diff --git a/src/common/utils/leb128.spec.ts b/src/common/utils/leb128.spec.ts
index 869e59e9c..c4d02142a 100644
--- a/src/common/utils/leb128.spec.ts
+++ b/src/common/utils/leb128.spec.ts
@@ -235,13 +235,35 @@ describe('leb128', () => {
         expect(() => decodeUInt32(data)).toThrow('Truncated LEB128 encoding');
       });
 
-      test('throws for encoding that exceeds uint32 range', () => {
+      test('throws for encoding longer than 5 bytes', () => {
         // 6 bytes with continuation bits (should never happen for uint32)
         const data = new Uint8Array([0x80, 0x80, 0x80, 0x80, 0x80, 0x01]);
         expect(() => decodeUInt32(data)).toThrow(
           'LEB128 sequence exceeds maximum length for uint32',
         );
       });
+
+      test('throws for a 5-byte encoding whose final byte overflows uint32', () => {
+        // 5 bytes, but the final byte carries data bits above bit 31
+        // (0x10 -> 2^32). Without a range check the 32-bit shift silently drops
+        // the overflow and returns 0 instead of rejecting the value.
+        const data = new Uint8Array([0x80, 0x80, 0x80, 0x80, 0x10]);
+        expect(() => decodeUInt32(data)).toThrow(
+          'LEB128 sequence exceeds uint32 range',
+        );
+      });
+
+      test('throws when the final byte uses all 7 data bits', () => {
+        const data = new Uint8Array([0xff, 0xff, 0xff, 0xff, 0x7f]);
+        expect(() => decodeUInt32(data)).toThrow(
+          'LEB128 sequence exceeds uint32 range',
+        );
+      });
+
+      test('accepts MAX_UINT32 whose final byte is exactly 0x0f', () => {
+        const data = new Uint8Array([0xff, 0xff, 0xff, 0xff, 0x0f]);
+        expect(decodeUInt32(data).value).toBe(4294967295);
+      });
     });
   });
 
diff --git a/src/common/utils/leb128.ts b/src/common/utils/leb128.ts
index 8f6978fa8..928b9e770 100644
--- a/src/common/utils/leb128.ts
+++ b/src/common/utils/leb128.ts
@@ -9,6 +9,11 @@ const CONTINUATION_BIT = 0x80;
 const DATA_BITS_MASK = 0x7f;
 const DATA_BITS_PER_BYTE = 7;
 const MAX_BYTES_FOR_UINT32 = 5;
+// The 5th byte is read at this shift; a uint32 only has 32 - 28 = 4 data bits
+// left for it, so its 7-bit payload must not exceed 0x0F. A larger payload
+// encodes a value above MAX_UINT32.
+const FINAL_BYTE_SHIFT = (MAX_BYTES_FOR_UINT32 - 1) * DATA_BITS_PER_BYTE;
+const FINAL_BYTE_MAX_DATA = (1 << (32 - FINAL_BYTE_SHIFT)) - 1;
 
 /**
  * Encodes an unsigned 32-bit integer into LEB128 format.
@@ -65,6 +70,16 @@ export function decodeUInt32(
       throw new Error('LEB128 sequence exceeds maximum length for uint32');
     }
 
+    // On the final (5th) byte only 4 of its 7 data bits fit in a uint32. Reject a
+    // larger payload instead of letting the 32-bit `<<` below silently drop the
+    // overflowing bits and return a wrong value for an out-of-range encoding.
+    if (
+      shift === FINAL_BYTE_SHIFT &&
+      (byte & DATA_BITS_MASK) > FINAL_BYTE_MAX_DATA
+    ) {
+      throw new Error('LEB128 sequence exceeds uint32 range');
+    }
+
     result |= (byte & DATA_BITS_MASK) << shift;
 
     if (!hasContinuationBit(byte)) {
diff --git a/src/vault/vault.spec.ts b/src/vault/vault.spec.ts
index 8903e7200..c2a52b829 100644
--- a/src/vault/vault.spec.ts
+++ b/src/vault/vault.spec.ts
@@ -274,6 +274,24 @@ describe('Vault', () => {
       expect(fetchMethod()).toBe('POST');
       expect(decrypted).toBe(originalText);
     });
+
+    it('rejects a payload whose key-length prefix overflows uint32', async () => {
+      // 12-byte IV + 16-byte tag as filler, then a 5-byte LEB128 length whose
+      // final byte (0x10) encodes 2^32, which is out of range for a uint32.
+      const payload = new Uint8Array([
+        ...new Array(28).fill(0),
+        0x80,
+        0x80,
+        0x80,
+        0x80,
+        0x10,
+      ]);
+      const encoded = Buffer.from(payload).toString('base64');
+
+      await expect(workos.vault.decrypt(encoded)).rejects.toThrow(
+        'LEB128 sequence exceeds uint32 range',
+      );
+    });
   });
   // @oagen-ignore-end
 });