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How CVE-2026-69248 Works
The vulnerability resides in the X.509 certificate chain verifier of the `python-cryptography` library. Under RFC 5280 §4.2.1.10, an intermediate Certificate Authority (CA) can impose Name Constraints to limit the DNS names for which it is authorized to issue certificates. For example, a constrained sub-CA may have a `permittedSubtrees` entry of dNSName = foo.example.com. This explicitly restricts the sub-CA to issuing certificates only for that exact domain.
The flaw, however, lies in DNSConstraint::matches—the core function responsible for evaluating whether a certificate’s DNS Subject Alternative Name (SAN) complies with a given name constraint. In versions of `cryptography` prior to 49.0.0, this function treated a wildcard pattern as matching a more‑specific permitted constraint. Specifically, if a leaf certificate contained the wildcard SAN .example.com, the verifier incorrectly considered this acceptable under a constraint that only permitted foo.example.com.
The logic error stems from reusing the same wildcard matching code for both `permittedSubtrees` and `excludedSubtrees` without distinguishing the semantic requirements of each. For permittedSubtrees, a wildcard like `.example.com` can expand to sibling domains such as `bar.example.com` or baz.example.com—names that were never explicitly permitted and lie outside the constrained scope. Yet `DNSConstraint::matches` approved the chain, allowing a leaf certificate signed by the restricted sub-CA to be validated for bar.example.com.
This bypass is triggered when a user builds a server verifier with `PolicyBuilder().build_server_verifier(DNSName(“bar.example.com”))` and calls verify(leaf,
)</code>. The verifier mistakenly trusts the leaf as authoritative for the target domain, even though the sub‑CA was never authorized to issue certificates for that name. The vulnerability thus enables an attacker to escape the scope of permitted names defined by an intermediate CA, effectively undermining the trust boundary established by Name Constraints. The issue is classified under CWE-295: Improper Certificate Validation and carries a CVSS v4.0 base score of 6.9 (Medium). <h2 style="color: blue;">DailyCVE Form:</h2> Platform: Python cryptography Version: < 49.0.0 Vulnerability: Name constraint bypass Severity: Medium (CVSS 6.9) Date: 2026-08-03 <h2 style="color: blue;">Prediction: 2026-08-04 (patch available)</h2> <h2 style="color: blue;">What Undercode Say (Analytics)</h2> The vulnerability was introduced in the Rust-based X.509 verification module (<code>src/rust/cryptography-x509-verification/src/lib.rs</code>). The fix was merged via commit `4d035a4225965edeffd312079a510ef25fcfdcb2` (PR 14888). <h2 style="color: blue;">To check your current version:</h2> [bash] pip show cryptography | grep Version
To upgrade to the patched version:
pip install --upgrade cryptography>=49.0.0
To verify a certificate chain against a specific peer name using the fixed verifier (Python):
from cryptography.x509.verification import PolicyBuilder, Store
from cryptography import x509
import datetime
Build verifier for target domain
verifier = (
PolicyBuilder()
.store(Store([bash]))
.time(datetime.datetime.now(tz=datetime.timezone.utc))
.build_server_verifier(x509.DNSName("bar.example.com"))
)
This should raise VerificationError if the chain is invalid
verifier.verify(leaf_cert, [bash])
Exploit
An attacker must obtain or forge a leaf certificate with a wildcard SAN (.example.com) signed by an intermediate CA that is itself constrained to a more specific name (foo.example.com). When this chain is presented to a service using the vulnerable `python-cryptography` verifier, the service will accept the certificate as valid for any sibling domain (e.g., bar.example.com), despite the sub‑CA lacking authorization. The Proof of Concept (PoC) provided in the advisory demonstrates this by building a full certificate chain and invoking `build_server_verifier` for bar.example.com—which the vulnerable verifier accepts.
Protection
- Upgrade `cryptography` to version 49.0.0 or later.
- If upgrading is not immediately possible, audit all certificate chains to ensure no leaf certificate contains a wildcard broader than the constraints of its issuing CA.
- Review wildcard certificate usage and consider replacing broad wildcards with specific SAN entries where feasible.
- Monitor for any unexpected certificate acceptances in your environment and revoke any suspicious certificates.
Impact
Successful exploitation allows an attacker to bypass DNS name constraints enforced by an intermediate CA. This can lead to:
- Unauthorized access to services that rely on certificate‑based authentication and name constraints.
- Man‑in‑the‑middle (MITM) attacks, as a malicious certificate chain could be accepted for domains outside the intended scope.
- Undermining of the certificate trust model, as the sub‑CA's scope escape violates the security boundaries established by the CA infrastructure.
- Potential credential exposure (aligned with ATT&CK technique T1552.002) if compromised certificates are used to gain access to protected systems.
Organizations using `python-cryptography` prior to `49.0.0` should treat this as a high‑priority update, as the flaw enables a clear violation of certificate chain validation rules.
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Sources:
Reported By: github.com
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