net/url: add urlmaxqueryparams GODEBUG to limit the number of query parameters

net/url does not currently limit the number of query parameters parsed by
url.ParseQuery or URL.Query.

When parsing a application/x-www-form-urlencoded form,
net/http.Request.ParseForm will parse up to 10 MB of query parameters.
An input consisting of a large number of small, unique parameters can
cause excessive memory consumption.

We now limit the number of query parameters parsed to 10000 by default.
The limit can be adjusted by setting GODEBUG=urlmaxqueryparams=<n>.
Setting urlmaxqueryparams to 0 disables the limit.

Thanks to jub0bs for reporting this issue.

Fixes #77101
Fixes CVE-2025-61726

Change-Id: Iee3374c7ee2d8586dbf158536d3ade424203ff66
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/3020
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Neal Patel <nealpatel@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/736712
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

5 files changed, 84 insertions, 0 deletions

diff --git a/doc/godebug.md b/doc/godebug.md
index 28a2dc506e..184e161c40 100644
--- a/doc/godebug.md
+++ b/doc/godebug.md
@@ -163,6 +163,13 @@ will fail early. The default value is `httpcookiemaxnum=3000`. Setting
 number of cookies. To avoid denial of service attacks, this setting and default
 was backported to Go 1.25.2 and Go 1.24.8.
 
+Go 1.26 added a new `urlmaxqueryparams` setting that controls the maximum number
+of query parameters that net/url will accept when parsing a URL-encoded query string.
+If the number of parameters exceeds the number set in `urlmaxqueryparams`,
+parsing will fail early. The default value is `urlmaxqueryparams=10000`.
+Setting `urlmaxqueryparams=0`bles the limit. To avoid denial of service attacks,
+this setting and default was backported to Go 1.25.4 and Go 1.24.10.
+
 Go 1.26 added a new `urlstrictcolons` setting that controls whether `net/url.Parse`
 allows malformed hostnames containing colons outside of a bracketed IPv6 address.
 The default `urlstrictcolons=1` rejects URLs such as `http://localhost:1:2` or `http://::1/`.
diff --git a/src/internal/godebugs/table.go b/src/internal/godebugs/table.go
index 8f6d8bbdda..87b499385a 100644
--- a/src/internal/godebugs/table.go
+++ b/src/internal/godebugs/table.go
@@ -69,6 +69,7 @@ var All = []Info{
 	{Name: "tlssha1", Package: "crypto/tls", Changed: 25, Old: "1"},
 	{Name: "tlsunsafeekm", Package: "crypto/tls", Changed: 22, Old: "1"},
 	{Name: "updatemaxprocs", Package: "runtime", Changed: 25, Old: "0"},
+	{Name: "urlmaxqueryparams", Package: "net/url", Changed: 24, Old: "0"},
 	{Name: "urlstrictcolons", Package: "net/url", Changed: 26, Old: "0"},
 	{Name: "winreadlinkvolume", Package: "os", Changed: 23, Old: "0"},
 	{Name: "winsymlink", Package: "os", Changed: 23, Old: "0"},
diff --git a/src/net/url/url.go b/src/net/url/url.go
index 3acd202c24..202957a3a2 100644
--- a/src/net/url/url.go
+++ b/src/net/url/url.go
@@ -929,7 +929,30 @@ func ParseQuery(query string) (Values, error) {
 	return m, err
 }
 
+var urlmaxqueryparams = godebug.New("urlmaxqueryparams")
+
+const defaultMaxParams = 10000
+
+func urlParamsWithinMax(params int) bool {
+	withinDefaultMax := params <= defaultMaxParams
+	if urlmaxqueryparams.Value() == "" {
+		return withinDefaultMax
+	}
+	customMax, err := strconv.Atoi(urlmaxqueryparams.Value())
+	if err != nil {
+		return withinDefaultMax
+	}
+	withinCustomMax := customMax == 0 || params < customMax
+	if withinDefaultMax != withinCustomMax {
+		urlmaxqueryparams.IncNonDefault()
+	}
+	return withinCustomMax
+}
+
 func parseQuery(m Values, query string) (err error) {
+	if !urlParamsWithinMax(strings.Count(query, "&") + 1) {
+		return errors.New("number of URL query parameters exceeded limit")
+	}
 	for query != "" {
 		var key string
 		key, query, _ = strings.Cut(query, "&")
diff --git a/src/net/url/url_test.go b/src/net/url/url_test.go
index bb48bb6bee..d099353eb2 100644
--- a/src/net/url/url_test.go
+++ b/src/net/url/url_test.go
@@ -1521,6 +1521,54 @@ func TestParseQuery(t *testing.T) {
 	}
 }
 
+func TestParseQueryLimits(t *testing.T) {
+	for _, test := range []struct {
+		params  int
+		godebug string
+		wantErr bool
+	}{{
+		params:  10,
+		wantErr: false,
+	}, {
+		params:  defaultMaxParams,
+		wantErr: false,
+	}, {
+		params:  defaultMaxParams + 1,
+		wantErr: true,
+	}, {
+		params:  10,
+		godebug: "urlmaxqueryparams=9",
+		wantErr: true,
+	}, {
+		params:  defaultMaxParams + 1,
+		godebug: "urlmaxqueryparams=0",
+		wantErr: false,
+	}} {
+		t.Setenv("GODEBUG", test.godebug)
+		want := Values{}
+		var b strings.Builder
+		for i := range test.params {
+			if i > 0 {
+				b.WriteString("&")
+			}
+			p := fmt.Sprintf("p%v", i)
+			b.WriteString(p)
+			want[p] = []string{""}
+		}
+		query := b.String()
+		got, err := ParseQuery(query)
+		if gotErr, wantErr := err != nil, test.wantErr; gotErr != wantErr {
+			t.Errorf("GODEBUG=%v ParseQuery(%v params) = %v, want error: %v", test.godebug, test.params, err, wantErr)
+		}
+		if err != nil {
+			continue
+		}
+		if got, want := len(got), test.params; got != want {
+			t.Errorf("GODEBUG=%v ParseQuery(%v params): got %v params, want %v", test.godebug, test.params, got, want)
+		}
+	}
+}
+
 type RequestURITest struct {
 	url *URL
 	out string
diff --git a/src/runtime/metrics/doc.go b/src/runtime/metrics/doc.go
index ca032f51b1..6b774c36f3 100644
--- a/src/runtime/metrics/doc.go
+++ b/src/runtime/metrics/doc.go
@@ -404,6 +404,11 @@ Below is the full list of supported metrics, ordered lexicographically.
 		The number of non-default behaviors executed by the runtime
 		package due to a non-default GODEBUG=updatemaxprocs=... setting.
 
+	/godebug/non-default-behavior/urlmaxqueryparams:events
+		The number of non-default behaviors executed by the net/url
+		package due to a non-default GODEBUG=urlmaxqueryparams=...
+		setting.
+
 	/godebug/non-default-behavior/urlstrictcolons:events
 		The number of non-default behaviors executed by the net/url
 		package due to a non-default GODEBUG=urlstrictcolons=...