The critical vulnerability that lets attackers read your database memory without authentication, one of the most critical MongoDB vulnerabilities in recent history

What Happened?

In December 2025, security researchers uncovered a severe vulnerability in MongoDB nicknamed "MongoBleed" (CVE-2025–14847). This bug affected nearly every MongoDB version since 2017, potentially exposing passwords, API keys, customer data and other sensitive information stored in database memory.

What makes this vulnerability particularly alarming is its simplicity: an attacker only needed network access to a MongoDB instance, no authentication required. With over 213,000 publicly accessible MongoDB databases on the internet, the potential impact was massive.

Understanding the Vulnerability: A Simple Analogy

Imagine you order a small coffee at a restaurant, but the waiter brings a big jug that still has other people's drinks in it, along with their names, card details and private conversations.​

You only wanted your coffee, but you also got everyone else's private information, this is what MongoBleed does, but with database memory instead of drinks.

The Technical Breakdown

How MongoDB Normally Works

MongoDB uses a custom protocol for communication and compresses messages using zlib to improve performance. When a compressed message arrives, the server:

  1. Reads the uncompressedSize field to know how much memory to allocate
  2. Allocates a buffer of that size
  3. Decompresses the message into the buffer
  4. Processes the decompressed data

Where Things Went Wrong

The critical bug was in step 3. After decompressing the message, MongoDB never verified that the actual decompressed size matched what the client claimed it would be. The server blindly trusted the uncompressedSize field provided by the client.

Here's what an attacker could do:

Step 1: Lie About the Size

  • Send a message claiming to be 1MB when uncompressed (uncompressedSize: 1MB)
  • But actually send only 1KB of real data

Step 2: Get Access to Leftover Memory

The server allocates 1MB of memory, decompresses 1KB of actual data, and leaves 999KB of uninitialized heap memory exposed.

[ 1KB Real Data | 999KB of Previous Database Operations' Memory ]

That 999KB could contain anything from previous operations:

  • Cleartext passwords
  • Session tokens and API keys
  • Customer personal information
  • Database credentials
  • IP addresses and system information

Step 3: Extract the Secrets

MongoDB uses BSON (Binary JSON) for message formatting. In BSON, strings are null-terminated, meaning they end with a special \0 character, a standard practice in C programming.

An attacker could send malformed BSON data without a null terminator. When MongoDB tried to parse the first field name, it would keep reading beyond the attacker's data into the leftover memory until it found a null terminator.

For example:

[ "fieldName | password: admin123\0 | apiKey: xyz789... ]

MongoDB would read "fieldName | password: admin123" as the field name, realize it's invalid BSON, and helpfully return an error message containing that exact string:

{
  "ok": 0,
  "errmsg": "invalid BSON field name 'fieldName | password: admin123'",
  "code": 2,
  "codeName": "BadValue"
}

Boom. The attacker just extracted a password from memory.

By repeating this thousands of times per second, attackers could systematically scan through the database's entire memory space, extracting sensitive data piece by piece.

Why This Was So Dangerous

1. Pre-Authentication Exploit

This vulnerability occurred during the message parsing phase, before any authentication checks. An attacker didn't need valid credentials, just network access to the database.

2. Eight Years of Exposure

The bug was introduced in May 2017 (MongoDB 3.6.0) and remained undetected until December 2025. That's nearly 8 years of potential exposure for millions of databases worldwide.

3. Trivial to Exploit

Unlike complex exploits requiring advanced techniques, MongoBleed was remarkably simple. Any attacker with basic networking knowledge could exploit it, making it accessible even to less sophisticated threat actors.

4. Massive Scale

With over 213,000 publicly accessible MongoDB instances on the internet (according to Shodan), the attack surface was enormous. Many organizations expose their databases to the internet despite security best practices advising against it.

The Response and Timeline

The vulnerability's disclosure and patching timeline raised some concerns in the security community:

  • May 2017: Bug introduced in MongoDB codebase
  • December 17, 2025: Fix initially committed (private repository)
  • December 18, 2025: MongoDB Atlas (cloud service) reportedly patched
  • December 19, 2025: CVE-2025–14847 published; MongoDB 8.0.17 released with fix
  • December 22, 2025: Fix merged in public repository
  • December 24, 2025: MongoDB officially announced Atlas was fully patched
  • December 26, 2025: Public proof-of-concept exploit released on GitHub

MongoDB stated they found "no evidence that this issue has been exploited or that any customer data has been compromised." However, given the simplicity of the exploit and the long exposure window, it's impossible to know for certain.

What You Should Do Right Now

If you're running MongoDB, take immediate action:

Immediate Actions

  1. Update to the latest patched version of MongoDB that includes the CVE-2025–14847 fix
  2. Disable zlib compression as a temporary mitigation if you cannot immediately update (set net.compression.compressors to an empty array)
  3. Audit your network exposure: Ensure your MongoDB instances are NOT directly accessible from the internet

Long-Term Security Practices

  1. Never expose databases directly to the internet — use VPNs, private networks, or bastion hosts
  2. Implement strong authentication with role-based access control (RBAC)
  3. Enable TLS encryption for all database connections
  4. Regular security audits: Schedule penetration testing and vulnerability assessments
  5. Monitor database logs for unusual access patterns or connection attempts
  6. Keep your software updated: Establish a regular patching schedule
  7. Use MongoDB Atlas (cloud service) if self-hosting is a security concern — it was patched before the CVE was published

The Bigger Picture

MongoBleed is a stark reminder that security vulnerabilities can hide in plain sight for years. A single line of missing validation code, failing to check that decompressed data matched the expected size — created an 8-year window of vulnerability affecting millions of databases.

This incident underscores several critical lessons:

Defense in Depth: Never rely on a single security control. Even with authentication, database exposure created unnecessary risk.

Secure by Default: Systems should be designed with security as a core principle, not an afterthought. Had MongoDB validated the decompression size, this vulnerability wouldn't have existed.

Assume Breach: Encrypt sensitive data at rest and in transit. If a breach occurs, encryption provides a critical last line of defense.

Visibility Matters: Regular security audits, code reviews, and penetration testing can uncover vulnerabilities before attackers do.

Conclusion

MongoBleed serves as a powerful reminder that even mature, widely-used technologies can harbor critical security flaws. The vulnerability's simplicity, long exposure window, and pre-authentication nature made it particularly dangerous.

For security professionals, developers, and organizations using MongoDB, the message is clear: security is not a one-time fix but an ongoing commitment. Regular updates, proper configuration, and defense-in-depth strategies are essential to protecting sensitive data in an increasingly hostile threat landscape.

Don't wait for the next MongoBleed. Review your MongoDB security posture today.

References:

  1. CVE-2025–14847 Official Entry: NIST National Vulnerability Database
  2. MongoDB Security Advisory: MongoDB Security Center
  3. MongoBleed Proof of Concept: GitHub — joe-desimone/mongobleed
  4. MongoDB Documentation on Compression: Network Compression
  5. Shodan Database Search: Shodan MongoDB Search (Search for publicly exposed databases)
  6. BSON Specification: BSON.org
  7. MongoDB Security Checklist: MongoDB Security Best Practices
  8. OWASP Database Security: OWASP Database Security Cheat Sheet

Stay safe, patch your systems, and never expose your databases to the internet.

About Me

Hi, I'm Dhanush Nehru an Engineer and Content Creator. I document my journey through articles and videos, sharing real-world insights about DevOps, automation, security, cloud engineering and more.

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