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CVE-2026-90668 – Denial of Service (Memory Exhaustion) – UnrealIRCd 6.0.5 through 6.2.6

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Summary

CVE-2026-90668 is a denial-of-service vulnerability in the webserver component of UnrealIRCd, affecting versions 6.0.5 through 6.2.6. The webserver fails to enforce a limit on the number of HTTP request headers, allowing a remote, unauthenticated attacker to exhaust server memory by sending a single HTTP request containing an excessive number of headers. Exploitation requires that a WebSocket or JSON-RPC listener be enabled, as these are disabled by default; the flaw carries a CVSS v3.1 base score of 7.5 (HIGH) and a CVSS v4.0 score of 8.7.

Technical details

  • Root cause: The function webserver_handle_request_header() in src/modules/webserver.c does not cap the number of HTTP headers processed per request, classified as CWE-770 (Allocation of Resources Without Limits or Throttling).
  • Trigger condition: The affected UnrealIRCd instance must have a WebSocket or JSON-RPC listener enabled in its configuration; these listener types are not enabled by default.
  • Attack vector: Network-based, unauthenticated, requiring no user interaction — an attacker sends a crafted HTTP request with an unbounded number of headers to a reachable WebSocket or JSON-RPC listener.
  • Impact: Uncontrolled memory consumption on the target server, leading to service unresponsiveness or crash (availability impact only; no confidentiality or integrity impact).

Affected software

  • UnrealIRCd versions 6.0.5 through 6.2.6 (inclusive), where a WebSocket or JSON-RPC listener is enabled.
  • Fixed in UnrealIRCd 6.2.7 and later.

Severity

  • CVSS v3.1 Base Score: 7.5 (HIGH) — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  • CVSS v4.0 Base Score: 8.7 (HIGH) — CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/AU:Y/R:A/RE:L

Mitigation and recommended actions

  • Immediate: Upgrade UnrealIRCd to version 6.2.7 or later, which enforces a limit on the number of HTTP request headers.
  • If immediate upgrade is not possible:
    • Apply the vendor-provided hot-patch by running ./unrealircd hot-patch webserver-header-dos, which mitigates the issue without requiring a restart.
    • Disable any WebSocket or JSON-RPC listeners in the UnrealIRCd configuration until the upgrade can be completed, since this removes the vulnerable code path from being reachable.
    • Consider network-level rate limiting or reverse-proxy/WAF rules that restrict the number or total size of HTTP headers as an additional layer of defense.

References

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