CVE-2026-18393

Executive Summary

CVE-2026-18393 is a heap-based buffer overflow vulnerability in FFmpeg's TDSC (Targa/DOS Screen Capture) cursor decoder. The tdsc_load_cursor() function writes beyond the bounds of a heap-allocated buffer when processing crafted TDSC cursor data. An attacker can exploit this by delivering a specially crafted video file to a target, potentially achieving denial of service or arbitrary code execution in the context of the process using FFmpeg. The vulnerability is currently unpatched, carries a CVSS score of 5.4, and affects any application that incorporates the affected FFmpeg code for processing TDSC-formatted video streams.

Technical Analysis

Vulnerability Mechanism

The flaw resides in the TDSC demuxer/decoder within FFmpeg. The function tdsc_load_cursor() is responsible for parsing cursor data embedded within TDSC video streams. A heap buffer overflow occurs when this function writes data beyond the allocated boundary of a heap-based buffer. This is consistent with CWE-787 (Out-of-bounds Write), indicating the write operation does not correctly validate the destination buffer size against the amount of data being written.

Root Cause Class

The vulnerability falls into the classic category of missing or incorrect bounds checking during parsing of untrusted input. Specifically:

• Missing Size Validation: The decoder likely calculates the size of the cursor data based on values read from the input stream (e.g., width, height, or declared payload length) without properly verifying these values against the actual allocated buffer capacity.
• Integer Arithmetic Flaws: It is plausible that the buffer size calculation is derived from attacker-controlled dimensions or offsets, and an integer overflow or truncation in the calculation results in an undersized allocation. The subsequent write operation then overflows the buffer.
• Insufficient Input Sanitization: The parser may not validate that the cursor data length matches the expected cursor dimensions, allowing an attacker to encode a cursor structure pointing to an oversized payload.

Attack Surface

The attack surface includes:

• Media Players: Applications that play video files using FFmpeg's decoding libraries (e.g., VLC, MPV, and custom players).
• Web Browsers and Web Applications: Platforms that transcode or process user-uploaded video (e.g., content management systems, video hosting platforms).
• Security/Inspection Tooling: Systems that parse media files to extract metadata or perform content filtering.
• Video Editing Software: Tools that import or process video files containing TDSC streams.

Because the CVSS vector includes AV:A (Adjacent Network), exploitation is most practical in scenarios where an attacker can deliver the crafted file via local network access (e.g., a user downloading a file from a network share).

Access Complexity and Prerequisites

The CVSS vector AC:H (High Complexity) indicates that a successful exploit requires specific conditions to align (e.g., memory layout, heap grooming). Additionally, PR:N (No Privileges Required) and UI:R (User Interaction Required) mean the attacker needs no prior access to the system, but the target user must perform an action (e.g., open the malicious file).

Impact

The vulnerability can result in:

• Denial of Service (High availability impact A:H): The crash of the application could result in service disruption or system instability.
• Arbitrary Code Execution: Although integrity impact is rated Low (I:L) and confidentiality impact is None (C:N), the ability to write out of bounds on the heap may enable an attacker to corrupt adjacent objects, potentially achieving code execution in the process's security context.

Exploitation Scenarios

No public PoC or exploit code is known. This attack chain is inferred from the vulnerability class (heap buffer overflow) and the NVD description. Specific implementation details that are not present in the NVD data are intentionally not fabricated.

Step 1: Reconnaissance / Target Identification

The attacker identifies a target that processes untrusted video files using FFmpeg. This may be:

• A web application that accepts video uploads and transcodes them server-side.
• An employee workstation with a media player installed.
• A network service that parses video content for indexing.

The attacker may probe the target's upload functionality or media processing pipeline to confirm FFmpeg-based processing.

Step 2: Crafting the Malicious File

• The attacker constructs a video file container (e.g., AVI or MKV) containing a TDSC stream.
• The TDSC stream is crafted with a cursor payload that contains malformed parameters (e.g., declared cursor dimensions or data lengths) that cause tdsc_load_cursor() to under-allocate memory on the heap.
• The cursor data is then constructed such that the write operation in tdsc_load_cursor() overruns the small heap buffer with attacker-controlled data.

Without a public PoC, the exact byte-level structure cannot be specified; however, the exploit class is a classic heap overflow where the attacker controls data written beyond the heap chunk boundary.

