Across the board, Casmer Labs has recorded the relative re-emerge of the “bleed” class of vulnerabilities, a category of memory safety errors made infamous by Heartbleed. At a high level, MongoBleed occurs because the MongoDB server trusts a client’s assertion of data size during the network message compression process. By claiming a payload is large while sending something small, an attacker can trick the server into reading beyond the legitimate data, returning a “slide” of the server’s working RAM. This memory contains the “dirty” residue of previous operations, including, but not limited to, database secrets.

Technical Breakdown: How It Works

The vulnerability is rooted in the C++ codebase, specifically within src/mongo/transport/message_compressor_zlib.cpp.

1. The Logic Error: When a client sends a message using the OP_COMPRESSED opcode, the server allocates a heap memory buffer based on the “Uncompressed Size” field provided in the header. Because high-performance memory allocators do not zero-out memory for efficiency, this new buffer is filled with “garbage” data from previous threads.
2. The Exploitation Mechanism:
   • An attacker sends a header claiming 1MB of uncompressed data, but provides a payload that only decompresses to to 1 byte.
   • The server allocates 1MB of memory, writes 1 byte, and then, due to the logic error, reports that the entire 1MB is “valid” data.
   • The server transmits the 1MB of “dirty” memory back to the client.
3. Weaponization Speed: The speed of weaponization for MongoBleed has been extremely rapid. Following public disclosure on December 19, functional PoC code was released on Christmas Day. Researchers have noted that Ai-assisted coding tools have significantly reduced the time required to develop these exploits.

Scope and Exposure

While managed services like MongoDB Atlas were patched preemptively, self-hosted and legacy instances remain vulnerable. According to Varonis, an approximate 87000 to 100000 MongoDB instances are currently exposed to the public internet.

Separate research indicates that 42% of cloud environments contain at least one vulnerable instance. This suggests that even if a database isn’t public, it remains a high-value target for lateral movement once an attacker gains an initial foothold within a victim’s network.

Relevant Controls

Casmer Labs recommends all organizations investigate properly and take the following steps, if necessary:

• Immediately update MongoDB binaries to the fixed versions (eg., 8.0.17, 7.0.28, or 6.0.27). For legacy versions (3.6, 4.0, 4.2) that are EOL, an upgrade to version 4.4 or higher is the only permanent solution
• If patching is delayed, disable zlib compression in the mongod.conf file by removing it from the net.compression.compressors list
• Harden the network by restricting port 27017 to known, trusted IP addresses via firewall rules
• Regularly rotate credentials
• Deploy IDS/IPS rules to detect the specificf anomaly in OP_COMPRESSED headers