diff --git a/doc/guix.texi b/doc/guix.texi
index 86dbe9f201..c116be8907 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -5448,7 +5448,9 @@ commit of a channel that should be authenticated.  The first time a
 channel is fetched with @command{guix pull} or @command{guix
 time-machine}, the command looks up the introductory commit and verifies
 that it is signed by the specified OpenPGP key.  From then on, it
-authenticates commits according to the rule above.
+authenticates commits according to the rule above.  Authentication fails
+if the target commit is neither a descendant nor an ancestor of the
+introductory commit.
 
 Additionally, your channel must provide all the OpenPGP keys that were
 ever mentioned in @file{.guix-authorizations}, stored as @file{.key}
diff --git a/guix/git-authenticate.scm b/guix/git-authenticate.scm
index ab3fcd8b2f..419cb85afc 100644
--- a/guix/git-authenticate.scm
+++ b/guix/git-authenticate.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2019, 2020, 2021, 2022 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -22,7 +22,9 @@
   #:use-module (guix base16)
   #:autoload   (guix base64) (base64-encode)
   #:use-module ((guix git)
-                #:select (commit-difference false-if-git-not-found))
+                #:select (commit-difference
+                          commit-descendant?
+                          false-if-git-not-found))
   #:use-module (guix i18n)
   #:use-module ((guix diagnostics) #:select (formatted-message))
   #:use-module (guix openpgp)
@@ -426,6 +428,17 @@ denoting the authorized keys for commits whose parent lack the
           (verify-introductory-commit repository keyring
                                       start-commit signer))
 
+        ;; Make sure END-COMMIT is a descendant of START-COMMIT or of one of
+        ;; AUTHENTICATED-COMMITS, which are known to be descendants of
+        ;; START-COMMIT.
+        (unless (commit-descendant? end-commit
+                                    (cons start-commit
+                                          authenticated-commits))
+          (raise (formatted-message
+                  (G_ "commit ~a is not a descendant of introductory commit ~a")
+                  (oid->string (commit-id end-commit))
+                  (oid->string (commit-id start-commit)))))
+
         (let ((stats (call-with-progress-reporter reporter
                        (lambda (report)
                          (authenticate-commits repository commits
diff --git a/tests/channels.scm b/tests/channels.scm
index d45c450241..0fe870dbaf 100644
--- a/tests/channels.scm
+++ b/tests/channels.scm
@@ -1,6 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2018 Ricardo Wurmus <rekado@elephly.net>
-;;; Copyright © 2019, 2020 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2019, 2020, 2022 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -525,6 +525,64 @@
                                   #:keyring-reference-prefix "")
             'failed))))))
 
+(unless (gpg+git-available?) (test-skip 1))
+(test-equal "authenticate-channel, not a descendant of introductory commit"
+  #t
+  (with-fresh-gnupg-setup (list %ed25519-public-key-file
+                                %ed25519-secret-key-file
+                                %ed25519-2-public-key-file
+                                %ed25519-2-secret-key-file)
+    (with-temporary-git-repository directory
+        `((add ".guix-channel"
+               ,(object->string
+                 '(channel (version 0)
+                           (keyring-reference "master"))))
+          (add ".guix-authorizations"
+               ,(object->string
+                 `(authorizations (version 0)
+                                  ((,(key-fingerprint
+                                      %ed25519-public-key-file)
+                                    (name "Charlie"))))))
+          (add "signer.key" ,(call-with-input-file %ed25519-public-key-file
+                               get-string-all))
+          (commit "first commit"
+                  (signer ,(key-fingerprint %ed25519-public-key-file)))
+          (branch "alternate-branch")
+          (checkout "alternate-branch")
+          (add "something.txt" ,(random-text))
+          (commit "intro commit"
+                  (signer ,(key-fingerprint %ed25519-public-key-file)))
+          (checkout "master")
+          (add "random" ,(random-text))
+          (commit "second commit"
+                  (signer ,(key-fingerprint %ed25519-public-key-file))))
+      (with-repository directory repository
+        (let* ((commit1 (find-commit repository "first"))
+               (commit2 (find-commit repository "second"))
+               (commit0 (commit-lookup
+                         repository
+                         (reference-target
+                          (branch-lookup repository "alternate-branch"))))
+               (intro   (make-channel-introduction
+                         (commit-id-string commit0)
+                         (openpgp-public-key-fingerprint
+                          (read-openpgp-packet
+                           %ed25519-public-key-file))))
+               (channel (channel (name 'example)
+                                 (url (string-append "file://" directory))
+                                 (introduction intro))))
+          (guard (c ((formatted-message? c)
+                     (and (string-contains (formatted-message-string c)
+                                           "not a descendant")
+                          (equal? (formatted-message-arguments c)
+                                  (list
+                                   (oid->string (commit-id commit2))
+                                   (oid->string (commit-id commit0)))))))
+            (authenticate-channel channel directory
+                                  (commit-id-string commit2)
+                                  #:keyring-reference-prefix "")
+            'failed))))))
+
 (unless (gpg+git-available?) (test-skip 1))
 (test-equal "authenticate-channel, .guix-authorizations"
   #t
diff --git a/tests/git-authenticate.scm b/tests/git-authenticate.scm
index 6ec55fb2e5..c063920c12 100644
--- a/tests/git-authenticate.scm
+++ b/tests/git-authenticate.scm
@@ -431,4 +431,48 @@
                                       #:keyring-reference "master"
                                       #:cache-key (random-text)))))))))
 
