Escalate on distinct rejected tokens, and make the auth budgets configurable - #897
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…gurable Ten failures in a minute is a stuck client retrying one bad token as much as it is an attack, and a minute's block answers both. Fifty distinct invalid tokens from one address in ten minutes is only an attack: a person has one token and a fleet behind a NAT has one each, so a distinct count is the one thing a legitimate neighbour never produces and a sprayer cannot avoid producing. What it protects is not this server, which refuses those requests cheaply either way. Every distinct credential that reaches verification is a request GitLab charges to the deployment's own address, and GitLab rate-limits failed authentication per address, so a sprayer is spending the deployment's standing upstream and the throttle lands on everybody using it. serverpool.DistinctTokenBudget holds the count, keyed by a truncated SHA-256 and never the credential, bounded in addresses and, because reaching the limit both blocks and clears the set, in digests per address. Reaching it raises a block of one window, then ten, then sixty, saturating there rather than growing without end: a block is a defence and not a punishment, and whoever inherits the address did nothing. Sixty windows of silence forgets the ladder. The ladder is derived from the fast window rather than given four more flags. At the defaults that is a minute, ten minutes and an hour, which is the ladder that was asked for, and a test can set the window to 200ms and watch the whole thing in seconds. Both guards consult the three budgets in one order and answer with the first that refuses, so the admitted-credential exemption keeps working unchanged: resolve already reads "blocked and not already admitted", and folding the new budget into blockedByBudget inherits it with no second decision to keep in step. Retry-After stops being a constant. An escalated block outlasts the window the other two are bounded by, so announcing the short one tells a client to come back while it is still refused, and a well-behaved one then knocks for the rest of the block. Telemetry counts the refusal, not the raising: a block is raised once and then refuses everything that arrives while it lasts, and what an operator acts on is what is being turned away. Keyed by the budget that refused, never by the address, which is the one dimension that would make the series identify people and grow without bound under exactly the traffic it measures. Four flags with GITLAB_MCP_ spellings, bounded in validateHTTPPoolAndRateBounds beside the rate ones, since HTTP mode never runs Config.validate and a bound enforced only there is a bound the flags escape.
Making the failure limit configurable introduced a trap of its own. The
limiter blocks once a record reaches its limit, so a limit of zero blocks an
address after a single failure: the most aggressive setting available, reached
by typing the figure every other budget here reads as 'no budget', and
documented as turning it off.
The limiter is therefore not built at all when the limit or the window is
zero. Every site that consults it already tolerates a nil one, so this is both
the smallest change and the only one that cannot be read two ways; the two
cleanup registrations are guarded to match.
Two figures were also still constants where they had just become settings. The
transport budget counted in the default window rather than the configured one,
and both 429s announced the default in Retry-After, so a deployment that
widened the window to five minutes told callers to come back in one. Each
guard now carries the window it was configured with.
The e2e pair drives the real binary: a repeated token is never blocked while
distinct ones are, with the fast budget off so the distinct count is the only
thing that can refuse; and the block lengthens on repetition, observed through
Retry-After with the window set to a second so the whole ladder fits in a
test. Flattening the ladder to {1,1,1} fails the second one, which is what
says it is measuring the escalation rather than the existence of a block.
security.md listed three layers in front of an unauthenticated request and now lists four. The new one is stated as a different question rather than a bigger version of the first, because that is what makes escalating on it safe: ten failures in a minute is a stuck client as much as an attack, while fifty distinct invalid tokens is only an attack, since a person has one token and a fleet behind a NAT has one each. The reference tables, CLAUDE.md and the HTTP guide gain the four settings, including the two things a reader would otherwise have to find out by running it: that zero turns a budget off rather than being its harshest setting, and that the escalation is derived from the failure window instead of configured. The em dashes went with it. Nine lines in the diff carried one, four in the layer list I was editing and five that the table formatter realigned when the new rows changed the column widths, and the gate counts added lines whatever made them move. The link texts lost theirs too; the anchors are unchanged, so every link still resolves.
The three new files are at 100% statement coverage: the budget and its cap recovery, the counters and the four bound checks, the environment reader and each of its eight refusals. Two gaps the first pass left were the ones worth closing, because both are paths a deployment reaches and no test had: the distinct-token table admitting new addresses again once its records lapse, which is what keeps a saturated table from refusing to count anybody until the process restarts, and every way the environment reader rejects a value. The telemetry test asserts the thing the instrument exists in this shape for: the reason is the only dimension a data point carries. An address as a metric label would make the series identify people and would grow without bound under exactly the traffic it measures, since a sprayer rotating addresses mints a new label per request. It reuses the pool test's refusing meter rather than declaring a second fake of the same thing. Both blockedByBudget results are named now, which gocritic asked for and which reads better at three returns, and the spray branch no longer shadows the named one.
