diff --git a/absl/log/CMakeLists.txt b/absl/log/CMakeLists.txt
index e0a307b3aff..f53a9ee6995 100644
--- a/absl/log/CMakeLists.txt
+++ b/absl/log/CMakeLists.txt
@@ -182,6 +182,22 @@ absl_cc_library(
     absl::span
 )
 
+absl_cc_test(
+  NAME
+    log_internal_proto_test
+  SRCS
+    "internal/proto_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::log_internal_proto
+    absl::span
+    GTest::gmock_main
+)
+
 absl_cc_library(
   NAME
     log_internal_message
diff --git a/absl/log/internal/BUILD.bazel b/absl/log/internal/BUILD.bazel
index 83c5eab5baf..732a145c5f5 100644
--- a/absl/log/internal/BUILD.bazel
+++ b/absl/log/internal/BUILD.bazel
@@ -446,6 +446,18 @@ cc_library(
     ],
 )
 
+cc_test(
+    name = "proto_test",
+    srcs = ["proto_test.cc"],
+    deps = [
+        ":proto",
+        "//absl/base:config",
+        "//absl/types:span",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
 cc_library(
     name = "fnmatch",
     srcs = ["fnmatch.cc"],
diff --git a/absl/log/internal/proto.cc b/absl/log/internal/proto.cc
index d0c57d69900..689c888aa12 100644
--- a/absl/log/internal/proto.cc
+++ b/absl/log/internal/proto.cc
@@ -163,12 +163,17 @@ void EncodeMessageLength(absl::Span<char> msg, const absl::Span<char> *buf) {
 
 namespace {
 uint64_t DecodeVarint(absl::Span<const char> *buf) {
+  // A base-128 varint for a 64-bit value occupies at most 10 bytes.  Stop after
+  // that many so the shift below can never reach the width of `value`, which
+  // would be undefined behavior on a malformed or over-long input.  The 64-bit
+  // and 32-bit decoders below cap their loops for the same reason.
+  constexpr size_t kMaxVarintBytes = 10;
   uint64_t value = 0;
   size_t s = 0;
   while (s < buf->size()) {
     value |= static_cast<uint64_t>(static_cast<unsigned char>((*buf)[s]) & 0x7f)
              << 7 * s;
-    if (!((*buf)[s++] & 0x80)) break;
+    if (!((*buf)[s++] & 0x80) || s == kMaxVarintBytes) break;
   }
   buf->remove_prefix(s);
   return value;
diff --git a/absl/log/internal/proto_test.cc b/absl/log/internal/proto_test.cc
new file mode 100644
index 00000000000..59dbb48cc53
--- /dev/null
+++ b/absl/log/internal/proto_test.cc
@@ -0,0 +1,68 @@
+//
+// Copyright 2024 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/log/internal/proto.h"
+
+#include <cstddef>
+#include <cstdint>
+#include <vector>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+namespace {
+
+TEST(ProtoTest, DecodeVarintRoundTrips) {
+  std::vector<char> buf(BufferSizeFor(1, WireType::kVarint));
+  absl::Span<char> encode_span(buf);
+  ASSERT_TRUE(EncodeVarint(1, uint64_t{300}, &encode_span));
+
+  absl::Span<const char> decode_span(buf.data(),
+                                     buf.size() - encode_span.size());
+  ProtoField field;
+  ASSERT_TRUE(field.DecodeFrom(&decode_span));
+  EXPECT_EQ(field.tag(), 1);
+  EXPECT_EQ(field.type(), WireType::kVarint);
+  EXPECT_EQ(field.uint64_value(), 300);
+  EXPECT_TRUE(decode_span.empty());
+}
+
+// A varint whose bytes all set the continuation bit is malformed, but the
+// decoder is meant to tolerate arbitrary input.  Before the length cap this
+// drove the shift exponent past the width of the accumulator (`<< 70`), which
+// is undefined behavior that UBSan flags.  The decoder must stop after the
+// 10-byte maximum of a 64-bit varint instead.
+TEST(ProtoTest, DecodeOverlongVarintDoesNotOverflowShift) {
+  std::vector<char> buf;
+  buf.push_back(static_cast<char>(MakeTagType(1, WireType::kVarint)));
+  for (int i = 0; i < 11; ++i) buf.push_back(static_cast<char>(0x80));
+  buf.push_back(static_cast<char>(0x01));
+
+  absl::Span<const char> decode_span(buf);
+  ProtoField field;
+  ASSERT_TRUE(field.DecodeFrom(&decode_span));
+  EXPECT_EQ(field.type(), WireType::kVarint);
+  // At most the 10 varint bytes are consumed, leaving the trailing bytes.
+  EXPECT_FALSE(decode_span.empty());
+}
+
+}  // namespace
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl