Firebird CVE-2026-27890

No special conditions - remote unauthenticated exploitation against default configurations of Firebird database server. The vulnerability triggers during the pre-authentication connection handshake phase, before any credential validation occurs. Exploitation requires only: (1) network connectivity to the Firebird service port (default TCP 3050), and (2) ability to send crafted TCP packets with out-of-order CNCT_specific_data segments. No user interaction (UI:N), no elevated privileges (PR:N), and no complex timing or race conditions (AC:L) are required. Exploitation is NOT limited by: authentication mechanisms (occurs before login), database configuration settings, enabled features, or specific deployment modes. The attack surface is maximally exposed for any Firebird instance reachable over a network. Limiting factors that reduce practical risk: most production Firebird deployments are on internal networks behind firewalls rather than directly Internet-facing; impact is limited to availability (crash) with no confidentiality or integrity compromise; attacker must have knowledge of target server IP and port through reconnaissance or prior knowledge.

Real-world risk is MODERATE-HIGH for denial-of-service but requires nuanced interpretation of available signals. CVSS 8.2 accurately reflects the unauthenticated network attack surface (AV:N/AC:L/PR:N/UI:N) and high availability impact (A:H), though the integrity impact is rated Low rather than None. The attack complexity is genuinely low - no special timing, race conditions, or configuration prerequisites beyond network reachability. However, multiple factors limit practical risk: (1) No CISA KEV listing indicates this is not being exploited at scale in the wild. (2) No public proof-of-concept code identified in references, raising the skill threshold for opportunistic attackers. (3) The impact is purely availability (server crash) with no data exfiltration, persistence, or lateral movement capability - this is a nuisance DoS, not a foothold for deeper compromise. (4) Firebird databases are typically deployed in internal/trusted networks rather than directly Internet-facing, reducing attacker reach. (5) Patches are available across all supported major versions (3.x, 4.x, 5.x), providing clear remediation. SSVC assessment: Exploitation=none (not observed), Automatable=yes (no user interaction), Technical Impact=partial (availability only) suggests scheduled patching rather than emergency response. Organizations with Internet-exposed Firebird instances or untrusted network segments should prioritize patching; those with perimeter controls and network segmentation can treat this as standard maintenance.

An attacker on the network (local LAN or Internet if database is exposed) identifies a Firebird database server listening on TCP port 3050 through port scanning. Without requiring any credentials, the attacker crafts malicious TCP packets containing CNCT_specific_data segments intentionally transmitted in reverse or random order during the initial connection handshake phase. The Firebird server's authentication parser attempts to process these out-of-sequence segments, triggering the Array.grow() integer underflow, which causes a segmentation fault and immediate server crash. Legitimate clients lose connectivity until an administrator manually restarts the database service. The attacker can repeat this process indefinitely to maintain denial-of-service, disrupting business operations dependent on the database. No public proof-of-concept code has been identified, but the attack mechanism is straightforward enough for skilled attackers to reproduce from the vulnerability description.