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Fix loopback redirect URI port matching per RFC 8252 §7.3#3589

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jlowin merged 1 commit intoPrefectHQ:mainfrom
radoshi:fix/loopback-redirect-uri-port-matching
Mar 22, 2026
Merged

Fix loopback redirect URI port matching per RFC 8252 §7.3#3589
jlowin merged 1 commit intoPrefectHQ:mainfrom
radoshi:fix/loopback-redirect-uri-port-matching

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@radoshi radoshi commented Mar 22, 2026

Claude wrote the PR and I reviewed and guided it for some minor fixups.


Summary

CIMD clients that declare redirect_uris without a port (e.g. http://localhost/callback) fail validation when the actual redirect uses a dynamic ephemeral port — standard behavior for MCP clients like Claude Code. Per RFC 8252 §7.3, the authorization server MUST allow any port for loopback redirect URIs.

The fix skips port matching in matches_allowed_pattern when the pattern host is a loopback address (localhost, 127.0.0.1, ::1) and no port is explicitly specified. Scheme, host, and path validation are unaffected.

from fastmcp.server.auth.redirect_validation import matches_allowed_pattern

# Before: False — pattern port defaults to 80, rejects dynamic port
# After: True — loopback pattern without port accepts any port
matches_allowed_pattern(
    "http://localhost:51353/callback",
    "http://localhost/callback",
)

Also updated CIMDFetcher.validate_redirect_uri to delegate to matches_allowed_pattern instead of raw fnmatch, so both validation paths get consistent behavior.

Closes #3588

🤖 Generated with Claude Code

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
@marvin-context-protocol marvin-context-protocol bot added bug Something isn't working. Reports of errors, unexpected behavior, or broken functionality. auth Related to authentication (Bearer, JWT, OAuth, WorkOS) for client or server. labels Mar 22, 2026
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Thanks!

@jlowin jlowin merged commit 204e566 into PrefectHQ:main Mar 22, 2026
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Thanks all! I was going to take a stab at this, but I'm so stoked that this has been merged already! Any idea when we can expect another release? 🚀

