diff --git a/clang-tools-extra/clangd/index/Serialization.cpp b/clang-tools-extra/clangd/index/Serialization.cpp
--- a/clang-tools-extra/clangd/index/Serialization.cpp
+++ b/clang-tools-extra/clangd/index/Serialization.cpp
@@ -21,6 +21,7 @@
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/raw_ostream.h"
+#include <cstdint>
 #include <vector>
 
 namespace clang {
@@ -81,12 +82,17 @@
 
   uint32_t consumeVar() {
     constexpr static uint8_t More = 1 << 7;
-    uint8_t B = consume8();
+
+    // Use a 32 bit unsigned here to prevent promotion to signed int (unless int
+    // is wider than 32 bits).
+    uint32_t B = consume8();
     if (LLVM_LIKELY(!(B & More)))
       return B;
     uint32_t Val = B & ~More;
     for (int Shift = 7; B & More && Shift < 32; Shift += 7) {
       B = consume8();
+      // 5th byte of a varint can only have lowest 4 bits set.
+      assert((Shift != 28 || B == (B & 0x0f)) && "Invalid varint encoding");
       Val |= (B & ~More) << Shift;
     }
     return Val;
diff --git a/clang-tools-extra/clangd/unittests/SerializationTests.cpp b/clang-tools-extra/clangd/unittests/SerializationTests.cpp
--- a/clang-tools-extra/clangd/unittests/SerializationTests.cpp
+++ b/clang-tools-extra/clangd/unittests/SerializationTests.cpp
@@ -366,9 +366,9 @@
   Pos += FileDigest.size();
 
   // Varints are little-endian base-128 numbers, where the top-bit of each byte
-  // indicates whether there are more. 8fffffff7f -> 0xffffffff.
+  // indicates whether there are more. ffffffff0f -> 0xffffffff.
   std::string CorruptSrcs =
-      (Srcs->Data.take_front(Pos) + llvm::fromHex("8fffffff7f") +
+      (Srcs->Data.take_front(Pos) + llvm::fromHex("ffffffff0f") +
        "some_random_garbage")
           .str();
   Srcs->Data = CorruptSrcs;