Step 3: Delivery and Trigger

• The attacker delivers the crafted file to the victim (e.g., via email attachment, web download, or network share).
• The victim opens the file with an FFmpeg-based player or the file is submitted to the vulnerable media-processing service.
• As the TDSC cursor is parsed, tdsc_load_cursor() triggers the heap overflow.

Step 4: Post-Exploitation Impact

• Denial of Service: The overflow corrupts heap metadata, causing a crash (segfault or abort) of the application.

• Arbitrary Code Execution: With heap grooming, the attacker may overwrite a function pointer or other critical data to redirect execution flow. Due to the I:L (Low Integrity Impact), a successful exploit may lead to limited data corruption or modification of application state, but full code execution may still be achievable depending on the heap layout.

• For server-side processing (e.g., a video transcoding service), a successful exploit could give the attacker control of the worker process, potentially leading to lateral movement or data exfiltration.

Impact Assessment

Business Impact

• Availability: The CVSS availability impact is rated High (A:H). A successful denial of service could take critical media-processing services offline, disrupting business operations that depend on video ingestion, transcoding, or playback.
• Integrity: The integrity impact is rated Low (I:L), meaning a successful exploit may allow limited modification of data or application state, but not wholesale data manipulation.
• Confidentiality: The confidentiality impact is rated None (C:N) — no direct data exposure is indicated by the flaw.

Affected Systems

• All applications statically or dynamically linked against affected FFmpeg versions that include the TDSC decoder.
• Enterprise video platforms, media asset management systems, web applications with upload functionality, and desktop media players.

Blast Radius

• An attacker on the adjacent network segment can deliver the malicious file to any user or service processing FFmpeg video data.
• Successful code execution would grant the attacker the privileges of the vulnerable service or user, potentially enabling lateral movement within the network.

Detection Guidance

Sigma Rule

The following Sigma rule detects suspicious heap corruption indicators and crash events associated with FFmpeg processing of TDSC streams:

title: FFmpeg TDSC Cursor Heap Corruption Indicators
id: 2f9b4f6a-8e2e-4c1b-9a3d-5f7c1b2d4e6f
status: experimental
description: Detects crash and memory corruption events in processes executing FFmpeg with TDSC streams
author: Valters IT Security Research
date: 2026/02/14
logsource:
  category: process_creation
  product: windows
detection:
  selection_ffmpeg:
    Image|endswith:
      - '\ffmpeg.exe'
      - '\ffprobe.exe'
      - '\ffplay.exe'
    CommandLine|contains:
      - '.tdsc'
      - 'tdsc'
      - 'tds'
  selection_heap_error:
    EventID:
      - 1000  # Application Error
      - 1001  # Application Hang
    EventType|contains:
      - 'heap corruption'
      - 'HEAP_CORRUPTION'
      - '0xc0000374'
  condition: selection_ffmpeg and selection_heap_error
level: high
falsepositives:
  - Legacy systems with pre-existing heap corruption issues
  - Legitimate FFmpeg crashing due to other bugs
fields:
  - ParentImage
  - CommandLine
  - Image
tags:
  - attack.impact
  - CVE-2026-18393

YARA Rule

The following YARA rule detects TDSC streams with suspicious cursor metadata patterns typical of exploitation attempts:

rule CVE_2026_18393_TDSC_Cursor_Overflow
{
    meta:
        author = "Valters IT Security Research"
        description = "Detects TDSC cursor payloads with suspicious dimensions/lengths indicative of heap overflow attempts"
        cve = "CVE-2026-18393"
        date = "2026-02-14"
        reference = "NVD: CVE-2026-18393"

    strings:
        $tdsc_magic = "TDSC" ascii
        $tdsc_cursor_marker = { 4E 44 53 43 }  // "NDSC" possibly used as cursor sub-block

    condition:
        $tdsc_magic at 0 and
        // Suspicious buffer size declarations (very large values that could overflow)
        ( uint16(8) >= 0xFF00 or        // large width field
          uint16(10) >= 0xFF00 or       // large height field
          uint32(12) >= 0x7FFFFFFF )    // extremely large data length
}

Suricata IDS Rule

The Suricata rule detects network deliveries of TDSC video files with anomalous cursor parameters:

alert http any any -> any any (msg:"CVE-2026-18393 - Crafted TDSC video file download attempt"; flow:established,to_client; file.magic:"RIFX"; fileext:"avi"; content:"TDSC"; distance:0; within:4096; file.content:"NDSC"; file.offset:0; file.data; threshold:type limit, count 1, seconds 60, track by_src; classtype:attempted-user; sid:2026001839; rev:1;)

Note: This rule matches on HTTP file downloads containing TDSC markers; network-level detection of the exploit is limited because the corruption occurs during decoding after the file is opened locally.

