CVE-2025-67433: Critical Heap Overflow in Open TFTP Server Exposes Systems to Remote Crash and Potential Code Execution

Open TFTP Server MultiThreaded – Heap Buffer Overflow

CVE ID: CVE-2025-67433
Affected Product: Open TFTP Server MultiThreaded
Affected Version: v1.7 (1.70 builds observed vulnerable)
Vulnerability Type: Heap-Based Buffer Overflow
Attack Vector: Remote (UDP – TFTP service)
Authentication Required: No
User Interaction Required: No
Impact: Denial of Service (confirmed), Potential Remote Code Execution (theoretical under specific configurations)
Exploit Availability: Public Proof-of-Concept available (educational/security research purposes)
Exposure Surface: UDP/69 (TFTP) and dynamically assigned TID ports

Official Project / Upgrade Location:
https://sourceforge.net/projects/tftp-server/files/tftp%20server%20multithreaded/

Technical Overview

A heap buffer overflow condition exists in the request handling logic of Open TFTP Server MultiThreaded within the processRequest routine. The vulnerability is triggered when the server processes specially crafted TFTP packets that manipulate negotiated block size values and data payload lengths.

The TFTP protocol allows a client to negotiate transfer parameters such as blksize through option extension mechanisms. When an unusually large block size is negotiated, memory is allocated dynamically on the heap to accommodate the requested transfer size. Improper bounds validation and unsafe string handling operations are later performed using this allocated memory.

Under certain packet sequences, attacker-controlled heap data is reused in formatted logging or internal processing routines where fixed-length stack buffers are used without sufficient boundary validation. This results in memory corruption that can overwrite adjacent memory structures.

The immediate result is typically process termination (access violation or segmentation fault). However, because memory corruption affects stack memory following heap manipulation, the potential exists for controlled memory overwrite conditions that could theoretically allow instruction pointer control in specific runtime environments.

Root Cause Analysis

The vulnerability is caused by:

Acceptance of arbitrarily large blksize values during TFTP option negotiation.
Dynamic heap allocation based on attacker-controlled size parameters.
Reuse of heap buffer contents in subsequent string formatting operations.
Use of unsafe copy mechanisms (e.g., unbounded formatted output into fixed stack buffers).
Lack of strict length validation before memory operations.

This condition aligns with:

CWE-122: Heap-Based Buffer Overflow
Improper Input Validation
Improper Bounds Checking

Exploitation Mechanics

Exploitation is performed entirely over UDP without authentication.

A typical exploitation sequence involves:

Sending a TFTP Write Request (WRQ) with a large blksize option (e.g., 60000 bytes).
Receiving an Option Acknowledgment (OACK) from the server.
Sending a large DATA packet designed to fill the allocated heap buffer with controlled content.
Triggering further request processing that causes the server to reuse heap data in stack-based operations.
Causing memory corruption during formatted logging or processing routines.

The result may be:

Immediate service crash (most common outcome).
Memory corruption with potential return address overwrite.
Uncontrolled exception leading to process termination.
Potential for exploit chaining in environments lacking memory protections.

Exploit code demonstrating this behavior has been published publicly for research and defensive validation purposes.

Impact Assessment

Confirmed Impact

Remote Denial of Service.
Service instability under repeated exploitation attempts.
Forced termination of TFTP process.

Potential Advanced Impact

Stack corruption.
Instruction pointer overwrite under controlled conditions.
Theoretical remote code execution if exploit primitives are stabilized.

Risk increases significantly if:

Write operations are enabled.
Large block sizes are permitted.
The service runs with elevated privileges.
The service is exposed externally.

Attack Surface

UDP port 69 (initial negotiation).
Dynamically assigned UDP TID ports.
Systems using TFTP for:
- Firmware deployment
- Network device configuration
- Backup distribution
- PXE environments

Internet-exposed TFTP services are considered high risk.

Indicators of Compromise

Network-Level Indicators

WRQ packets containing unusually large blksize values.
DATA packets significantly larger than 512 bytes (default TFTP block).
Repeated WRQ/DATA/ERROR sequences within short intervals.
Abnormal UDP session bursts targeting TFTP ports.
Multiple crash-triggering attempts from same source IP.

Host-Level Indicators

Application crashes referencing OpenTFTPServer executable.
Access violation errors.
Unexpected process restarts.
Corrupted or partially written files in TFTP directory.
High CPU spikes before crash.

Detection Rules

Suricata / Snort Rule – Large Block Size Negotiation

alert udp any any -> $HOME_NET 69 (
msg:"TFTP WRQ with abnormal large blksize - CVE-2025-67433 attempt";
content:"|00 02|"; depth:2;
content:"blksize"; nocase; distance:0; within:50;
pcre:"/blksize\x00([5-9][0-9]{3,}|[1-9][0-9]{4,})\x00/";
classtype:attempted-dos;
sid:300001;
rev:1;
)

Suricata Rule – Oversized TFTP DATA Packet

alert udp any any -> $HOME_NET any (
msg:"TFTP oversized DATA packet possible heap overflow attempt";
content:"|00 03|"; depth:2;
dsize:>1400;
classtype:attempted-dos;
sid:300002;
rev:1;
)

Zeek Detection Logic

event tftp_request(c: connection, opcode: count, payload: string)
{
    if ( opcode == 2 && /blksize/ in payload )
    {
        # Extract numeric value and alert if > 4096
    }
}

Splunk Query – TFTP Crash Correlation

index=windows_logs
(EventCode=1000 OR EventCode=1001)
"OpenTFTPServer"
| stats count by ComputerName, FaultingApplicationName, Source_IP

Firewall Log Detection Query

SELECT src_ip, COUNT(*) as attempts
FROM firewall_logs
WHERE dest_port=69
AND protocol='UDP'
GROUP BY src_ip
HAVING COUNT(*) > 50;

Log Sources for Monitoring

Network IDS/IPS logs
Firewall logs (UDP 69 traffic)
NetFlow records
Windows Application Event Logs
TFTP application logs
Endpoint Detection and Response telemetry

Mitigation

Until a patched release is officially provided:

Disable TFTP write functionality if not required.
Restrict UDP/69 access to trusted internal networks.
Block external exposure at firewall.
Limit maximum allowed block size in configuration (if supported).
Deploy IDS rules provided above.
Run service under least-privilege account.
Enable process crash monitoring.

Remediation

Monitor the official project repository for updated secure builds.

Official project location for upgrade or patch information:

https://sourceforge.net/projects/tftp-server/files/tftp%20server%20multithreaded

If a newer secure version becomes available, immediate upgrade is strongly recommended.

Security Advisory Classification

Vulnerability Category: Memory Corruption
Weakness Type: Heap-Based Buffer Overflow
Exploit Maturity: Public Proof-of-Concept
Attack Complexity: Low to Moderate
Privileges Required: None
Network Accessible: Yes

Final Takeaway

Proof-of-concept exploit code has been made public for defensive research and vulnerability validation. Testing should only be conducted in controlled lab environments. Unauthorized exploitation of systems without explicit permission may violate applicable laws and regulations.