diff -urN b/creds/creds.go a/creds/creds.go
--- b/creds/creds.go	2023-12-13 19:56:25.000000000 +0100
+++ a/creds/creds.go	2025-01-17 08:55:10.175959181 +0100
@@ -53,11 +53,20 @@
 // as input.
 type Creds map[string][]string
 
-func bufferCreds(c Creds) *bytes.Buffer {
+func (c Creds) buffer(protectProtocol bool) (*bytes.Buffer, error) {
 	buf := new(bytes.Buffer)
 
 	for k, v := range c {
 		for _, item := range v {
+			if strings.Contains(item, "\n") {
+				return nil, errors.Errorf(tr.Tr.Get("credential value for %s contains newline: %q", k, item))
+			}
+			if protectProtocol && strings.Contains(item, "\r") {
+				return nil, errors.Errorf(tr.Tr.Get("credential value for %s contains carriage return: %q\nIf this is intended, set `credential.protectProtocol=false`", k, item))
+			}
+			if strings.Contains(item, string(rune(0))) {
+				return nil, errors.Errorf(tr.Tr.Get("credential value for %s contains null byte: %q", k, item))
+			}
 			buf.Write([]byte(k))
 			buf.Write([]byte("="))
 			buf.Write([]byte(item))
@@ -65,7 +74,7 @@
 		}
 	}
 
-	return buf
+	return buf, nil
 }
 
 type CredentialHelperContext struct {
@@ -153,6 +162,9 @@
 			helpers = append(helpers, ctxt.askpassCredHelper)
 		}
 	}
+
+	ctxt.commandCredHelper.protectProtocol = ctxt.urlConfig.Bool("credential", rawurl, "protectProtocol", true)
+
 	return CredentialHelperWrapper{CredentialHelper: NewCredentialHelpers(append(helpers, ctxt.commandCredHelper)), Input: input, Url: u}
 }
 
@@ -292,7 +304,8 @@
 }
 
 type commandCredentialHelper struct {
-	SkipPrompt bool
+	SkipPrompt      bool
+	protectProtocol bool
 }
 
 func (h *commandCredentialHelper) Fill(creds Creds) (Creds, error) {
@@ -323,7 +336,10 @@
 	if err != nil {
 		return nil, errors.New(tr.Tr.Get("failed to find `git credential %s`: %v", subcommand, err))
 	}
-	cmd.Stdin = bufferCreds(input)
+	cmd.Stdin, err = input.buffer(h.protectProtocol)
+	if err != nil {
+		return nil, errors.New(tr.Tr.Get("invalid input to `git credential %s`: %v", subcommand, err))
+	}
 	cmd.Stdout = output
 	/*
 	   There is a reason we don't read from stderr here:
diff -urN b/creds/creds_test.go a/creds/creds_test.go
--- b/creds/creds_test.go	2023-12-13 19:56:25.000000000 +0100
+++ a/creds/creds_test.go	2025-01-17 08:55:21.318023782 +0100
@@ -1,12 +1,89 @@
 package creds
 
 import (
+	"bytes"
 	"errors"
+	"slices"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
 )
 
+func assertCredsLinesMatch(t *testing.T, expected []string, buf *bytes.Buffer) {
+	actual := strings.SplitAfter(buf.String(), "\n")
+
+	slices.Sort(expected)
+	slices.Sort(actual)
+
+	assert.Equal(t, expected, actual)
+}
+func TestCredsBufferFormat(t *testing.T) {
+	creds := make(Creds)
+
+	expected := []string{""}
+
+	buf, err := creds.buffer(true)
+	assert.NoError(t, err)
+	assertCredsLinesMatch(t, expected, buf)
+
+	creds["protocol"] = []string{"https"}
+	creds["host"] = []string{"example.com"}
+
+	expected = []string{"host=example.com\n", "protocol=https\n", ""}
+
+	buf, err = creds.buffer(true)
+	assert.NoError(t, err)
+	assertCredsLinesMatch(t, expected, buf)
+
+	creds["wwwauth[]"] = []string{"Basic realm=test", "Negotiate"}
+
+	expected = append(expected, "wwwauth[]=Basic realm=test\n", "wwwauth[]=Negotiate\n")
+	buf, err = creds.buffer(true)
+	assert.NoError(t, err)
+	assertCredsLinesMatch(t, expected, buf)
+}
+
+func TestCredsBufferProtect(t *testing.T) {
+	creds := make(Creds)
+
+	// Always disallow LF characters
+	creds["protocol"] = []string{"https"}
+	creds["host"] = []string{"one.example.com\nhost=two.example.com"}
+
+	buf, err := creds.buffer(false)
+	assert.Error(t, err)
+	assert.Nil(t, buf)
+
+	buf, err = creds.buffer(true)
+	assert.Error(t, err)
+	assert.Nil(t, buf)
+
+	// Disallow CR characters unless protocol protection disabled
+	creds["host"] = []string{"one.example.com\rhost=two.example.com"}
+
+	expected := []string{"", "protocol=https\n", "host=one.example.com\rhost=two.example.com\n"}
+
+	buf, err = creds.buffer(false)
+	assert.NoError(t, err)
+	assertCredsLinesMatch(t, expected, buf)
+
+	buf, err = creds.buffer(true)
+	assert.Error(t, err)
+	assert.Nil(t, buf)
+
+	// Always disallow null bytes
+	creds["host"] = []string{"one.example.com\x00host=two.example.com"}
+
+	buf, err = creds.buffer(false)
+	assert.Error(t, err)
+	assert.Nil(t, buf)
+
+	buf, err = creds.buffer(true)
+	assert.Error(t, err)
+	assert.Nil(t, buf)
+}
+
 type testCredHelper struct {
 	fillErr    error
 	approveErr error
diff -urN b/t/cmd/lfstest-gitserver.go a/t/cmd/lfstest-gitserver.go
--- b/t/cmd/lfstest-gitserver.go	2023-12-13 19:56:25.000000000 +0100
+++ a/t/cmd/lfstest-gitserver.go	2025-01-16 14:33:23.825991696 +0100
@@ -27,6 +27,7 @@
 	"net/http"
 	"net/http/httptest"
 	"net/textproto"
+	"net/url"
 	"os"
 	"os/exec"
 	"regexp"
@@ -252,6 +253,7 @@
 }
 