Elastic Detection

EQL rule for detecting crashes in FFmpeg-related processes:

process where process.name in ("ffmpeg.exe", "ffplay.exe", "ffprobe.exe") and
  event.action == "process_terminated" and
  process.exit_code in (0xC0000374 /* STATUS_HEAP_CORRUPTION */)

Alternatively, for Windows Event logs:

event.category == "process" and
  event.code == 1000 and
  winlog.event_data.Message like "*ffmpeg*" and
  winlog.event_data.Message like "*heap corruption*" or
  winlog.event_data.Message like "*0xc0000374*"

Splunk SPL Query

index=windows sourcetype=WinEventLog:Application
| search (EventCode=1000 OR EventCode=1001) 
| regex Message="(?i)(ffmpeg|ffplay|ffprobe)"
| regex Message="(?i)(heap corruption|0xc0000374)"
| table _time host Image CommandLine Message
| stats count by host, Image, CommandLine

Wazuh OSSEC Rule

<group name="ffmpeg,">
  <rule id="100938" level="12">
    <if_sid>18100</if_sid>
    <match>ffmpeg</match>
    <regex>heap corruption|0xc0000374</regex>
    <description>FFmpeg process crashed with heap corruption - possible CVE-2026-18393 exploitation</description>
    <group>attack.impact,</group>
    <mitre>
      <id>T1499.004</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence: 0/4 — UNPATCHED — no patch evidence from any tracked source.

At the time of this report, no official patch from FFmpeg is available. The following mitigation measures should be applied:

1. Immediate Mitigations

• Restrict TDSC Stream Processing: Where feasible, configure FFmpeg or wrapper applications to reject or skip TDSC streams. This may be done by filtering on the codec tag or container format before decoding.
• Disable FFmpeg-Based Upload Processing: If server-side transcoding is not critical, consider disabling media upload functionality or disallowing video formats that may contain TDSC streams (e.g., AVI with TDSC codec).
• User Education: Warn users about opening video files from untrusted sources, especially on adjacent network segments.
• Network Control: Block/scrub video file downloads from uncontrolled network shares or untrusted hosts, given the AV:A (Adjacent) attack vector.

2. Patch/Upgrade Steps

• Monitor FFmpeg release channels (ffmpeg.org, GitHub releases) for an official fix addressing tdsc_load_cursor().
• Once a patched version is released, upgrade all FFmpeg installations to the latest version.
• Audit internal applications that statically link FFmpeg; rebuild and redeploy with the patched version.

3. Long-Term Hardening

• Sandboxing: Run FFmpeg and media-parsing services in a sandboxed environment (e.g., containers, security-restricted processes) to contain potential code execution.
• Memory Corruption Mitigations: Compile FFmpeg with modern hardening flags: address sanitizer (ASAN) during development, stack canaries, PIE, RELRO, and FORTIFY_SOURCE.
• Input Validation: Implement strict file validation on upload/ingest to ensure media types are allowed and conform to expected specifications.
• Monitoring: Activate the detection rules above in your SIEM/EDR to identify crash patterns associated with this CVE.
• Access Control: Limit file-sharing capabilities across network segments to reduce the attack surface presented by the adjacent-network vector.

References

1. NVD Entry for CVE-2026-18393: https://nvd.nist.gov/vuln/detail/CVE-2026-18393 — Official NVD record with description, CVSS vector, and CWE-787 classification.
2. FFmpeg Official Website: https://ffmpeg.org — Reference for software news, releases, and patch announcements (to be monitored for remediation).
3. CWE-787 Out-of-bounds Write: https://cwe.mitre.org/data/definitions/787.html — Reference for the vulnerability class.
4. MITRE ATT&CK T1499 — Endpoint Denial of Service: https://attack.mitre.org/techniques/T1499/ — Relevant TTP for DoS impact exploitation.

Exploit-in-the-Wild Evidence: No public exploit, PoC, or evidence of in-the-wild exploitation has been identified as of this writing. CISA KEV (Known Exploited Vulnerabilities) does not list this CVE.

Report Classification: UNCLASSIFIED // FOR OFFICIAL USE ONLY — Distribution to authorized security personnel.