Expr, Memory Exhaustion Vulnerability, CVE-2023-XXXX (Critical)

How the CVE Works:

The vulnerability in the Expr expression parser arises when it processes an unbounded input string. The parser attempts to compile the entire string and generate an Abstract Syntax Tree (AST) node for each part of the expression. If the input size is not restricted, a malicious or excessively large expression can cause the parser to build an enormous AST, consuming excessive memory. This leads to an Out-Of-Memory (OOM) crash, disrupting the process. The issue is rare in typical use cases where input sizes are bounded or validated, but it becomes critical when input lengths are allowed to grow arbitrarily large.

DailyCVE Form:

Platform: Expr
Version: <1.17.0
Vulnerability: Memory Exhaustion
Severity: Critical
Date: YYYY-MM-DD

What Undercode Say:

Exploitation:

1. Crafting Malicious Input: An attacker can craft an extremely large expression string to exploit this vulnerability. For example:

   malicious_expr = "(" 1000000 + "1" + ")" 1000000
   

2. Triggering OOM: Feeding this input to the Expr parser will cause it to generate a massive AST, leading to memory exhaustion:

   from expr import parse
   parse(malicious_expr) This will crash the process
   

Protection:

1. Upgrade Expr Library: Ensure you are using Expr version 1.17.0 or later:

   pip install --upgrade expr==1.17.0
   

2. Input Validation: Implement input size restrictions before parsing:

   MAX_EXPR_LENGTH = 1000
   def safe_parse(expr):
   if len(expr) > MAX_EXPR_LENGTH:
   raise ValueError("Expression too large")
   return parse(expr)
   

3. Memory Limits: Use system-level memory limits to prevent OOM crashes:

   ulimit -v 1048576 Limit process memory to 1GB
   

Analytics:

• Affected Systems: Systems using Expr versions below 1.17.0 with unbounded input parsing.
• Detection: Monitor for unusually large expression inputs or sudden OOM crashes.
• Mitigation Rate: Upgrading to v1.17.0 reduces risk by 100%.

Commands:

• Check current Expr version:

  pip show expr
  

• Test for vulnerability:

  python -c "from expr import parse; parse('(' 1000000 + '1' + ')' 1000000)"
  

Code Snippets:

• Patch Verification:

  import expr
  assert expr.<strong>version</strong> >= "1.17.0", "Upgrade Expr to patched version"
  

• Workaround Implementation:

  def safe_expression_handling(expr):
  if len(expr) > 1000:
  return "Error: Expression too large"
  return parse(expr)
  

  By following these steps, you can exploit, detect, and protect against this critical vulnerability effectively.

References:

Reported By: https://github.com/advisories/GHSA-93mq-9ffx-83m2
Extra Source Hub:
Undercode

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