jlowin added a commit that referenced this pull request Mar 30, 2026
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Jeremiah Lowin <jlowin@users.noreply.github.com>
Co-authored-by: Marvin Context Protocol <41898282+Marvin Context Protocol@users.noreply.github.com>
Co-authored-by: voidborne-d <voidborne-d@users.noreply.github.com>
Co-authored-by: marvin-context-protocol[bot] <225465937+marvin-context-protocol[bot]@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>
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Co-authored-by: d 🔹 <258577966+voidborne-d@users.noreply.github.com>
Co-authored-by: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>
Co-authored-by: nightcityblade <nightcityblade@gmail.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Bill Easton <strawgate@users.noreply.github.com>
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Fix auto-close MRE script posting comment without closing (#3386)
Fix WorkOS token scope verification bypass 🤖 Generated with Codex (#3407)
Fix initialize McpError fallthrough 🤖 Generated with Codex (#3413)
Fix transform arg collisions with passthrough params (#3431)
Fix get_* returning None when latest version is disabled (#3439)
Fix get_* returning None when latest version is disabled (#3421)
Fix server lifespan overlap teardown (#3415)
Fix $ref output schema object detection regression (#3420)
resolved annotations (#3429)
Fix async partial callables rejected by iscoroutinefunction (#3438)
Fix async partial callables rejected by iscoroutinefunction (#3423)
fix: add version to components (#3458)
fix: use intent-based flag for OIDC scope patch in load_access_token (#3465)
Fixes #3461
fix: normalize Google scope shorthands and surface valid_scopes (#3477)
fix: resolve ty 0.0.23 type-checking errors and bump pin (#3481)
fix: shield lifespan teardown from cancellation (#3480)
fix: forward custom_route endpoints from mounted servers (#3462)
fix updates _get_additional_http_routes() to traverse providers,
Fixes #3457
fix: remove hardcoded version from CLI help text (#3456)
fix: monty 0.0.8 compatibility, drop external_functions from constructor (#3468)
fix: task test teardown hanging 5s per test (#3499)
Closes #3498
fix: validate workspace path is a directory before cursor install (#3440)
Fixes #3426
fix: handle re.error from malformed URI templates in build_regex (#3501)
fix: reject empty/OIDC-only required_scopes in AzureProvider (#3503)
fix: restrict $ref resolution to local refs only (SSRF/LFI) (#3502)
fix warnings and timeouts (#3504)
close upgrade check issue when build passes (#3505)
Closes #3484
fix: URL-encode path params to prevent SSRF/path traversal (GHSA-vv7q-7jx5-f767) (#3507)
fix: prevent path traversal in skill download (#3493)
fix: prefer IdP-granted scopes over client-requested scopes in OAuthProxy (#3492)
fix: remove unrelated transform and http.py changes from PR scope
fix: remove forced follow_redirects from httpx_client_factory calls (#3496)
fix: stop passing follow_redirects to httpx_client_factory
fix: restore follow_redirects=True for custom httpx client factories
Closes #3509
fix: CSRF double-submit cookie check in consent flow (#3519)
fix: validate server names in install commands (#3522)
fix: use raw strings for regex in pytest.raises match (#3523)
fix: reject refresh tokens used as Bearer access tokens (#3524)
fix: route ResourcesAsTools/PromptsAsTools through server middleware (#3495)
fix: resolve Pyright "Module is not callable" on @tool, @resource, @prompt decorators (#3540)
fix: filter warnings by message in KEY_PREFIX test (#3549)
fix: suppress output schema for ToolResult subclass annotations (#3548)
fix: increase sleep duration in proxy cache tests (#3567)
fix: store absolute token expiry to prevent stale expires_in on reload (#3572)
fix: preserve tool properties named 'title' during schema compression (#3582)
Fix loopback redirect URI port matching per RFC 8252 §7.3 (#3589)
Fix app tool routing: visibility check and middleware propagation (#3591)
Fix query parameter serialization to respect OpenAPI explode/style settings (#3595)
Fix dev apps form: union types, textarea support, JSON parsing (#3597)
fix(google): replace deprecated /oauth2/v1/tokeninfo with /oauth2/v3/userinfo (#3603)
fix: resolve EntraOBOToken dependency injection through MultiAuth (#3609)
fix(docs): correct misleading stateless_http header (#3622)
fix: filesystem provider import machinery (#3626)
Closes #3625 (issues 2, 3, 6)
fix: recover StdioTransport after subprocess exits (#3630)
fix(server): preserve mounted tool task metadata (#3632)
fix: scope deprecation warning filter to FastMCPDeprecationWarning (#3649)
fix imports, add PrefabAppConfig (#3650)
fix: resolve CurrentFastMCP/ctx.fastmcp to child server in mounted background tasks (#3651)
Fix blocking docs issues: chart imports, Select API, Rx consistency (#3652)
closed by default (#3657)
Fix prompt caching middleware missing wrap/unwrap round-trip (#3666)
fix: serialize object query params per OpenAPI style/explode rules (#3662)
Fixes #2857
fix: HTTP request headers not accessible in background task workers (#3631)
fix: restore HTTP headers in worker execution path for background tasks (#3681)
fix: strip discriminator after dereferencing schemas (#3682)
fix: remove stale ty:ignore directives for ty 0.0.26 (#3684)
Fix docs gaps in app provider pages (#3690)
fix: dev apps log panel UX improvements (#3698)
fix dev server empty string args (#3700)
DrFaust92 added a commit to DrFaust92/okta-mcp-server that referenced this pull request Apr 1, 2026
… to 3.2.0

- Port validation.py (validate_ids decorator) from upstream commit 104c46d
- Apply @validate_ids decorators to all tool functions with ID parameters,
  including fork-added list_user_groups in users.py
- Bump fastmcp>=3.2.0 to include RFC 8252 §7.3 loopback redirect URI fix
  (PrefectHQ/fastmcp#3589) which resolves the CIMD redirect_uri port mismatch
- Fix typo in system_logs.py docstring

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>
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CIMD redirect_uri validation rejects loopback URIs with dynamic ports (RFC 8252 §7.3)

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