 func lfsUrl(repo, oid string, redirect bool) string {
+	repo = url.QueryEscape(repo)
 	if redirect {
 		return server.URL + "/redirect307/objects/" + oid + "?r=" + repo
 	}
diff -urN b/t/t-credentials-protect.sh a/t/t-credentials-protect.sh
--- b/t/t-credentials-protect.sh	1970-01-01 01:00:00.000000000 +0100
+++ a/t/t-credentials-protect.sh	2025-01-16 14:03:23.597029590 +0100
@@ -0,0 +1,146 @@
+#!/usr/bin/env bash
+
+. "$(dirname "$0")/testlib.sh"
+
+ensure_git_version_isnt $VERSION_LOWER "2.3.0"
+
+export CREDSDIR="$REMOTEDIR/creds-credentials-protect"
+setup_creds
+
+# Copy the default record file for the test credential helper to match the
+# hostname used in the Git LFS configurations of the tests.
+cp "$CREDSDIR/127.0.0.1" "$CREDSDIR/localhost"
+
+begin_test "credentials rejected with line feed"
+(
+  set -e
+
+  reponame="protect-linefeed"
+  setup_remote_repo "$reponame"
+  clone_repo "$reponame" "$reponame"
+
+  contents="a"
+  contents_oid=$(calc_oid "$contents")
+
+  git lfs track "*.dat"
+  printf "%s" "$contents" >a.dat
+  git add .gitattributes a.dat
+  git commit -m "add a.dat"
+
+  # Using localhost instead of 127.0.0.1 in the LFS API URL ensures this URL
+  # is used when filling credentials rather than the Git remote URL, which
+  # would otherwise be used since it would have the same scheme and hostname.
+  gitserver="$(echo "$GITSERVER" | sed 's/127\.0\.0\.1/localhost/')"
+  testreponame="test%0a$reponame"
+  git config lfs.url "$gitserver/$testreponame.git/info/lfs"
+
+  GIT_TRACE=1 git lfs push origin main 2>&1 | tee push.log
+  if [ "0" -eq "${PIPESTATUS[0]}" ]; then
+    echo >&2 "fatal: expected 'git lfs push' to fail ..."
+    exit 1
+  fi
+  grep "batch response: Git credentials for $gitserver.* not found" push.log
+  grep "credential value for path contains newline" push.log
+  refute_server_object "$testreponame" "$contents_oid"
+
+  git config credential.protectProtocol false
+
+  GIT_TRACE=1 git lfs push origin main 2>&1 | tee push.log
+  if [ "0" -eq "${PIPESTATUS[0]}" ]; then
+    echo >&2 "fatal: expected 'git lfs push' to fail ..."
+    exit 1
+  fi
+  grep "batch response: Git credentials for $gitserver.* not found" push.log
+  grep "credential value for path contains newline" push.log
+  refute_server_object "$testreponame" "$contents_oid"
+)
+end_test
+
+begin_test "credentials rejected with carriage return"
+(
+  set -e
+
+  reponame="protect-return"
+  setup_remote_repo "$reponame"
+  clone_repo "$reponame" "$reponame"
+
+  contents="a"
+  contents_oid=$(calc_oid "$contents")
+
+  git lfs track "*.dat"
+  printf "%s" "$contents" >a.dat
+  git add .gitattributes a.dat
+  git commit -m "add a.dat"
+
+  # Using localhost instead of 127.0.0.1 in the LFS API URL ensures this URL
+  # is used when filling credentials rather than the Git remote URL, which
+  # would otherwise be used since it would have the same scheme and hostname.
+  gitserver="$(echo "$GITSERVER" | sed 's/127\.0\.0\.1/localhost/')"
+  testreponame="test%0d$reponame"
+  git config lfs.url "$gitserver/$testreponame.git/info/lfs"
+
+  GIT_TRACE=1 git lfs push origin main 2>&1 | tee push.log
+  if [ "0" -eq "${PIPESTATUS[0]}" ]; then
