CVE-2020-36049: Allocation of Resources Without Limits or Throttling (CWE-770)
value: socket.io-parser before 3.4.1 allows attackers to cause a denial of service (memory consumption) via a large packet because a concatenation approach is used.
CWE description: The product allocates a reusable resource or group of resources on behalf of an actor without imposing any restrictions on the size or number of resources that can be allocated, in violation of the intended security policy for that actor.
CVSS v3 base score: 7.5
Published: 2021-01-08T00:15Z
Repository: https://github.com/socketio/socket.io-parser
Language: JavaScript
Files changed: index.js
Commit date: 2020-05-13 07:37:32 +0200
Commit message: fix: prevent DoS (OOM) via massive packets (#95)

When maxHttpBufferSize is large (1e8 bytes), a payload of length 100MB
can be sent like so:

99999991:422222222222222222222222222222222222222222222...

This massive packet can cause OOM via building up many many
`ConsOneByteString` objects due to concatenation:
99999989 `ConsOneByteString`s and then converting the massive integer to
a `Number`.

The performance can be improved to avoid this by using `substring`
rather than building the string via concatenation.

Below I tried one payload of length 7e7 as the 1e8 payload took so
long to process that it timed out before running out of memory.

```
==== JS stack trace =========================================

    0: ExitFrame [pc: 0x13c5b79]
Security context: 0x152fe7b808d1 <JSObject>
    1: decodeString [0x2dd385fb5d1] [/node_modules/socket.io-parser/index.js:~276] [pc=0xf59746881be](this=0x175d34c42b69 <JSGlobal Object>,0x14eccff10fe1 <Very long string[69999990]>)
    2: add [0x31fc2693da29] [/node_modules/socket.io-parser/index.js:242] [bytecode=0xa7ed6554889 offset=11](this=0x0a2881be5069 <Decoder map = 0x3ceaa8bf48c9>,0x14eccff10fe1 <Very...

FATAL ERROR: Ineffective mark-compacts near heap limit Allocation failed - JavaScript heap out of memory
 1: 0xa09830 node::Abort() [node]
 2: 0xa09c55 node::OnFatalError(char const*, char const*) [node]
 3: 0xb7d71e v8::Utils::ReportOOMFailure(v8::internal::Isolate*, char const*, bool) [node]
 4: 0xb7da99 v8::internal::V8::FatalProcessOutOfMemory(v8::internal::Isolate*, char const*, bool) [node]
 5: 0xd2a1f5  [node]
 6: 0xd2a886 v8::internal::Heap::RecomputeLimits(v8::internal::GarbageCollector) [node]
 7: 0xd37105 v8::internal::Heap::PerformGarbageCollection(v8::internal::GarbageCollector, v8::GCCallbackFlags) [node]
 8: 0xd37fb5 v8::internal::Heap::CollectGarbage(v8::internal::AllocationSpace, v8::internal::GarbageCollectionReason, v8::GCCallbackFlags) [node]
 9: 0xd3965f v8::internal::Heap::HandleGCRequest() [node]
10: 0xce8395 v8::internal::StackGuard::HandleInterrupts() [node]
11: 0x1042cb6 v8::internal::Runtime_StackGuard(int, unsigned long*, v8::internal::Isolate*) [node]
12: 0x13c5b79  [node]
```

Diff: diff --git a/index.js b/index.js
index 102615a..ff613cc 100644
--- a/index.js
+++ b/index.js
@@ -284,48 +284,45 @@ function decodeString(str) {
     return error('unknown packet type ' + p.type);
   }
 
   // look up attachments if type binary
   if (exports.BINARY_EVENT === p.type || exports.BINARY_ACK === p.type) {
-    var buf = '';
-    while (str.charAt(++i) !== '-') {
-      buf += str.charAt(i);
-      if (i == str.length) break;
-    }
+    var start = i + 1;
+    while (str.charAt(++i) !== '-' && i != str.length) {}
+    var buf = str.substring(start, i);
     if (buf != Number(buf) || str.charAt(i) !== '-') {
       throw new Error('Illegal attachments');
     }
     p.attachments = Number(buf);
   }
 
   // look up namespace (if any)
   if ('/' === str.charAt(i + 1)) {
-    p.nsp = '';
+    var start = i + 1;
     while (++i) {
       var c = str.charAt(i);
       if (',' === c) break;
-      p.nsp += c;
       if (i === str.length) break;
     }
+    p.nsp = str.substring(start, i);
   } else {
     p.nsp = '/';
   }
 
   // look up id
   var next = str.charAt(i + 1);
   if ('' !== next && Number(next) == next) {
-    p.id = '';
+    var start = i + 1;
     while (++i) {
       var c = str.charAt(i);
       if (null == c || Number(c) != c) {
         --i;
         break;
       }
-      p.id += str.charAt(i);
       if (i === str.length) break;
     }
-    p.id = Number(p.id);
+    p.id = Number(str.substring(start, i + 1));
   }
 
   // look up json data
   if (str.charAt(++i)) {
     var payload = tryParse(str.substr(i));

Vulnerable code:     var buf = '';
    while (str.charAt(++i) !== '-') {
      buf += str.charAt(i);
      if (i == str.length) break;
    }
    p.nsp = '';
      p.nsp += c;
    p.id = '';
      p.id += str.charAt(i);
    p.id = Number(p.id);
Fixed code:     var start = i + 1;
    while (str.charAt(++i) !== '-' && i != str.length) {}
    var buf = str.substring(start, i);
    var start = i + 1;
    p.nsp = str.substring(start, i);
    var start = i + 1;
    p.id = Number(str.substring(start, i + 1));