The three new source files and their tests move the counts the README and the testing reference publish.
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📝 SummarySummary by CodeRabbit
WalkthroughThe server now supports configurable per-address authentication-failure and distinct-token budgets. Distinct credentials use truncated SHA-256 digests and escalating blocks. HTTP and environment configuration, telemetry, tests, and security documentation were updated. ChangesAuthentication budget controls
Priority: ➖ Normal Estimated code review effort: 4 (Complex) | ~60 minutes Change: Feature Sequence Diagram(s)sequenceDiagram
participant Client
participant bearerGuard
participant DistinctTokenBudget
participant GitLab
Client->>bearerGuard: Submit credential
bearerGuard->>DistinctTokenBudget: Check address and token budget
DistinctTokenBudget-->>bearerGuard: Block status and retry duration
bearerGuard->>GitLab: Verify credential when not blocked
GitLab-->>bearerGuard: Authentication result
bearerGuard->>DistinctTokenBudget: Charge refused distinct token
bearerGuard-->>Client: Authentication response and Retry-After
Merge Risk: 🟠 High · up to Persistent spraying can prematurely reset escalation, while legitimate clients may receive an excessive Retry-After. Correct these authentication-budget behaviors before merging. 🚥 Pre-merge checks | ✅ 3 | ❌ 2❌ Failed checks (2 warnings)
✅ Passed checks (3 passed)
Full details: Linked Issues checkExplanation Issue Full details: Out of Scope Changes checkExplanation Most changes support Issue ✨ Finishing Touches📝 Generate docstrings
🧪 Generate unit tests (beta)
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The curated help is what -h prints, and it did not name the four budget flags. A flag missing from it is discoverable only through the flag package's own output, which is the output -h exists to replace: an operator who does not already know a flag's name cannot find it there. The tests beside it checked individual entries that had each been wrong once, and nothing checked the set, so the omission was silent. TestPrintHelp_Docume ntsEveryFlag now reads every flag main.go registers, out of the source with go/ast since they are registered in main() and a test cannot call that, and holds each to an entry. One flag is declared as deliberately undocumented, -h itself, and a declaration for a flag that no longer exists fails too. Suppressing one help entry fails the gate, which is what says it measures the set rather than the file being non-empty. The site gains a section on the two budgets in both languages, with the table an operator reads first and the reason the second budget is worth having: every distinct credential that reaches verification is a request GitLab charges to the deployment's own address. Two sentences that described the failure lockout as the whole story were corrected rather than left beside it.
The Markdown table formatter re-pads a table when a row changes its column widths, and the budgets section added one to each language.
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Inline comments:
In `@CLAUDE.md`:
- Line 618: Update the --auth-failure-window table description to state that
Retry-After reports the remaining duration of an escalated distinct-token block,
rather than the failure-window value; remove the contradictory claim that both
429 responses announce this window.
In `@cmd/server/auth_gate.go`:
- Around line 654-663: Update blockedByBudget in both mcpServerGate and the
corresponding bearer guard to evaluate every active limiter, source budget, and
distinct-token block before returning. Select the block with the longest retry
duration and return its matching reason, preserving the existing blocked status
and behavior when no budget is active.
In `@cmd/server/main.go`:
- Around line 563-573: Add the four missing authentication environment-variable
entries to the HTTP environment help section: GITLAB_MCP_AUTH_FAILURE_LIMIT,
GITLAB_MCP_AUTH_FAILURE_WINDOW, GITLAB_MCP_AUTH_DISTINCT_TOKEN_LIMIT, and
GITLAB_MCP_AUTH_DISTINCT_TOKEN_WINDOW. Document each default and preserve the
zero-disables-budget semantics for the two limit variables, matching the
corresponding flag descriptions.
- Line 3327: Update transportBudget initialization to store the effective
transport-budget window from transportFailureBudget, then use that stored window
in transportBudget.charge, transportBudget.cleanup, and the source-budget
branches of both blockedByBudget methods. Keep the configured AuthFailureWindow
for the primary AuthRateLimiter response, while ensuring zero-window
source-budget responses use the effective default window.
In `@docs/concepts/security.md`:
- Line 338: Update the response table’s `429` condition to include both the
existing failure budget and the distinct-credential budget, while preserving the
documented `Retry-After` behavior.
