pgjdbc: Unbounded PBKDF2 iterations in SCRAM authentication allows CPU exhaustion DoS

Summary

pgjdbc is vulnerable to a client-side denial of service during SCRAM-SHA-256 authentication.

Impact

A malicious server can instruct the driver to perform SCRAM authentication with a very large iteration count. With a large enough value, the client spends an unbounded amount of CPU time inside PBKDF2 before authentication can fail. A single attempt ties up a CPU core. Repeated or concurrent attempts exhaust client CPU and can wedge connection pools.

In affected versions, loginTimeout did not fully mitigate this problem. When loginTimeout expired, the caller could stop waiting, but the worker thread performing the connection attempt could continue running and burning CPU inside the SCRAM PBKDF2 computation.

This issue affects availability. It does not provide authentication bypass, privilege escalation, or direct password disclosure.

A user is vulnerable when all of the following are true:

1. The connection uses SCRAM-SHA-256 authentication.
2. The client reaches a malicious, compromised, or attacker-controlled PostgreSQL endpoint.
3. That endpoint sends a very large SCRAM PBKDF2 iteration count in the server-first-message.

In practice, that can happen in these situations:

• the application lets end users or tenants supply their own database connection details (as in many BI, reporting, analytics, ETL, and low-code platforms), so a user can point the shared client host at a server they control
• the application accepts connection strings, hostnames, or JDBC URLs from user input, configuration uploaded by users, or other untrusted sources
• the application is configured to connect to a PostgreSQL server that is itself malicious or later becomes compromised
• the application connects through an untrusted proxy, relay, tunnel, bastion, or connection-pooling service that can act as the PostgreSQL server
• an attacker can redirect the client to a fake PostgreSQL endpoint by manipulating DNS, service discovery, Kubernetes service resolution, /etc/hosts, environment variables, or similar indirection
• an active network attacker on the path can impersonate the server because the connection does not strongly verify server identity (for example, sslmode lower than verify-full, or trusting a CA that signs hosts outside the operator's control)

The issue is more damaging when the application uses connection retries, many parallel connection attempts, or loginTimeout and assumes the timeout fully stops the work.

Patches

The patch introduces a new connection property, scramMaxIterations, with a default of 100K. The client now rejects SCRAM server messages that advertise more PBKDF2 iterations than the configured cap before starting the PBKDF2 computation begins.

Workarounds

Until a patched version of pgjdbc is deployed, the following measures reduce exposure:

1. Only connect to trusted PostgreSQL servers whose identity is verified.
   Connect only to trusted PostgreSQL servers, and verify server identity with TLS using sslmode=verify-full and a trusted CA. TLS without certificate and hostname verification is not sufficient as an active network attacker can still impersonate the server.

2. Do not rely on loginTimeout as a complete mitigation on unpatched versions.
   On affected versions, loginTimeout can stop the waiting caller while the worker thread continues spending CPU.

3. Avoid SCRAM on untrusted or interceptable connection paths.
   For those paths, use an authentication method that does not let the server choose a SCRAM PBKDF2 iteration count.

4. Reduce blast radius operationally.
   Limit parallel connection attempts, add retry backoff, isolate connection establishment in a separate worker or process when possible, and apply CPU or container limits where appropriate.

5. On trusted servers you control, keep SCRAM iteration counts at ordinary values.
   This does not defend against an attacker-controlled server, but it avoids unnecessary client cost when talking to legitimate servers.