diff --git a/Documentation/BreakingChanges.adoc b/Documentation/BreakingChanges.adoc
index 73bb939359..dbc46d14e3 100644
--- a/Documentation/BreakingChanges.adoc
+++ b/Documentation/BreakingChanges.adoc
@@ -171,6 +171,11 @@ JGit, libgit2 and Gitoxide need to support it.
   matches the default branch name used in new repositories by many of the
   big Git forges.
 
+* Git will accept hex object IDs only in lowercase. The fact that Git has
+	historically allowed uppercase characters in hex object IDs has been the
+	source of a variety of bugs and security problems in software using Git. We
+	don't expect most users to notice any change.
+
 * Git will require Rust as a mandatory part of the build process. While Git
   already started to adopt Rust in Git 2.49, all parts written in Rust are
   optional for the time being. This includes:
diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index 6b2a87e2d3..44ac72ef1e 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -1866,7 +1866,7 @@ static void repack_local_links(void)
 	while (strbuf_getline_lf(&line, out) != EOF) {
 		unsigned char binary[GIT_MAX_RAWSZ];
 		if (line.len != the_hash_algo->hexsz ||
-		    !hex_to_bytes(binary, line.buf, line.len))
+		    !hex_to_bytes(binary, line.buf, line.len, HEX_KIND_MIXED))
 			die(_("index-pack: Expecting full hex object ID lines only from pack-objects."));
 
 		/*
diff --git a/color.c b/color.c
index 00b53f97ac..9015d0faf1 100644
--- a/color.c
+++ b/color.c
@@ -72,7 +72,7 @@ static int get_hex_color(const char **inp, int width, unsigned char *out)
 	unsigned int val;
 
 	assert(width == 1 || width == 2);
-	val = (hexval(in[0]) << 4) | hexval(in[width - 1]);
+	val = (hexval(in[0], HEX_KIND_MIXED) << 4) | hexval(in[width - 1], HEX_KIND_MIXED);
 	if (val & ~0xff)
 		return -1;
 	*inp += width;
diff --git a/diagnose.c b/diagnose.c
index 5092bf80d3..9c652d36a6 100644
--- a/diagnose.c
+++ b/diagnose.c
@@ -112,7 +112,7 @@ static void loose_objs_stats(struct strbuf *buf, const char *path)
 	while ((e = readdir_skip_dot_and_dotdot(dir)) != NULL)
 		if (get_dtype(e, &count_path, 0) == DT_DIR &&
 		    strlen(e->d_name) == 2 &&
-		    !hex_to_bytes(&c, e->d_name, 1)) {
+		    !hex_to_bytes(&c, e->d_name, 1, HEX_KIND_OID)) {
 			strbuf_setlen(&count_path, base_path_len);
 			strbuf_addf(&count_path, "%s/", e->d_name);
 			total += (count = count_files(&count_path));
diff --git a/hex-ll.c b/hex-ll.c
index 4d7ece1de5..b2e9684693 100644
--- a/hex-ll.c
+++ b/hex-ll.c
@@ -36,10 +36,45 @@ const signed char hexval_table[256] = {
 	 -1, -1, -1, -1, -1, -1, -1, -1,		/* f8-ff */
 };
 
-int hex_to_bytes(unsigned char *binary, const char *hex, size_t len)
+const signed char hexval_lc_table[256] = {
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* 00-07 */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* 08-0f */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* 10-17 */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* 18-1f */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* 20-27 */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* 28-2f */
+	  0,  1,  2,  3,  4,  5,  6,  7,		/* 30-37 */
+	  8,  9, -1, -1, -1, -1, -1, -1,		/* 38-3f */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* 40-47 */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* 48-4f */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* 50-57 */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* 58-5f */
+	 -1, 10, 11, 12, 13, 14, 15, -1,		/* 60-67 */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* 68-67 */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* 70-77 */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* 78-7f */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* 80-87 */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* 88-8f */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* 90-97 */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* 98-9f */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* a0-a7 */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* a8-af */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* b0-b7 */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* b8-bf */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* c0-c7 */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* c8-cf */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* d0-d7 */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* d8-df */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* e0-e7 */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* e8-ef */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* f0-f7 */
+	 -1, -1, -1, -1, -1, -1, -1, -1,		/* f8-ff */
+};
+
+int hex_to_bytes(unsigned char *binary, const char *hex, size_t len, enum hexkind kind)
 {
 	for (; len; len--, hex += 2) {
-		unsigned int val = (hexval(hex[0]) << 4) | hexval(hex[1]);
+		unsigned int val = (hexval(hex[0], kind) << 4) | hexval(hex[1], kind);
 
