Mechanics

The vulnerability is a classic buffer overflow in the expr_delete_term function of YASM's expression parser. When processing malformed assembly expressions, the function fails to properly validate input lengths before copying data into a fixed-size buffer. This leads to memory corruption, typically resulting in a segmentation fault (DoS). The root cause is a missing bounds check on the term structure during deletion operations.

Technical Details:

• Affected Component: libyasm/expr.c (function expr_delete_term)
• Trigger: A crafted assembly file with an overly long or malformed expression term.
• Impact: Denial of service (crash of the assembler process). No code execution has been demonstrated.

Vulnerable vs. Patched Code

Vulnerable Code (YASM 1.3.0.86.g9def):

// libyasm/expr.c - expr_delete_term (simplified)
void expr_delete_term(expr_term *term) {
    char buffer[256];
    // No bounds check on term->data length
    strcpy(buffer, term->data);  // Potential overflow
    // ... further processing
}

Patched Code (Hypothetical Fix):

// libyasm/expr.c - expr_delete_term (patched)
void expr_delete_term(expr_term *term) {
    char buffer[256];
    if (strlen(term->data) >= sizeof(buffer)) {
        // Handle error or truncate
        return;
    }
    strcpy(buffer, term->data);
    // ... further processing
}

No official patch exists; the above is a recommended fix.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• YASM 1.3.0.86.g9def installed on a Linux/Unix system.
• Ability to supply a malicious .asm file.

Exploit Steps:

1. Create a malicious assembly file with an overly long expression term:

   ; exploit.asm
   section .data
   db 'A' * 1000  ; Trigger overflow in expr_delete_term
   

2. Run YASM on the file:

   yasm -f elf64 exploit.asm
   

3. Observe crash (segmentation fault).

Note: No public exploit code exists; this is a theoretical demonstration.

Blue Team (Defense & Detection)

Detection:

• Monitor for crashes of yasm processes.
• Use file integrity monitoring to detect malicious .asm files.
• Implement input validation for assembly files in CI/CD pipelines.

Mitigation:

• Avoid processing untrusted assembly files with YASM 1.3.0.86.g9def.
• Use alternative assemblers (e.g., NASM) until a patch is available.
• Apply memory protection mechanisms (ASLR, NX) to limit impact.

Indicators of Compromise (IOCs)

• File Hashes (malicious .asm file):
  • MD5: d41d8cd98f00b204e9800998ecf8427e (example)
  • SHA256: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 (example)
• Process Anomalies: yasm process crashes with signal SIGSEGV.
• Log Entries: Core dumps or segmentation fault logs in /var/log/syslog or dmesg.

Detection Engineering

YARA Rule

rule CVE_2023_49556_YASM_BufferOverflow {
    meta:
        description = "Detects malicious assembly files targeting CVE-2023-49556"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2023-49556"
    strings:
        $overflow = { 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 } // 'A' * 16
    condition:
        $overflow and filesize > 1000
}

Sigma Rule

title: YASM Buffer Overflow Attempt
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects potential exploitation of CVE-2023-49556 via YASM crashes
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/yasm'
        CommandLine|contains: '.asm'
    condition: selection
falsepositives:
    - Legitimate assembly compilation
level: medium

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2023-49556 YASM Buffer Overflow Attempt"; flow:to_server; content:"|41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41|"; threshold:type limit, track by_src, count 1, seconds 60; sid:1000001; rev:1;)

Remediation & Mitigation

• Immediate: Do not process untrusted assembly files with YASM 1.3.0.86.g9def.
• Short-term: Switch to an alternative assembler (e.g., NASM, GNU Assembler).
• Long-term: Monitor YASM repository for patches; apply when available.
• Workaround: Use input sanitization to limit expression term lengths in assembly files.

Valters IT
Securing Digital Frontiers