PostgreSQL is one of the world’s most trusted open-source relational databases, and libpq is its C client library that many applications depend on for database communication. However, a recently disclosed vulnerability—CVE-2025-12818—has raised serious concerns, as it could lead to massive memory corruption, crashes, or even potential code execution in systems using outdated PostgreSQL releases.

Let’s dive deep into what this bug is, how it works, and how developers and sysadmins can protect their applications.

What is CVE-2025-12818?

This vulnerability is an integer wraparound bug in multiple functions within the libpq library (the official PostgreSQL C client library). When an application provides crafted input, or if a malicious network peer interacts with libpq, the bug can cause an undersized memory allocation. Then, by feeding more data than allocated, libpq writes far outside the allocated memory—sometimes by hundreds of megabytes!

The immediate consequence is a segmentation fault and application crash, but depending on the environment, it could also lead to unpredictable behavior or be combined with other issues for code execution.

Fixed in: 18.1, 17.7, 16.11, 15.15, 14.20, 13.23

> Check your database client library version—not just your server!

Integer Wraparound Explainer

In C programming, integers have fixed limits. If a calculation exceeds that limit, it "wraps around"—for example, UINT_MAX + 1 becomes zero. Attackers can feed data that tricks the code into believing a required buffer is small, when in reality, much more data is coming.

If code like this exists

// Vulnerable snippet (hypothetical example)
int size = get_int_from_packet(packet);
char *buf = malloc(size); // what if size is very large, or negative?
memcpy(buf, packet->data, size); // booom! buffer overflow!

If get_int_from_packet() returns a large enough value that, when combined with some addition or multiplication, wraps around and becomes a small (or even negative) number, malloc allocates a tiny buffer but memcpy copies a huge amount. This overwrites adjacent memory—classic out-of-bounds (OOB) write.

Real-World in libpq

Several internal functions in libpq did not strictly check their allocation sizes against possible wraparounds, especially given values coming from remote or application sources.

Example (simplified)

int length = read_from_network();
int alloc_size = length + HEADER_SIZE; // Wraparound not checked

char *buf = malloc(alloc_size);
read_bytes(sock, buf, alloc_size);

If length is large enough, alloc_size wraps around. The result: the buffer is much too small for incoming data, leading to a buffer overflow.

Exploitation Scenario

The attacker lets their input (or packet from the network) set a length value close to the maximum integer (like 2,147,483,647 for 32-bit signed int). When some header or offset is added to this, it wraps around to a small positive value or even a negative one.

The application then allocates a buffer of the wrapped value and copies data—or lets libpq process a much larger chunk. This causes writing outside the intended buffer, corrupting hundreds of megabytes of RAM, overwriting application state, or simply crashing with a segmentation fault.

Example Exploit (Python, Proof of Crash - NOT Payload Delivery)

import socket

def send_exploit():
    s = socket.create_connection(('target-postgres', 5432))
    # Crafted oversized length field; actual bytes may differ by protocol version
    length = x7fffffff    # Largest positive signed 32-bit int
    # + 8 (header) will wrap if code isn't careful
    # Let's send something similar (simplified protocol)
    packet = length.to_bytes(4, 'big') + b'A' * 10000
    s.sendall(packet)
    s.close()

send_exploit()

*This example is illustrative; real exploitation would require precise protocol knowledge and likely a much larger payload to crash the app reliably.*

Impact

- Denial of Service: The application (such as web server, backend service, or automation tool) which uses the affected libpq will crash when exploited.
- Potential Arbitrary Code Execution: If other conditions are right, overwriting large regions of memory could be chained with further vulnerabilities to execute code.
- Systems At Risk: Any application (not just PostgreSQL servers) linked against affected libpq—so think Python psycopg2, PHP's pgsql extension, Java JDBC/native bridge, C/C++ binaries, etc.

How to Fix

Update your client library now! Even if your PostgreSQL SERVER is patched, any client software using an old libpq is at risk.

For system packages:

  sudo apt-get update
  sudo apt-get upgrade libpq5

For source installs:

  wget https://ftp.postgresql.org/pub/source/v16.11/postgresql-16.11.tar.gz
  # build, install, and replace client library

Also update your downstream drivers:

Node.js: Update node-postgres

- PHP: Update PHP/pgsql

References

- PostgreSQL Security Release Details
- Official Announcement
- CVE Entry: CVE-2025-12818
- libpq Documentation

Conclusion

CVE-2025-12818 is a critical bug due to its ease of exploitation and severe outcome (out-of-bounds writes of hundreds of megabytes). Unlike server-side vulnerabilities, this one hits wherever the PostgreSQL client library is used—including all sorts of applications, tools, and services.

Patch now—don’t wait for a crash, or a more motivated attacker!


*If this writeup helped you understand CVE-2025-12818, please share it with your engineering team or client developers. Stay safe and keep your libraries up-to-date!*

Timeline

Published on: 11/13/2025 13:15:45 UTC
Last modified on: 11/14/2025 16:42:03 UTC