 		if (val & ~0xff)
 			return -1;
diff --git a/hex-ll.h b/hex-ll.h
index a381fa8556..2f9c8d7c25 100644
--- a/hex-ll.h
+++ b/hex-ll.h
@@ -1,20 +1,32 @@
 #ifndef HEX_LL_H
 #define HEX_LL_H
 
+enum hexkind {
+	HEX_KIND_MIXED = 0,
+	HEX_KIND_LOWER = 1,
+};
+
+#ifdef WITH_BREAKING_CHANGES
+#define HEX_KIND_OID HEX_KIND_LOWER
+#else
+#define HEX_KIND_OID HEX_KIND_MIXED
+#endif
+
 extern const signed char hexval_table[256];
-static inline unsigned int hexval(unsigned char c)
+extern const signed char hexval_lc_table[256];
+static inline unsigned int hexval(unsigned char c, enum hexkind kind)
 {
-	return hexval_table[c];
+	return kind == HEX_KIND_MIXED ? hexval_table[c] : hexval_lc_table[c];
 }
 
 /*
  * Convert two consecutive hexadecimal digits into a char.  Return a
  * negative value on error.  Don't run over the end of short strings.
  */
-static inline int hex2chr(const char *s)
+static inline int hex2chr(const char *s, enum hexkind kind)
 {
-	unsigned int val = hexval(s[0]);
-	return (val & ~0xf) ? val : (val << 4) | hexval(s[1]);
+	unsigned int val = hexval(s[0], kind);
+	return (val & ~0xf) ? val : (val << 4) | hexval(s[1], kind);
 }
 
 /*
@@ -22,6 +34,6 @@ static inline int hex2chr(const char *s)
  * values to `binary` as `len` bytes. Return 0 on success, or -1 if
  * the input does not consist of hex digits).
  */
-int hex_to_bytes(unsigned char *binary, const char *hex, size_t len);
+int hex_to_bytes(unsigned char *binary, const char *hex, size_t len, enum hexkind kind);
 
 #endif
diff --git a/hex.c b/hex.c
index f02832140d..4e1e81af3f 100644
--- a/hex.c
+++ b/hex.c
@@ -9,7 +9,7 @@ static int get_hash_hex_algop(const char *hex, unsigned char *hash,
 			      const struct git_hash_algo *algop)
 {
 	for (size_t i = 0; i < algop->rawsz; i++) {
-		int val = hex2chr(hex);
+		int val = hex2chr(hex, HEX_KIND_OID);
 		if (val < 0)
 			return -1;
 		*hash++ = val;
diff --git a/http-push.c b/http-push.c
index b8f3faaed9..43b4b61c70 100644
--- a/http-push.c
+++ b/http-push.c
@@ -1031,12 +1031,13 @@ static int get_oid_hex_from_objpath(const char *path, struct object_id *oid)
 	if (strlen(path) != the_hash_algo->hexsz + 1)
 		return -1;
 
-	if (hex_to_bytes(oid->hash, path, 1))
+	if (hex_to_bytes(oid->hash, path, 1, HEX_KIND_OID))
 		return -1;
 	path += 2;
 	path++; /* skip '/' */
 
