CVE-2026-33337: Firebird Database Buffer Overflow Vulnerability

CVE-2026-33337 Overview

CVE-2026-33337 is a buffer overflow vulnerability in Firebird, a widely-used open-source relational database management system. The vulnerability exists in the xdr_datum() function during the deserialization of slice packets, where improper validation of cstring length against slice descriptor bounds allows an attacker to overflow the allocated buffer. This flaw can be exploited by an unauthenticated remote attacker who sends a specially crafted network packet to the Firebird server, potentially causing a denial of service through application crash or other security impacts.

Critical Impact

Unauthenticated remote attackers can crash Firebird database servers by sending malicious packets, causing service disruption without requiring any credentials.

Affected Products

• Firebird versions prior to 5.0.4
• Firebird versions prior to 4.0.7
• Firebird versions prior to 3.0.14

Discovery Timeline

• 2026-04-17 - CVE-2026-33337 published to NVD
• 2026-04-20 - Last updated in NVD database

Technical Details for CVE-2026-33337

Vulnerability Analysis

This vulnerability is classified under CWE-120 (Buffer Copy without Checking Size of Input), a classic buffer overflow weakness. The flaw resides in the XDR (External Data Representation) deserialization code that handles slice packets in Firebird's network protocol layer.

When the xdr_datum() function processes incoming slice packet data, it reads a cstring length value from the packet without validating whether this length fits within the boundaries defined by the slice descriptor. An attacker can craft a malicious packet containing an oversized length value, causing the function to write beyond the allocated buffer boundaries.

The attack is particularly dangerous because it can be executed over the network without authentication. Any system with Firebird exposed to network access is potentially vulnerable, making this a significant risk for database servers accessible from untrusted networks.

Root Cause

The root cause is insufficient input validation in the xdr_datum() function. Specifically, the function fails to check that the cstring length specified in the incoming packet conforms to the slice descriptor bounds before copying data into the buffer. This missing bounds check allows an attacker-controlled length value to dictate how much data is written, potentially overflowing the destination buffer.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Establishing a network connection to the Firebird server's listening port (default: 3050)
2. Crafting a malicious slice packet with a cstring length value exceeding the slice descriptor bounds
3. Sending the crafted packet to trigger the buffer overflow in xdr_datum()
4. The overflow can cause the server process to crash, resulting in denial of service

The vulnerability resides in the packet deserialization layer, meaning exploitation occurs before any authentication checks. See the GitHub Security Advisory GHSA-89mq-229g-x47p for additional technical details on the vulnerability mechanism.

Detection Methods for CVE-2026-33337

Indicators of Compromise

• Unexpected Firebird server crashes or service restarts without clear cause
• Abnormal network traffic patterns to Firebird port 3050 containing malformed slice packets
• Core dump files indicating crashes in the XDR deserialization functions
• Log entries showing connection attempts followed by immediate process termination

Detection Strategies

• Deploy network intrusion detection rules to monitor for malformed packets targeting Firebird's default port (3050)
• Implement application-level monitoring to detect abnormal crash patterns in the Firebird process
• Configure endpoint detection to alert on repeated Firebird service restarts within short time windows
• Monitor for connections from untrusted sources attempting to send large or malformed slice packets

Monitoring Recommendations

• Enable verbose logging on Firebird servers to capture connection details before potential crash events
• Set up process monitoring to track Firebird service availability and restart frequency
• Configure network traffic analysis to baseline normal Firebird protocol behavior and alert on anomalies
• Implement centralized log collection to correlate crash events across multiple database servers

How to Mitigate CVE-2026-33337

Immediate Actions Required

• Upgrade Firebird immediately to a patched version: 5.0.4, 4.0.7, or 3.0.14 depending on your deployment
• Restrict network access to Firebird servers using firewall rules to allow only trusted hosts
• If upgrade is not immediately possible, consider placing Firebird behind a VPN or application firewall
• Review and audit which systems have network access to your Firebird instances

Patch Information

The Firebird project has released security patches addressing this vulnerability. Organizations should upgrade to the following fixed versions based on their current deployment:

• Firebird 5.x users: Upgrade to version 5.0.4
• Firebird 4.x users: Upgrade to version 4.0.7
• Firebird 3.x users: Upgrade to version 3.0.14

For complete details on the security fix, refer to the GitHub Security Advisory GHSA-89mq-229g-x47p.

Workarounds

• Implement strict firewall rules to limit Firebird port access to only authorized IP addresses
• Deploy a network-level application firewall capable of inspecting Firebird protocol traffic
• Consider network segmentation to isolate database servers from untrusted network zones
• Monitor for abnormal connection patterns and implement rate limiting at the network level

# Example firewall configuration to restrict Firebird access
# Allow only specific trusted hosts to connect to Firebird (port 3050)
iptables -A INPUT -p tcp --dport 3050 -s 10.0.1.100 -j ACCEPT
iptables -A INPUT -p tcp --dport 3050 -s 10.0.1.101 -j ACCEPT
iptables -A INPUT -p tcp --dport 3050 -j DROP