CVE-2026-27890: Firebird Database DoS Vulnerability

CVE-2026-27890 Overview

CVE-2026-27890 is a denial of service vulnerability in Firebird, an open-source relational database management system. The flaw exists in the server's handling of CNCT_specific_data segments during the authentication process. When processing these segments, the server incorrectly assumes they arrive in strictly ascending order. If an attacker sends segments out of order, the Array class's grow() method computes a negative size value, resulting in a SIGSEGV crash that terminates the database server.

This vulnerability is particularly dangerous because it can be exploited by an unauthenticated attacker who only needs to know the server's IP address and port number. No valid credentials or prior access is required to trigger the crash, making any internet-exposed Firebird instance vulnerable to remote denial of service attacks.

Critical Impact

An unauthenticated remote attacker can crash the Firebird database server, causing complete denial of service for all connected applications and users.

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

• April 17, 2026 - CVE-2026-27890 published to NVD
• April 20, 2026 - Last updated in NVD database

Technical Details for CVE-2026-27890

Vulnerability Analysis

This vulnerability falls under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The flaw originates in Firebird's network authentication handling code, specifically in how the server processes CNCT_specific_data segments during client connection establishment.

The authentication protocol expects data segments to arrive in a specific ascending order. However, the server implementation does not validate segment ordering before performing memory operations. When segments arrive out of sequence, the Array class's grow() method receives parameters that cause it to calculate a negative size value for memory allocation. This negative value triggers undefined behavior, ultimately causing a segmentation fault (SIGSEGV) that crashes the database server process.

The vulnerability can be exploited over the network without any authentication, making it accessible to any attacker who can reach the Firebird service port (typically port 3050).

Root Cause

The root cause is improper input validation in the CNCT_specific_data segment processing logic. The code assumes that incoming segments will always arrive in ascending order and fails to validate this assumption before using segment indices in arithmetic operations. When this assumption is violated by deliberately crafted network packets, integer underflow occurs in the grow() method's size calculation, leading to a null pointer dereference or invalid memory access that terminates the server.

Attack Vector

The attack vector is network-based and requires no authentication. An attacker constructs a malformed connection request containing CNCT_specific_data segments in non-ascending order. When this packet is sent to the Firebird server's listening port, the authentication handler processes the segments without proper ordering validation. The server's Array class attempts to resize its internal buffer based on the out-of-order segment data, computes an invalid negative size, and crashes with SIGSEGV.

The attack can be performed remotely by any attacker with network access to the Firebird service. Since no valid credentials are required, the attack surface includes any Firebird instance accessible from untrusted networks.

Detection Methods for CVE-2026-27890

Indicators of Compromise

• Unexpected Firebird server process terminations with SIGSEGV signal
• Repeated crash-restart cycles of the firebird or fbserver service
• Abnormal connection attempts followed by immediate service failure
• Core dump files generated by the Firebird process indicating memory access violations

Detection Strategies

• Monitor system logs for SIGSEGV crashes in firebird, fbserver, or fbguard processes
• Implement network intrusion detection rules to identify malformed Firebird authentication packets
• Deploy application-level monitoring to detect rapid service restarts
• Use SentinelOne's behavioral detection to identify anomalous process termination patterns

Monitoring Recommendations

• Enable Firebird audit logging to capture connection attempt details before crashes
• Configure process monitoring to alert on repeated crashes of database services
• Implement network traffic analysis on port 3050 (default Firebird port) for suspicious patterns
• Set up database availability monitoring to detect service disruptions immediately

How to Mitigate CVE-2026-27890

Immediate Actions Required

• Upgrade to Firebird 5.0.4, 4.0.7, or 3.0.14 depending on your version branch
• Restrict network access to the Firebird server port using firewall rules
• Ensure Firebird is not directly exposed to the internet without VPN or other access controls
• Implement network segmentation to limit attack surface

Patch Information

FirebirdSQL has released patched versions that address this vulnerability:

• 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 technical details, refer to the GitHub Security Advisory GHSA-6crx-4g37-7j49.

Workarounds

• Place Firebird servers behind a firewall that restricts access to trusted IP addresses only
• Use a VPN or SSH tunnel for all remote database connections
• Configure host-based firewall rules on the database server to whitelist authorized clients
• Consider deploying a reverse proxy or load balancer with connection filtering capabilities

# Example firewall configuration to restrict Firebird access
# Allow only trusted network to access Firebird (default port 3050)
iptables -A INPUT -p tcp --dport 3050 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 3050 -j DROP