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ssh: don't skip the source-address critical option in CheckCert

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CertChecker.CheckCert ignored source-address on the assumption that
serverAuthenticate would enforce it, but that only happens in the
server-side user authentication path. Nothing enforced it in the host key
path, so CheckHostKey accepted CA-signed host certificates carrying
source-address regardless of the server's address, and the same applied to
applications calling CheckCert directly.

Drop the special case: every critical option must be listed in
SupportedCriticalOptions, host and user certificates alike. Authenticate,
the only path where serverAuthenticate does enforce source-address, checks
against a copy of the CertChecker with that option appended.

Fixes golang/go#80872

Change-Id: I3c3554b71ea2a4ce4b17696a9596b973b7d74b00
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/816840
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
This commit is contained in:
Nicola Murino
2026-08-30 20:04:51 -07:00
committed by Gopher Robot
parent afebf4cb4e
commit 39dc44e69c
2 changed files with 93 additions and 21 deletions
Download Patch File Download Diff File Expand all files Collapse all files
ssh/certs.go
+17 -21
View File
@@ -10,6 +10,7 @@ import (
"fmt"
"io"
"net"
"slices"
"sort"
"time"
)
@@ -305,8 +306,11 @@ const sourceAddressCriticalOption = "source-address"
// minimally, the IsAuthority callback should be set.
type CertChecker struct {
// SupportedCriticalOptions lists the CriticalOptions that the
// server application layer understands. These are only used
// for user certificates.
// application layer understands. A certificate carrying a critical
// option that is not listed here is rejected.
// CertChecker.Authenticate additionally accepts the source-address
// option, which the server enforces on the Permissions that
// Authenticate returns.
SupportedCriticalOptions []string
// IsUserAuthority should return true if the key is recognized as an
@@ -369,8 +373,9 @@ func (c *CertChecker) CheckHostKey(addr string, remote net.Addr, key PublicKey)
return c.CheckCert(hostname, cert)
}
// Authenticate checks a user certificate. Authenticate can be used as
// a value for ServerConfig.PublicKeyCallback.
// Authenticate checks a user certificate. Authenticate can be used as a value
// for ServerConfig.PublicKeyCallback. The source-address critical option is
// allowed, as it will be enforced by the server.
func (c *CertChecker) Authenticate(conn ConnMetadata, pubKey PublicKey) (*Permissions, error) {
cert, ok := pubKey.(*Certificate)
if !ok {
@@ -389,8 +394,11 @@ func (c *CertChecker) Authenticate(conn ConnMetadata, pubKey PublicKey) (*Permis
if !c.IsUserAuthority(cert.SignatureKey) {
return nil, fmt.Errorf("ssh: certificate signed by unrecognized authority")
}
if err := c.CheckCert(conn.User(), cert); err != nil {
// The source-address critical option is enforced by serverAuthenticate,
// so it is supported regardless of SupportedCriticalOptions
cc := *c
cc.SupportedCriticalOptions = append(slices.Clip(cc.SupportedCriticalOptions), sourceAddressCriticalOption)
if err := cc.CheckCert(conn.User(), cert); err != nil {
return nil, err
}
@@ -398,27 +406,15 @@ func (c *CertChecker) Authenticate(conn ConnMetadata, pubKey PublicKey) (*Permis
}
// CheckCert checks CriticalOptions, ValidPrincipals, revocation, timestamp and
// the signature of the certificate.
// the signature of the certificate. Critical options that are not listed in
// SupportedCriticalOptions are rejected.
func (c *CertChecker) CheckCert(principal string, cert *Certificate) error {
if c.IsRevoked != nil && c.IsRevoked(cert) {
return fmt.Errorf("ssh: certificate serial %d revoked", cert.Serial)
}
for opt := range cert.CriticalOptions {
// sourceAddressCriticalOption will be enforced by
// serverAuthenticate
if opt == sourceAddressCriticalOption {
continue
}
found := false
for _, supp := range c.SupportedCriticalOptions {
if supp == opt {
found = true
break
}
}
if !found {
if !slices.Contains(c.SupportedCriticalOptions, opt) {
return fmt.Errorf("ssh: unsupported critical option %q in certificate", opt)
}
}