ssh: curb GSSAPI DoS risk by limiting number of specified OIDs

Previously, an attacker could specify an integer up to 0xFFFFFFFF
that would directly allocate memory despite the observability of
the rest of the payload. This change places a hard cap on the
amount of mechanisms that can be specified and encoded in the
payload. Additionally, it performs a small sanity check to deny
payloads whose stated size is contradictory to the observed payload.

Thank you to Jakub Ciolek for reporting this issue.

Fixes CVE-2025-58181
Fixes golang/go#76363

Change-Id: I0307ab3e906a3f2ae763b5f9f0310f7073f84485
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/721961
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

2 files changed, 38 insertions, 1 deletions

diff --git a/ssh/ssh_gss.go b/ssh/ssh_gss.go
index 24bd7c8..a6249a1 100644
--- a/ssh/ssh_gss.go
+++ b/ssh/ssh_gss.go
@@ -106,6 +106,13 @@ func parseGSSAPIPayload(payload []byte) (*userAuthRequestGSSAPI, error) {
 	if !ok {
 		return nil, errors.New("parse uint32 failed")
 	}
+	// Each ASN.1 encoded OID must have a minimum
+	// of 2 bytes; 64 maximum mechanisms is an
+	// arbitrary, but reasonable ceiling.
+	const maxMechs = 64
+	if n > maxMechs || int(n)*2 > len(rest) {
+		return nil, errors.New("invalid mechanism count")
+	}
 	s := &userAuthRequestGSSAPI{
 		N:    n,
 		OIDS: make([]asn1.ObjectIdentifier, n),
@@ -122,7 +129,6 @@ func parseGSSAPIPayload(payload []byte) (*userAuthRequestGSSAPI, error) {
 		if rest, err = asn1.Unmarshal(desiredMech, &s.OIDS[i]); err != nil {
 			return nil, err
 		}
-
 	}
 	return s, nil
 }
diff --git a/ssh/ssh_gss_test.go b/ssh/ssh_gss_test.go
index 39a1112..9e3ea8c 100644
--- a/ssh/ssh_gss_test.go
+++ b/ssh/ssh_gss_test.go
@@ -17,6 +17,37 @@ func TestParseGSSAPIPayload(t *testing.T) {
 	}
 }
 
+func TestParseDubiousGSSAPIPayload(t *testing.T) {
+	for _, tc := range []struct {
+		name    string
+		payload []byte
+		wanterr bool
+	}{
+		{
+			"num mechanisms is unrealistic",
+			[]byte{0xFF, 0x00, 0x00, 0xFF,
+				0x00, 0x00, 0x00, 0x0b, 0x06, 0x09, 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x12, 0x01, 0x02, 0x02},
+			true,
+		},
+		{
+			"num mechanisms greater than payload",
+			[]byte{0x00, 0x00, 0x00, 0x40, // 64, |rest| too small
+				0x00, 0x00, 0x00, 0x0b, 0x06, 0x09, 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x12, 0x01, 0x02, 0x02},
+			true,
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			_, err := parseGSSAPIPayload(tc.payload)
+			if tc.wanterr && err == nil {
+				t.Errorf("got nil, want error")
+			}
+			if !tc.wanterr && err != nil {
+				t.Errorf("got %v, want nil", err)
+			}
+		})
+	}
+}
+
 func TestBuildMIC(t *testing.T) {
 	sessionID := []byte{134, 180, 134, 194, 62, 145, 171, 82, 119, 149, 254, 196, 125, 173, 177, 145, 187, 85, 53,
 		183, 44, 150, 219, 129, 166, 195, 19, 33, 209, 246, 175, 121}