+(unless (gpg+git-available?) (test-skip 1))
+(test-equal "authenticate-repository, target not a descendant of intro"
+  'target-commit-not-a-descendant-of-intro
+  (with-fresh-gnupg-setup (list %ed25519-public-key-file
+                                %ed25519-secret-key-file)
+    (let ((fingerprint (key-fingerprint %ed25519-public-key-file)))
+      (with-temporary-git-repository directory
+          `((add "signer.key" ,(call-with-input-file %ed25519-public-key-file
+                                 get-string-all))
+            (add ".guix-authorizations"
+                 ,(object->string
+                   `(authorizations (version 0)
+                                    ((,(key-fingerprint
+                                        %ed25519-public-key-file)
+                                      (name "Charlie"))))))
+            (commit "zeroth commit" (signer ,fingerprint))
+            (branch "pre-intro-branch")
+            (checkout "pre-intro-branch")
+            (add "b.txt" "B")
+            (commit "alternate commit" (signer ,fingerprint))
+            (checkout "master")
+            (add "a.txt" "A")
+            (commit "first commit" (signer ,fingerprint))
+            (add "c.txt" "C")
+            (commit "second commit" (signer ,fingerprint)))
+        (with-repository directory repository
+          (let ((commit1 (find-commit repository "first"))
+                (commit-alt
+                 (commit-lookup repository
+                                (reference-target
+                                 (branch-lookup repository
+                                                "pre-intro-branch")))))
+            (guard (c ((formatted-message? c)
+                       (and (equal? (formatted-message-arguments c)
+                                    (list (oid->string (commit-id commit-alt))
+                                          (oid->string (commit-id commit1))))
+                            'target-commit-not-a-descendant-of-intro)))
+              (authenticate-repository repository
+                                       (commit-id commit1)
+                                       (openpgp-fingerprint fingerprint)
+                                       #:end (commit-id commit-alt)
+                                       #:keyring-reference "master"
+                                       #:cache-key (random-text)))))))))
+
 (test-end "git-authenticate")
diff --git a/tests/guix-git-authenticate.sh b/tests/guix-git-authenticate.sh
index 8ebbea398b..2b90d8a4af 100644
--- a/tests/guix-git-authenticate.sh
+++ b/tests/guix-git-authenticate.sh
@@ -1,5 +1,5 @@
 # GNU Guix --- Functional package management for GNU
-# Copyright © 2020 Ludovic Courtès <ludo@gnu.org>
+# Copyright © 2020, 2022 Ludovic Courtès <ludo@gnu.org>
 #
 # This file is part of GNU Guix.
 #
@@ -34,10 +34,18 @@ intro_signer="BBB0 2DDF 2CEA F6A8 0D1D  E643 A2A0 6DF2 A33A 54FA"
 
 cache_key="test-$$"
 
-guix git authenticate "$intro_commit" "$intro_signer"	\
+# This must fail because the end commit is not a descendant of $intro_commit.
+! guix git authenticate "$intro_commit" "$intro_signer"	\
      --cache-key="$cache_key" --stats			\
      --end=9549f0283a78fe36f2d4ff2a04ef8ad6b0c02604
 
+# The v1.2.0 commit is a descendant of $intro_commit and it satisfies the
+# authorization invariant.
+v1_2_0_commit="a099685659b4bfa6b3218f84953cbb7ff9e88063"
+guix git authenticate "$intro_commit" "$intro_signer"	\
+     --cache-key="$cache_key" --stats			\
+     --end="$v1_2_0_commit"
+
 rm "$XDG_CACHE_HOME/guix/authentication/$cache_key"
 
 # Commit and signer of the 'v1.0.0' tag.
@@ -45,6 +53,11 @@ v1_0_0_commit="6298c3ffd9654d3231a6f25390b056483e8f407c"
 v1_0_0_signer="3CE4 6455 8A84 FDC6 9DB4  0CFB 090B 1199 3D9A EBB5" # civodul
 v1_0_1_commit="d68de958b60426798ed62797ff7c96c327a672ac"
 
+# This should succeed because v1.0.0 is an ancestor of $intro_commit.
+guix git authenticate "$intro_commit" "$intro_signer"	\
+     --cache-key="$cache_key" --stats			\
+     --end="$v1_0_0_commit"
+
 # This should fail because these commits lack '.guix-authorizations'.
 ! guix git authenticate "$v1_0_0_commit" "$v1_0_0_signer" \
        --cache-key="$cache_key" --end="$v1_0_1_commit"