GHSA-5qpg-rh4j-qp35: Pycares Pycares Has A Use-After-Free Vulnerability Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-5qpg-rh4j-qp35",
  "modified" : "2025-06-16T16:09:47Z",
  "published" : "2025-06-16T16:09:47Z",
  "aliases" : [ ],
  "summary" : "pycares has a Use-After-Free Vulnerability",
  "details" : "## Summary\n\npycares is vulnerable to a use-after-free condition that occurs when a Channel object is garbage collected while DNS queries are still pending. This results in a fatal Python error and interpreter crash.\n\n## Details\n\n### Root Cause\n\nThe vulnerability stems from improper handling of callback references when the Channel object is destroyed:\n\n1. When a DNS query is initiated, pycares stores a callback reference using `ffi.new_handle()`\n2. If the Channel object is garbage collected while queries are pending, the callback references become invalid\n3. When c-ares attempts to invoke the callback, it accesses freed memory, causing a fatal error\n\nThis issue was much more likely to occur when using `event_thread=True` but could happen without it under the right circumstances.\n\n### Technical Details\n\nThe core issue is a race condition between Python's garbage collector and c-ares's callback execution:\n\n1. When `__del__` is called from within a c-ares callback context, we cannot immediately call `ares_destroy()` because c-ares is still executing code after the callback returns\n2. c-ares needs to execute cleanup code after our Python callback returns (specifically at lines 1422-1429 in ares_process.c)\n3. If we destroy the channel too quickly, c-ares accesses freed memory\n\n### Impact\n\nApplications using `pycares` can be crashed remotely by triggering DNS queries that result in `Channel` objects being garbage collected before query completion. This is particularly problematic in scenarios where:\n\n- Channel objects are created per-request\n- Multiple failed DNS queries are processed rapidly\n- The application doesn't properly manage Channel lifecycle\n\nThe error manifests as:\n```\nFatal Python error: b_from_handle: ffi.from_handle() detected that the address passed points to garbage\n```\n\n## Fix\n\nThe vulnerability has been fixed in pycares 4.9.0 by implementing a safe channel destruction mechanism\n\n## Mitigation\n\n### For Application Developers\n\n1. **Upgrade to pycares >= 4.9.0** - This version includes the fix and requires no code changes\n2. **Best practices** (optional but recommended):\n   ```python\n   # Explicit cleanup\n   channel.close()\n   \n   # Or use context manager\n   with pycares.Channel() as channel:\n       # ... use channel ...\n   # Automatically closed\n   ```\n3. **Avoid creating Channel objects per-request** - Prefer long-lived instances for better performance and safety\n\nThe fix is completely transparent - no API changes or code modifications are required.\n\n## Credit\n\nThis vulnerability was reported by @vEpiphyte through the aio-libs security program.",
  "severity" : [ ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "pycares"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "4.9.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 4.8.0"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/saghul/pycares/security/advisories/GHSA-5qpg-rh4j-qp35"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/saghul/pycares/commit/ebfd7d71eb8e74bc1057a361ea79a5906db510d4"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/saghul/pycares"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-416" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2025-06-16T16:09:47Z",
    "nvd_published_at" : null
  }
}