GHSA-WCHH-MRV2-5H4Q
Vulnerability from github – Published: 2026-11-09 21:51 – Updated: 2026-11-09 21:51Summary
A remote attacker from any IP address defeats the operator-configured source-IP allowlist (mcp_allowed_remote_ips / api_token_allowed_remote_ips) by sending a single X-Forwarded-For: <allowlisted-ip> header. The router is created with gin.New() and never calls SetTrustedProxies, so gin trusts every proxy by default and c.ClientIP() returns the attacker-supplied header verbatim. The same forged value is also written to Transaction.CreatedIp and the access log, so the attacker's real IP never appears in any record.
Details
cmd/webserver.go:104 builds the router with gin.New(). A grep of cmd/ and pkg/ confirms SetTrustedProxies is never called, leaving gin's default trusted-proxy range of 0.0.0.0/0 and ::/0. WebContext (pkg/core/context_web.go:39) embeds *gin.Context and does not override ClientIP().
Both allowlist middlewares match the operator allowlist against this attacker-controlled value:
// pkg/middlewares/mcp_server_ip_limit.go:19
if config.MCPAllowedRemoteIPs[i].Match(c.ClientIP()) { c.Next(); return }
The identical c.ClientIP() is consumed by the audit trail at pkg/api/transactions.go:1078 (createNewTransactionModel(..., c.ClientIP()) → CreatedIp) and the access log (pkg/middlewares/request_log.go:28), so one header both bypasses the gate and forges attribution.
PoC
Runtime PoC (no live deployment required) — pkg/middlewares/xff_bypass_runtime_test.go, pkg/middlewares/xff_chain_runtime_test.go:
go test ./pkg/middlewares/ -run 'TestACCESSCONTROL001_XFFBypass|TestCHAIN_AllowlistBypass' -v
Output:
[control] no XFF, RemoteAddr=1.2.3.4 -> status=400 (correctly blocked)
[exploit] XFF=10.0.0.1, RemoteAddr=1.2.3.4 -> status=200, ClientIP=10.0.0.1 (bypassed)
gate status = 200 | created_ip that would be persisted = "10.0.0.1"
Against a live instance with mcp_allowed_remote_ips=10.0.0.1:
curl -s -o /dev/null -w "%{http_code}\n" https://target/mcp # 403 (blocked)
curl -s -o /dev/null -w "%{http_code}\n" -H "X-Forwarded-For: 10.0.0.1" https://target/mcp # passes IP gate
Impact
Defeats an explicitly advertised, operator-configured security control on the MCP and API-token surfaces, from any source IP, with a single header and no authentication to the gate itself. It also destroys non-repudiation: every action is logged under the spoofed allowlisted IP, so audit trails point at the trusted host instead of the attacker. (The MCP/API actions themselves still require a valid token — this neutralizes the compensating IP control, not the JWT layer.)
{
"affected": [
{
"package": {
"ecosystem": "Go",
"name": "github.com/mayswind/ezbookkeeping"
},
"ranges": [
{
"events": [
{
"introduced": "0.10.0"
},
{
"fixed": "1.6.0"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-86041"
],
"database_specific": {
"cwe_ids": [
"CWE-348"
],
"github_reviewed": true,
"github_reviewed_at": "2026-11-09T21:51:28Z",
"nvd_published_at": "2026-11-04T23:17:58Z",
"severity": "MODERATE"
},
"details": "### Summary\nA remote attacker from any IP address defeats the operator-configured source-IP allowlist (`mcp_allowed_remote_ips` / `api_token_allowed_remote_ips`) by sending a single `X-Forwarded-For: \u003callowlisted-ip\u003e` header. The router is created with `gin.New()` and never calls `SetTrustedProxies`, so gin trusts every proxy by default and `c.ClientIP()` returns the attacker-supplied header verbatim. The same forged value is also written to `Transaction.CreatedIp` and the access log, so the attacker\u0027s real IP never appears in any record.\n\n### Details\n`cmd/webserver.go:104` builds the router with `gin.New()`. A grep of `cmd/` and `pkg/` confirms `SetTrustedProxies` is never called, leaving gin\u0027s default trusted-proxy range of `0.0.0.0/0` and `::/0`. `WebContext` (`pkg/core/context_web.go:39`) embeds `*gin.Context` and does not override `ClientIP()`.\n\nBoth allowlist middlewares match the operator allowlist against this attacker-controlled value:\n\n```go\n// pkg/middlewares/mcp_server_ip_limit.go:19\nif config.MCPAllowedRemoteIPs[i].Match(c.ClientIP()) { c.Next(); return }\n```\n\nThe identical `c.ClientIP()` is consumed by the audit trail at `pkg/api/transactions.go:1078` (`createNewTransactionModel(..., c.ClientIP())` \u2192 `CreatedIp`) and the access log (`pkg/middlewares/request_log.go:28`), so one header both bypasses the gate and forges attribution.\n\n### PoC\nRuntime PoC (no live deployment required) \u2014 `pkg/middlewares/xff_bypass_runtime_test.go`, `pkg/middlewares/xff_chain_runtime_test.go`:\n\n```\ngo test ./pkg/middlewares/ -run \u0027TestACCESSCONTROL001_XFFBypass|TestCHAIN_AllowlistBypass\u0027 -v\n```\n\nOutput:\n```\n[control] no XFF, RemoteAddr=1.2.3.4 -\u003e status=400 (correctly blocked)\n[exploit] XFF=10.0.0.1, RemoteAddr=1.2.3.4 -\u003e status=200, ClientIP=10.0.0.1 (bypassed)\ngate status = 200 | created_ip that would be persisted = \"10.0.0.1\"\n```\n\nAgainst a live instance with `mcp_allowed_remote_ips=10.0.0.1`:\n```\ncurl -s -o /dev/null -w \"%{http_code}\\n\" https://target/mcp # 403 (blocked)\ncurl -s -o /dev/null -w \"%{http_code}\\n\" -H \"X-Forwarded-For: 10.0.0.1\" https://target/mcp # passes IP gate\n```\n\n### Impact\nDefeats an explicitly advertised, operator-configured security control on the MCP and API-token surfaces, from any source IP, with a single header and no authentication to the gate itself. It also destroys non-repudiation: every action is logged under the spoofed allowlisted IP, so audit trails point at the trusted host instead of the attacker. (The MCP/API actions themselves still require a valid token \u2014 this neutralizes the compensating IP control, not the JWT layer.)",
"id": "GHSA-wchh-mrv2-5h4q",
"modified": "2026-11-09T21:51:28Z",
"published": "2026-11-09T21:51:28Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/mayswind/ezbookkeeping/security/advisories/GHSA-wchh-mrv2-5h4q"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-86041"
},
{
"type": "WEB",
"url": "https://github.com/mayswind/ezbookkeeping/commit/d79a5beb8037e529dc4dc64a6d22bd2d02b2e986"
},
{
"type": "PACKAGE",
"url": "https://github.com/mayswind/ezbookkeeping"
},
{
"type": "WEB",
"url": "https://github.com/mayswind/ezbookkeeping/releases/tag/v1.6.0"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N",
"type": "CVSS_V4"
}
],
"summary": "ezBookkeeping: Source-IP allowlist (MCP \u0026 API token) is bypassable via a spoofed `X-Forwarded-For` header"
}
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