CVE-2026-55391: CVE-2026-55391: Server-Side Request Forgery Bypass via DNS Rebinding in datamodel-code-generator

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CVE-2026-55391: Server-Side Request Forgery Bypass via DNS Rebinding in datamodel-code-generator

Vulnerability ID: CVE-2026-55391
CVSS Score: 7.5
Published: 2026-07-28

CVE-2026-55391 is a server-side request forgery (SSRF) bypass vulnerability in the datamodel-code-generator package. The vulnerability occurs due to a Time-of-Check to Time-of-Use (TOCTOU) race condition during DNS resolution, combined with a failure to inspect embedded IPv4-in-IPv6 address mappings. By deploying a malicious DNS server with a low Time-To-Live (TTL) configuration, an attacker can bypass private address blocklists and coerce the application to connect to internal services or local endpoints.

TL;DR

A Time-of-Check to Time-of-Use (TOCTOU) DNS vulnerability in datamodel-code-generator allows remote attackers to bypass SSRF protections using DNS rebinding or IPv6-mapped IPv4 addresses, exposing internal networks.

⚠️ Exploit Status: POC

Technical Details

CWE ID: CWE-918 (SSRF), CWE-367 (TOCTOU)
Attack Vector: Network
Attack Complexity: High
CVSS v3.1 Score: 7.5
Exploit Status: Proof-of-Concept
CISA KEV Status: Not Listed

Affected Systems

datamodel-code-generator python package
datamodel-code-generator: >= 0.50.0, < 0.63.0 (Fixed in: 0.63.0)

Code Analysis

Commit: 25c8b7e

Implement socket-level DNS pinning and embedded IPv4 unwrapping in fetch modules.

--- a/src/datamodel_code_generator/http.py
+++ b/src/datamodel_code_generator/http.py
@@ -8,24 +8,119 @@
-def _get_httpx() -> Any:
+def _get_httpx() -> _HTTPXModule:
+class _PinnedNetworkBackend:
+    def connect_tcp(...) -> httpcore.NetworkStream:
+        if _normalize_dns_host(host) != self._pinned_host:
+            raise OSError(...)
+        for ip in self._pinned_ips:
+            return self._backend.connect_tcp(str(ip), ...)

Mitigation Strategies

Implement DNS pinning at the socket or transport layer to prevent independent DNS resolution passes.
Unwrap IPv6-encapsulated IPv4 configurations to detect hidden private addresses.
Use isolated container networks with egress rules to block private IP space connections.

Remediation Steps:

Update the datamodel-code-generator dependency to version 0.63.0 or greater.
Ensure validation protocols explicitly block legacy IP notations and IPv6 mapping formats.
Configure cloud host firewalls to restrict local loopback and metadata access from worker environments.