SSL options ISSUERCERT, EC_CURVES and CRLFILE silently ignored by non-OpenSSL backends

How it was found

Summary:

The SSL options ISSUERCERT, EC_CURVES and CRLFILE are silently ignored for e.g. the mbedTLS backend, which allows MITM attacks for the ISSUERCERT and CRLFILE bug, and can reduce the security and compliance by ignoring the specified curve for the EC_CURVES bug.

Affected version

Tested with latest master version, commit 2d5a063121202acaa23bb77975b2739bec4551ce, and mbedTLS 3.6 and 4.0.

Steps To Reproduce:

Install Docker on a Linux system, and run the test.sh script. It compiles 2 versions of curl: with mbedTLS, and with OpenSSL. It then tests all 3 bugs with a custom C program which is needed for the bug, and in addition the EC_CURVES bug from the command line. Expected output:

=== mbedTLS ===
===========================================
curl 8.19.0-DEV (mbedTLS/4.0.0)
===========================================

--- CURLOPT_ISSUERCERT ---
  Setting: "fake_issuer.pem"
  setopt: No error
  perform: No error
  Result: VULNERABLE - silently ignored!

--- CURLOPT_SSL_EC_CURVES ---
  Setting: "INVALID_CURVE_NAME"
  setopt: No error
  perform: No error
  Result: VULNERABLE - silently ignored!

--- CURLOPT_CRLFILE ---
  Setting: "fake_crl.pem"
  setopt: No error
  perform: Failed to load CRL file (path? access rights?, format?)
  Result: CORRECT - CRL check enforced

===========================================
SUMMARY
===========================================
  CURLOPT_ISSUERCERT        VULNERABLE
  CURLOPT_SSL_EC_CURVES     VULNERABLE
  CURLOPT_CRLFILE           OK

FOUND 2 VULNERABILITIES

=== OpenSSL ===
===========================================
curl 8.19.0-DEV (OpenSSL/3.0.13)
===========================================

--- CURLOPT_ISSUERCERT ---
  Setting: "fake_issuer.pem"
  setopt: No error
  perform: Issuer check against peer certificate failed
  Result: CORRECT - issuer check enforced

--- CURLOPT_SSL_EC_CURVES ---
  Setting: "INVALID_CURVE_NAME"
  setopt: No error
  perform: Could not use specified SSL cipher
  Result: CORRECT - EC curve validated

--- CURLOPT_CRLFILE ---
  Setting: "fake_crl.pem"
  setopt: No error
  perform: Failed to load CRL file (path? access rights?, format?)
  Result: CORRECT - CRL check enforced

===========================================
SUMMARY
===========================================
  CURLOPT_ISSUERCERT        OK
  CURLOPT_SSL_EC_CURVES     OK
  CURLOPT_CRLFILE           OK

No vulnerabilities found.

########################################
# CLI TEST (--curves)
########################################

=== mbedTLS CLI ===
Running: curl --curves INVALID_CURVE https://curl.se
200 -> VULNERABLE (silently ignored)

=== OpenSSL CLI ===
Running: curl --curves INVALID_CURVE https://curl.se
000 -> CORRECT (rejected invalid curve)

########################################
# RESULT
########################################

VULNERABILITIES CONFIRMED: mbedTLS ignores SSL options that OpenSSL enforces

It probably is also a problem for Windows with the Schannel backend, but I didn't test this.

Supporting Material/References:

Attached files:

• Dockerfile: Builds curl with both mbedTLS and OpenSSL backends
• test_ssl_options.c: Test program for all SSL options
• run_test.sh: Test script (runs inside container)
• test.sh: Build and run (host entry point)

The fix should be easy: Return CURLE_NOT_BUILT_IN for backends that don't implement these features.

case CURLOPT_ISSUERCERT:
  if(!Curl_ssl_supports(data, SSLSUPP_ISSUERCERT))
    return CURLE_NOT_BUILT_IN;
  return Curl_setstropt(&data->set.str[STRING_SSL_ISSUERCERT], va_arg(param, char *));

This requires:

1. Adding feature flags (e.g., SSLSUPP_ISSUERCERT, SSLSUPP_EC_CURVES) to the SSL backend interface
2. Each backend declaring which features it supports
3. Checking support before accepting the option

Impact

Summary

MITM Attack Enablement

Applications and scripts that rely on CURLOPT_ISSUERCERT or CURLOPT_CRLFILE for certificate validation are vulnerable to MITM attacks when built against affected backends. An attacker with any valid certificate can intercept supposedly-pinned connections.

Cryptographic Downgrade

The CURLOPT_SSL_EC_CURVES/--curves option being ignored allows servers (or MITM attackers) to negotiate weaker elliptic curves than intended, potentially exploiting known cryptographic weaknesses.

Silent Failure

The most critical aspect is that these options fail silently, returning CURLE_OK and proceeding without the security check. Applications have no way to detect that their security requirements are not being enforced.

Affected Platforms

All systems which use affected backends. For example OpenWrt (which replaced WolfSSL with mbedTLS in newer releases), Windows builds using Schannel, and many IoT devices and Docker containers. WolfSSL probably has the same bug.

Attack Surface

The bugs affect both libcurl applications (via curl_easy_setopt()) and curl command line users (via --curves and --crlfile flags). Two of three vulnerabilities are exploitable from the CLI, not just custom programs.