-	return hex_to_bytes(oid->hash + 1, path, the_hash_algo->rawsz - 1);
+	return hex_to_bytes(oid->hash + 1, path, the_hash_algo->rawsz - 1,
+			    HEX_KIND_OID);
 }
 
 static void process_ls_object(struct remote_ls_ctx *ls)
diff --git a/mailinfo.c b/mailinfo.c
index 13949ff31e..85c3119048 100644
--- a/mailinfo.c
+++ b/mailinfo.c
@@ -396,7 +396,7 @@ static int decode_q_segment(struct strbuf *out, const struct strbuf *q_seg,
 			int ch, d = *in;
 			if (d == '\n' || !d)
 				break; /* drop trailing newline */
-			ch = hex2chr(in);
+			ch = hex2chr(in, HEX_KIND_MIXED);
 			if (ch >= 0) {
 				strbuf_addch(out, ch);
 				in += 2;
diff --git a/notes.c b/notes.c
index ec9c2cb150..7e9e3eb2d2 100644
--- a/notes.c
+++ b/notes.c
@@ -428,7 +428,7 @@ static void load_subtree(struct notes_tree *t, struct leaf_node *subtree,
 				goto handle_non_note;
 
 			if (hex_to_bytes(object_oid.hash + prefix_len, entry.path,
-					 hashsz - prefix_len))
+					 hashsz - prefix_len, HEX_KIND_MIXED))
 				goto handle_non_note; /* entry.path is not a SHA1 */
 
 			memset(object_oid.hash + hashsz, 0, GIT_MAX_RAWSZ - hashsz);
@@ -442,7 +442,8 @@ static void load_subtree(struct notes_tree *t, struct leaf_node *subtree,
 				/* internal nodes must be trees */
 				goto handle_non_note;
 
-			if (hex_to_bytes(object_oid.hash + len++, entry.path, 1))
+			if (hex_to_bytes(object_oid.hash + len++, entry.path, 1,
+					 HEX_KIND_OID))
 				goto handle_non_note; /* entry.path is not a SHA1 */
 
 			/*
diff --git a/object-file.c b/object-file.c
index 9123593126..d72c77cab6 100644
--- a/object-file.c
+++ b/object-file.c
@@ -1081,7 +1081,7 @@ int for_each_file_in_obj_subdir(unsigned int subdir_nr,
 		strbuf_add(path, de->d_name, namelen);
 		if (namelen == algop->hexsz - 2 &&
 		    !hex_to_bytes(oid.hash + 1, de->d_name,
-				  algop->rawsz - 1)) {
+				  algop->rawsz - 1, HEX_KIND_OID)) {
 			oid_set_algo(&oid, algop);
 			memset(oid.hash + algop->rawsz, 0,
 			       GIT_MAX_RAWSZ - algop->rawsz);
diff --git a/object-name.c b/object-name.c
index 4eda8c8eac..d6da559e0e 100644
--- a/object-name.c
+++ b/object-name.c
@@ -236,17 +236,10 @@ static int parse_oid_prefix(const char *name, int len,
 {
 	for (int i = 0; i < len; i++) {
 		unsigned char c = name[i];
-		unsigned char val;
-		if (c >= '0' && c <= '9') {
-			val = c - '0';
-		} else if (c >= 'a' && c <= 'f') {
-			val = c - 'a' + 10;
-		} else if (c >= 'A' && c <='F') {
-			val = c - 'A' + 10;
-			c -= 'A' - 'a';
-		} else {
+		int val = hexval(c, HEX_KIND_OID);
+
+		if (val < 0)
 			return -1;
-		}
 
 		if (hex_out)
 			hex_out[i] = c;
diff --git a/pkt-line.c b/pkt-line.c
index 3fc3e9ea70..338075558c 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -378,10 +378,10 @@ int packet_length(const char lenbuf_hex[4], size_t size)
 {
 	if (size < 4)
 		BUG("buffer too small");
-	return	hexval(lenbuf_hex[0]) << 12 |
-		hexval(lenbuf_hex[1]) <<  8 |
-		hexval(lenbuf_hex[2]) <<  4 |
-		hexval(lenbuf_hex[3]);
+	return	hexval(lenbuf_hex[0], HEX_KIND_MIXED) << 12 |
+		hexval(lenbuf_hex[1], HEX_KIND_MIXED) <<  8 |
+		hexval(lenbuf_hex[2], HEX_KIND_MIXED) <<  4 |
+		hexval(lenbuf_hex[3], HEX_KIND_MIXED);
 }
 
