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CyberChef MCP

ADR 0001: GPL-3.0-or-later

Date: 2026-08-30 Status: Accepted

Context

Section titled “Context”

CyberChef-MCP has been distributed under Apache-2.0 since v1.0.0, inherited from upstream gchq/CyberChef.

v2.0.0 incorporates algorithms ported from eight reference security tools (see docs/planning/ext-proj-int/) to add 80–120 native MCP tools. Two of those tools are GPL-licensed, which is incompatible with continuing to distribute the combined work under Apache-2.0. Either the GPL-derived work is excluded, or the combined work is relicensed.

The licence texts were read directly rather than inferred from summaries:

Component Licence, verbatim Consequence
katana COPYING.txt: “either version 3 of the License, or (at your option) any later version” GPL-3.0-or-later. Forces v3 — GPLv2 is not an option.
John the Ripper LICENSE: “either version 2 of the License, or (at your option) any later version” GPL-2.0-or-later, so lawfully usable under GPLv3.
upstream CyberChef Apache-2.0 One-way compatible into GPLv3 only. Apache-2.0 is famously incompatible with GPLv2, so GPLv2 was never viable regardless of katana.
Ares, Ciphey, cryptii, pwntools, RsaCtfTool, xortool MIT GPLv3-compatible.

The npm dependency tree (1,694 packages) was audited: 1,379 MIT, 131 ISC, 56 Apache-2.0, 71 BSD-2/3-Clause, plus six entries needing individual review (LGPL-3.0+, MPL-2.0, and three dual licences, and CC-BY-SA-4.0). Every one is GPLv3-compatible. No blocker was found.

Decision

Section titled “Decision”

Distribute CyberChef-MCP v2.0.0 and later under GPL-3.0-or-later.

GPL-3.0-or-later is not a preference among several workable options — it is the only licence that simultaneously admits katana (v3+), John the Ripper (v2+), and the Apache-2.0 upstream.

We do not relicense GCHQ’s code, and do not claim to. Upstream files keep their Apache-2.0 headers and copyright. What changes is the licence of the combined work, which Apache-2.0’s one-way compatibility with GPLv3 expressly permits. The full Apache-2.0 text and the original combined notice are preserved in LICENSE.Apache-2.0.

AGPL-3.0 was considered and rejected. Both GPL tools permit it, and it would extend copyleft to network use — relevant for a server. But CyberChef-MCP is overwhelmingly run locally over stdio by a single user, so AGPL’s network clause would impose obligations on essentially nobody while deterring adoption in environments with blanket AGPL policies.

Consequences

Section titled “Consequences”

Accepted:

  • Anyone distributing a derivative of CyberChef-MCP v2.0.0+ must do so under GPLv3. Merely running it — including serving it over HTTP — carries no obligation; GPLv3, unlike AGPL, has no network-use clause.
  • Upstream contribution becomes one-way, permanently. GCHQ cannot accept GPLv3 code into an Apache-2.0 project, so MCP-layer improvements can no longer be offered upstream. In practice the sync has always been pull-only, so this codifies existing behaviour rather than changing it.
  • The change is effectively irreversible. Once GPL-derived code from john/katana is merged, returning to Apache-2.0 would require excising it. This ADR and the relicense therefore land as a single deliberate, dated commit before any GPL-derived code is imported, rather than happening implicitly when that code arrives.
  • Downstream users with policies against GPLv3 must remain on the v1.9.x line, which stays Apache-2.0 and receives security-only patches through the LTS window.

No contributor consent was required. Every prior contribution arrived under Apache-2.0, which already permits incorporation into a GPLv3 work. The five non-doublegate commits touching MCP paths are covered by that grant.

References

Section titled “References”