bypass of this Fixed #2437131 [ Inadequate Protocol Restriction Enforcement in curl ]

How it was found

Summary:

A flaw has been identified in the curl command-line tool related to its protocol selection mechanism. Specifically, the protocol restrictions set by the --proto option can be bypassed, allowing unintended protocols to be used despite explicit restrictions. This flaw can result in plaintext communication being used even when the user has attempted to disable all protocols except encrypted ones.

Vulnerability Details

Command Triggering the Issue: curl --proto -all,-http http://example.com

Observed Behavior:

The command was intended to disable all protocols and then explicitly disable HTTP.

However, the actual behavior allowed HTTP requests to be made, indicating that the protocol restrictions were not enforced correctly.

Bypass Example:

    curl --proto =all http://example.com
    curl --proto http http://evil.com

The command curl --proto =all appears to override the protocol restrictions, allowing HTTP requests to bypass the intended restrictions.

Steps to Reproduce

Execute Command with Protocol Disabling: curl --proto -all,-http http://example.com

Observe Error Message: Error: curl: (1) Protocol "http" disabled

Execute Bypass Command: curl --proto http http://evil.com

Observe Successful Request: The request is made over HTTP despite the initial restrictions.

Potential Risks

Security Breach: Users may believe they have enforced secure communication protocols, but the flaw could result in unencrypted communication, increasing the risk of eavesdropping or man-in-the-middle attacks.

User Trust: Users trusting curl for secure communication may be misled by incorrect protocol enforcement.

Mitigation

Update curl: Ensure that you are using the latest version of curl, as this issue might be addressed in newer releases.

Use Explicit Protocols: Instead of disabling protocols, explicitly specify the protocols you want to allow: curl --proto https https://example.com

Recommendation

Report to Maintainters: Consider reporting this issue to the curl maintainers if it has not been already reported. Provide them with detailed reproduction steps and observed behavior.

Impact

Summary:

Unencrypted Communication: The flaw allows requests to be made over unencrypted protocols (such as HTTP) even when the user has explicitly disabled plaintext protocols. Data Exposure Risk: This can lead to potential exposure of sensitive data if transmitted over an unencrypted link.