In `@docs/reference/cli.md`:
- Line 63: Update the `-allow-any-gitlab-url` CLI option description to state
that caller-selected instances are allowed, while private and link-local
destinations additionally require `-allow-private-instances`; retain the
existing loopback/unix-socket restriction and other accurate behavior.
In `@internal/serverpool/distinct_token_budget.go`:
- Around line 138-164: Update DistinctTokenBudget.Charge to check an existing
record’s blockedUntil under its mutex immediately after retrieving the address
record and before any reset or digest updates; return false without charging
when the block is active. Add a concurrent regression test proving charges that
passed an earlier Blocked check do not escalate the ladder after the first
Charge activates a block.
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CLAUDE.mdREADME.mdcmd/server/auth_blocks.gocmd/server/auth_blocks_test.gocmd/server/auth_gate.gocmd/server/bearer_guard.gocmd/server/env_overlay.gocmd/server/help_coverage_test.gocmd/server/main.gocmd/server/main_test.godocs/concepts/security.mddocs/development/testing/testing.mddocs/guides/http-server-mode.mddocs/reference/cli.mddocs/reference/env.mdinternal/config/auth_budgets.gointernal/config/auth_budgets_test.gointernal/config/config.gointernal/config/env_name.gointernal/config/http_overlay.gointernal/mcpotel/auth_blocks.gointernal/mcpotel/auth_blocks_test.gointernal/serverpool/distinct_token_budget.gointernal/serverpool/distinct_token_budget_test.gosite/src/content/docs/es/operations/security.mdxsite/src/content/docs/operations/security.mdxtest/e2e/http/gate_test.go
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Three findings, all in test files I wrote after the last full lint and gate run, which is the lesson rather than the defects: run them again after adding a file, not only after changing one. A shadowed err in the help gate's parser. Two case loops asserting without a subtest, both of which are genuinely sequential rather than cases: the reason vocabulary was checking uniqueness by accumulating into a map as it went, and is now a set comparison with no asserting loop at all, and the escalation rounds each climb the ladder the previous round raised, which is what the sequential declaration exists for. The declaration has to be the line directly above the loop; splitting it across two comment lines left it unread.
check-test-file-names refused cmd/server/help_coverage_test.go: a _test.go exists only under the name of a module it tests, and there is no help_coverage.go. printHelp lives in main.go, so main_test.go is where its tests belong, and the gate says so. The move is verbatim apart from sort.Strings becoming slices.Sort, which main_test.go already imports.
Three were defects the configurability introduced, and each is the same shape: a value that used to be a constant is now a setting, and a reader of it was left behind. A charge that lands while an address is already blocked no longer counts. Both guards check Blocked before authenticating and charge afterwards, so a burst that passes the check together arrives after the block is on; counting it let 3*limit concurrent refusals reach the second and third rungs at once, earning the hour without the sender ever being told it was blocked. The ladder answers persistence after a block, which a charge during one is not. Both guards now answer with the longest active block rather than the first one found. One failure can raise the minute-long lockout and an hour-long distinct-token block together, and a client told to come back in a minute spent the other fifty-nine being refused. Retry-After is a promise about when the next attempt can succeed. transportBudget carries the window its limiter was built with, instead of reading the package default in charge and cleanup. A configured window longer than the default let one (source, key) pair recharge inside a single window and blocked a proxy's legitimate clients early; a shorter one deduplicated for longer than the limiter remembered. The four documentation findings: the Retry-After description said both things at once, the four new environment variables were missing from the help's HTTP section, the 429 row named only the failure budget, and --allow-any-gitlab-url read as if it permitted private destinations by itself, which still need --allow-private-instances.
check-test-subtests refused the loop: it asserts without opening a subtest per case. The four tokens are not four cases, they are one burst accumulating against a block that is already on, and each charge only means anything after the one before it. That is what the sequential declaration is for, and it sits on the line directly above the loop because that is where the gate reads it.
# Conflicts: # README.md # docs/development/testing/testing.md
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Inline comments:
In `@cmd/server/auth_gate.go`:
- Around line 657-658: Expose remaining block durations from AuthRateLimiter and
add the equivalent remaining-duration query for the transport budget, then
update both authentication guards to pass those durations to longestAuthBlock
instead of full failure and transport windows. Preserve
DistinctTokenBudget.Blocked as the duration source for the distinct-token budget
and retain existing blocked-state behavior.
In `@internal/serverpool/distinct_token_budget.go`:
- Line 166: Update DistinctTokenBudget to track blocked-request activity with a
separate timestamp updated by the active-block path in Blocked; use that
timestamp for reset and cleanup decisions, while keeping blocked charges free
and preventing them from advancing the ladder. Do not rely on Charge, which
returns before recording activity.
