Some wincred fixes

These were rolled out as part of the security fix release Git for Windows v2.55.0(3).

Johannes Schindelin (2):
wincred: avoid memory corruption when erasing a credential
wincred: prevent silent credential loss when storing OAuth tokens

 contrib/credential/wincred/git-credential-wincred.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

base-commit: 94f057755b7941b321fd11fec1b2e3ca5313a4e0
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2182%2Fdscho%2Fwincred-fixes-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2182/dscho/wincred-fixes-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/2182

gitgitgadget

From: Johannes Schindelin <[email protected]>

The earlier d22a488482 (wincred: avoid memory corruption, 2025-11-17) repaired only get_credential(); match_cred_password() has the same defect and is reached on git credential reject. When Git asks the helper to erase a stored credential whose password was supplied by the caller, the helper copies the candidate's password into a freshly allocated buffer for comparison. That copy overruns the allocation by one WCHAR of NUL, which on uninstrumented Windows manifests as process termination with status 0xC0000374. Because the helper can die before reaching CredDeleteW(), git credential reject masks the failure and the rejected credential remains stored.

CredentialBlobSize is documented as a byte count, so for an N-WCHAR blob it equals N * sizeof(WCHAR). The pre-fix code allocated that many bytes and asked wcsncpy_s to copy N wide characters, but wcsncpy_s always appends a terminating NUL WCHAR, writing one WCHAR past the allocation. The destination-capacity argument was also passed in bytes rather than in WCHAR elements as the API requires, so the safe-CRT runtime never rejected the copy.

See GHSA-rxqw-wxqg-g7hw.

---
 contrib/credential/wincred/git-credential-wincred.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

index 73c2b9b72a..190bbccdf9 100644
@@ -121,10 +121,10 @@ static int match_part_last(LPCWSTR *ptarget, LPCWSTR want, LPCWSTR delim)
 
 static int match_cred_password(const CREDENTIALW *cred) {
 	int ret;
-	WCHAR *cred_password = xmalloc(cred->CredentialBlobSize);
-	wcsncpy_s(cred_password, cred->CredentialBlobSize,
-		(LPCWSTR)cred->CredentialBlob,
-		cred->CredentialBlobSize / sizeof(WCHAR));
+	size_t wlen = cred->CredentialBlobSize / sizeof(WCHAR);
+	WCHAR *cred_password = xmalloc((wlen + 1) * sizeof(WCHAR));
+	wcsncpy_s(cred_password, wlen + 1,
+		(LPCWSTR)cred->CredentialBlob, wlen);
 	ret = !wcscmp(cred_password, password);
 	free(cred_password);
 	return ret;

gitgitgadget

From: Johannes Schindelin <[email protected]>

When git credential approve hands the wincred helper a password together with an oauth_refresh_token, the OAuth branch of store_credential() writes one WCHAR past the allocation while formatting both fields into a single CredentialBlob. On Windows this trips heap verification and tears the helper down with status 0xC0000374; approve masks the failure, so the credential the user meant to save never reaches CredWriteW() and the next session prompts for it again.

The bug has the same shape as the one fixed in the previous commit:
the allocation leaves no room for the terminating NUL, and the sizeOfBuffer argument to _snwprintf_s() is a byte count where the API expects a WCHAR count, which lets the safe-CRT runtime write the terminator out of bounds.

Apply the same remedy d22a488482 (wincred: avoid memory corruption, 2025-11-17) applied in get_credential(): allocate (wlen + 1) * sizeof(WCHAR) bytes and pass wlen + 1 as the destination capacity in WCHARs.

This closes the second of the two heap writes tracked under GHSA-rxqw-wxqg-g7hw.

---
 contrib/credential/wincred/git-credential-wincred.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index 190bbccdf9..22eb27ca31 100644
@@ -208,8 +208,8 @@ static void store_credential(void)
 
 	if (oauth_refresh_token) {
 		wlen = _scwprintf(L"%s\r\noauth_refresh_token=%s", password, oauth_refresh_token);
-		secret = xmalloc(sizeof(WCHAR) * wlen);
-		_snwprintf_s(secret, sizeof(WCHAR) * wlen, wlen, L"%s\r\noauth_refresh_token=%s", password, oauth_refresh_token);
+		secret = xmalloc((wlen + 1) * sizeof(WCHAR));
+		_snwprintf_s(secret, wlen + 1, wlen, L"%s\r\noauth_refresh_token=%s", password, oauth_refresh_token);
 	} else {
 		secret = _wcsdup(password);
 	}

gitgitgadget

"Johannes Schindelin via GitGitGadget" <[email protected]>

Thanks. Let me merge it down fast.