+    echo >&2 "fatal: expected 'git lfs push' to fail ..."
+    exit 1
+  fi
+  grep "batch response: Git credentials for $gitserver.* not found" push.log
+  grep "credential value for path contains carriage return" push.log
+  refute_server_object "$testreponame" "$contents_oid"
+
+  git config credential.protectProtocol false
+
+  git lfs push origin main 2>&1 | tee push.log
+  if [ "0" -ne "${PIPESTATUS[0]}" ]; then
+    echo >&2 "fatal: expected 'git lfs push' to succeed ..."
+    exit 1
+  fi
+  [ $(grep -c "Uploading LFS objects: 100% (1/1)" push.log) -eq 1 ]
+  assert_server_object "$testreponame" "$contents_oid"
+)
+end_test
+
+begin_test "credentials rejected with null byte"
+(
+  set -e
+
+  reponame="protect-null"
+  setup_remote_repo "$reponame"
+  clone_repo "$reponame" "$reponame"
+
+  contents="a"
+  contents_oid=$(calc_oid "$contents")
+
+  git lfs track "*.dat"
+  printf "%s" "$contents" >a.dat
+  git add .gitattributes a.dat
+  git commit -m "add a.dat"
+
+  # Using localhost instead of 127.0.0.1 in the LFS API URL ensures this URL
+  # is used when filling credentials rather than the Git remote URL, which
+  # would otherwise be used since it would have the same scheme and hostname.
+  gitserver="$(echo "$GITSERVER" | sed 's/127\.0\.0\.1/localhost/')"
+  testreponame="test%00$reponame"
+  git config lfs.url "$gitserver/$testreponame.git/info/lfs"
+
+  GIT_TRACE=1 git lfs push origin main 2>&1 | tee push.log
+  if [ "0" -eq "${PIPESTATUS[0]}" ]; then
+    echo >&2 "fatal: expected 'git lfs push' to fail ..."
+    exit 1
+  fi
+  grep "batch response: Git credentials for $gitserver.* not found" push.log
+  grep "credential value for path contains null byte" push.log
+  refute_server_object "$testreponame" "$contents_oid"
+
+  git config credential.protectProtocol false
+
+  GIT_TRACE=1 git lfs push origin main 2>&1 | tee push.log
+  if [ "0" -eq "${PIPESTATUS[0]}" ]; then
+    echo >&2 "fatal: expected 'git lfs push' to fail ..."
+    exit 1
+  fi
+  grep "batch response: Git credentials for $gitserver.* not found" push.log
+  grep "credential value for path contains null byte" push.log
+  refute_server_object "$testreponame" "$contents_oid"
+)
+end_test
diff -urN b/t/testhelpers.sh a/t/testhelpers.sh
--- b/t/testhelpers.sh	2023-12-13 19:56:25.000000000 +0100
+++ a/t/testhelpers.sh	2025-01-16 14:15:19.240279305 +0100
@@ -557,6 +557,14 @@
   fi
 }
 
+
+setup_creds() {
+  mkdir -p "$CREDSDIR"
+  write_creds_file "user:pass" "$CREDSDIR/127.0.0.1"
+  write_creds_file ":pass" "$CREDSDIR/--$certpath"
+  write_creds_file ":pass" "$CREDSDIR/--$keypath"
+}
+
 # setup initializes the clean, isolated environment for integration tests.
 setup() {
   cd "$ROOTDIR"
@@ -613,10 +621,7 @@
   # setup the git credential password storage
   local certpath="$(echo "$LFS_CLIENT_CERT_FILE" | tr / -)"
   local keypath="$(echo "$LFS_CLIENT_KEY_FILE_ENCRYPTED" | tr / -)"
-  mkdir -p "$CREDSDIR"
-  write_creds_file "user:pass" "$CREDSDIR/127.0.0.1"
-  write_creds_file ":pass" "$CREDSDIR/--$certpath"
-  write_creds_file ":pass" "$CREDSDIR/--$keypath"
+  setup_creds
 
   echo "#"
   echo "# HOME: $HOME"