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CLAUDE.mdREADME.mdcmd/server/auth_gate.gocmd/server/auth_gate_test.gocmd/server/bearer_guard.gocmd/server/bearer_guard_test.gocmd/server/main.gocmd/server/main_test.godocs/concepts/security.mddocs/development/testing/testing.mddocs/reference/cli.mdinternal/mcpotel/auth_blocks_test.gointernal/serverpool/distinct_token_budget.gointernal/serverpool/distinct_token_budget_test.go
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Two defects the last round introduced, both found in review. A blocked caller is refused before its credential is read, so Charge never sees it and only Blocked does. Measuring the silence that forgives the ladder from the last charge therefore read an address hammering through an hour-long block as an hour of silence, and handed it a clean ladder the moment the block lifted. The record now tracks when the address was last heard from at all, updated on both paths, and the reset and the sweep read that. Both guards passed the configured windows to longestAuthBlock while the distinct-token budget passed its remaining time, so the comparison was between a window and a countdown and the answer could be the wrong reason with an excessive Retry-After. AuthRateLimiter exposes BlockedFor and transportBudget blockedFor, so all three arms are now time left on the block. A block runs from the first failure of its window, so this is also shorter than the window by however much of it had passed. transportBudget.blocked had no caller left once both guards moved, so it is gone rather than kept for the tests that were its only users.
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Closes #790.
#789 fixed the defect that issue was opened for, and deliberately left the two additive halves out so the security change stayed the smallest one that is clearly correct. This is those two.
The distinct count, and why it is safe to escalate on
Ten failures in a minute is a stuck client retrying one bad token as much as it is an attack, and a minute's block is the right answer to both. Fifty distinct invalid tokens from one address inside ten minutes is only an attack: a person has one token and a fleet behind a NAT has one each, so the distinct count is the one thing a legitimate neighbor never produces and a sprayer cannot avoid producing.
What it protects is not really this server, which refuses those requests cheaply either way. Every distinct credential that reaches verification is a request to GitLab from the deployment's own address, and GitLab rate-limits failed authentication per source address, so a sprayer is spending the deployment's standing upstream and the throttle GitLab eventually applies lands on everyone using it.
serverpool.DistinctTokenBudgetholds the count as truncated SHA-256 digests, never the credential. Both tables are bounded, and the digest set needs no cap of its own because reaching the limit both blocks the address and clears the set. Reaching it raises a block of one window, then ten, then sixty, saturating there rather than growing without end: a block is a defense and not a punishment, and whoever inherits the address did nothing. Sixty windows of silence forgets the ladder.The ladder is derived from the failure window rather than given four more settings. At the defaults that is a minute, ten minutes and an hour, which is the ladder the issue asked for, and a test can set the window to a second and watch the whole thing in seconds.
What the wiring bought, and what it cost
Both guards consult the three budgets in one order and answer with the first that refuses, so the admitted-credential exemption keeps working with no second decision to keep in step:
resolvealready reads "blocked and not already admitted", and folding the new budget intoblockedByBudgetinherits that.Three defects turned up while wiring it, all introduced by this change and all fixed here:
0blocked an address after a single failure. The limiter blocks once a record reaches its limit, so zero was the harshest setting available, reached by typing the figure every other budget here reads as "none", and documented as turning the budget off. It is not built at all now; every consulting site already tolerated a nil one.Retry-Afterwas a constant. An escalated block outlasts the window the other two are bounded by, so announcing that window tells a client to come back while it is still refused, and a well-behaved one then knocks for the rest of the block. Both guards carry the window they were configured with.Telemetry
A counter of refusals, keyed by the budget that refused. The refusal rather than the raising, because a block is raised once and then refuses everything that arrives while it lasts, and what an operator acts on is what is being turned away. The address is deliberately not a dimension: it would make the series identify people, and it would grow without bound under exactly the traffic the counter exists to measure.
Configuration
Four flags with
GITLAB_MCP_spellings and an HTTP overlay, bounded invalidateHTTPPoolAndRateBoundsbeside the rate ones, since HTTP mode never runsConfig.validateand a bound enforced only there is a bound the flags escape.Tests
Two e2e cases drive the real binary: a repeated token is never blocked while distinct ones are, with the fast budget off so the distinct count is the only thing that can refuse, and the block lengthens on repetition, observed through
Retry-After. Flattening the ladder to{1, 1, 1}fails the second one, which is what says it measures the escalation rather than the existence of a block.The three new source files are at 100% statement coverage, including the table admitting new addresses again once its records lapse and every way the environment reader rejects a value.