CRITICAL - DNS Rebinding (TOCTOU Race Condition)

[harshal-paltse]

Title: TOCTOU Race Condition in SSRF Protection (DNS Rebinding Attack)
Severity: CRITICAL
Component: src/extension_shield/utils/http_safety.py (Lines 137-182)
Impact: Supply chain compromise in cloud deployments

Problem
The safe_get() function validates DNS resolution once, then requests.get() performs an independent second DNS resolution. An attacker can serve a public IP during validation and a private/cloud-metadata IP during the actual request.

Python

❌ VULNERABLE CODE:

def safe_get(url, allowed_hosts, ...):
# First DNS resolution + validation
validate_outbound_url(url, allowed_hosts) # ✓ Resolves to 1.2.3.4

# Second independent DNS resolution (VULNERABLE)
response = requests.get(url, ...)  # ❌ Can resolve to 169.254.169.254!

Attack Scenario
Code

1. Attacker controls attacker.com DNS
2. First check: attacker.com → 1.2.3.4 (public IP) ✓ passes
3. Request sent: attacker.com → 169.254.169.254 (AWS metadata)
4. ExtensionShield gets: EC2 instance credentials, IAM tokens, etc.

[Stanzin7]

@harshal-paltse this is not a valid critical bug in the current code path. While safe_get() does validate first and then call requests.get(), it is only used with a small set of trusted allowlisted hosts such as Google and chrome-stats domains. The report’s attack example depends on attacker control of an allowed host’s DNS, which is not how this code is used. If you want to keep this issue open, it should be reframed as a lower-severity hardening improvement around DNS pinning / redirect revalidation, not a critical SSRF exploit.