 static const char *find_packfile_uri_path(const char *buffer)
diff --git a/ref-filter.c b/ref-filter.c
index bdf54f6f59..cd02677e42 100644
--- a/ref-filter.c
+++ b/ref-filter.c
@@ -3636,7 +3636,7 @@ static void append_literal(const char *cp, const char *ep, struct ref_formatting
 			if (cp[1] == '%')
 				cp++;
 			else {
-				int ch = hex2chr(cp + 1);
+				int ch = hex2chr(cp + 1, HEX_KIND_MIXED);
 				if (0 <= ch) {
 					strbuf_addch(s, ch);
 					cp += 3;
diff --git a/strbuf.c b/strbuf.c
index 44955669e8..88d23f8ac5 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -457,7 +457,7 @@ size_t strbuf_expand_literal(struct strbuf *sb, const char *placeholder)
 		return 1;
 	case 'x':
 		/* %x00 == NUL, %x0a == LF, etc. */
-		ch = hex2chr(placeholder + 1);
+		ch = hex2chr(placeholder + 1, HEX_KIND_MIXED);
 		if (ch < 0)
 			return 0;
 		strbuf_addch(sb, ch);
diff --git a/t/t1503-rev-parse-verify.sh b/t/t1503-rev-parse-verify.sh
index 87638a4a2c..f07b45de5a 100755
--- a/t/t1503-rev-parse-verify.sh
+++ b/t/t1503-rev-parse-verify.sh
@@ -60,6 +60,11 @@ test_expect_success 'works with one good rev' '
 	test "$rev_head" = "$HASH4"
 '
 
+test_expect_success WITH_BREAKING_CHANGES 'rejects uppercase revs' '
+	UC_HASH=$(echo "$HASH1" | tr a-f A-F) &&
+	test_must_fail git rev-parse --verify "$UC_HASH"
+'
+
 test_expect_success 'fails with any bad rev or many good revs' '
 	test_must_fail git rev-parse --verify 2>error &&
 	test_grep "single revision" error &&
diff --git a/t/t5324-split-commit-graph.sh b/t/t5324-split-commit-graph.sh
index bf7ba0e558..29db815c77 100755
--- a/t/t5324-split-commit-graph.sh
+++ b/t/t5324-split-commit-graph.sh
@@ -349,7 +349,7 @@ test_expect_success 'verify after commit-graph-chain corruption (base)' '
 		test_must_fail git commit-graph verify 2>test_err &&
 		grep -v "^+" test_err >err &&
 		test_grep "invalid commit-graph chain" err &&
-		corrupt_file "$graphdir/commit-graph-chain" 30 "A" &&
+		corrupt_file "$graphdir/commit-graph-chain" 30 "a" &&
 		test_must_fail git commit-graph verify 2>test_err &&
 		grep -v "^+" test_err >err &&
 		test_grep "unable to find all commit-graph files" err
@@ -364,7 +364,7 @@ test_expect_success 'verify after commit-graph-chain corruption (tip)' '
 		test_must_fail git commit-graph verify 2>test_err &&
 		grep -v "^+" test_err >err &&
 		test_grep "invalid commit-graph chain" err &&
-		corrupt_file "$graphdir/commit-graph-chain" 70 "A" &&
+		corrupt_file "$graphdir/commit-graph-chain" 70 "b" &&
 		test_must_fail git commit-graph verify 2>test_err &&
 		grep -v "^+" test_err >err &&
 		test_grep "unable to find all commit-graph files" err
diff --git a/url.c b/url.c
index a59818278f..b4d72f784a 100644
--- a/url.c
+++ b/url.c
@@ -62,7 +62,7 @@ static char *url_decode_internal(const char **query, int len,
 		}
 
 		if (c == '%' && (len < 0 || len >= 3)) {
-			int val = hex2chr(q + 1);
+			int val = hex2chr(q + 1, HEX_KIND_MIXED);
 			if (0 < val) {
 				strbuf_addch(out, val);
 				q += 3;
diff --git a/urlmatch.c b/urlmatch.c
index 20bc2d009c..989f1d794b 100644
--- a/urlmatch.c
+++ b/urlmatch.c
@@ -50,7 +50,7 @@ static int append_normalized_escapes(struct strbuf *buf,
 		if (ch == '%') {
 			if (from_len < 2)
 				return 0;
-			ch = hex2chr(from);
+			ch = hex2chr(from, HEX_KIND_MIXED);
 			if (ch < 0)
 				return 0;
 			from += 2;