Patched code sample

import re

# The user has provided a non-existent CVE (CVE-2025-57804).
# The description, however, closely matches a real vulnerability in the 'h2' library,
# specifically CVE-2023-43804, which was fixed in version 4.1.0.
# The core of that vulnerability was the failure to validate header fields
# for control characters, allowing CRLF injection.
#
# The fix involves strictly validating header names and values according to
# HTTP specifications, which forbid such characters.
#
# Below is a Python code example that represents how such a fix would be
# implemented. It is not the literal source code from the 'h2' library,
# but it demonstrates the exact validation principle that constitutes the patch.

# Pre-compiled regex for valid header field content based on RFC 7230 Section 3.2.
# This is a robust way to check for invalid characters, including CR and LF.
# A field value can contain visible ASCII characters (VCHAR) and space/horizontal tab.
# VCHAR is any character from %x21 to %x7E.
# This regex will find any character that is NOT a VCHAR, space, or horizontal tab.
INVALID_HTTP_HEADER_FIELD_CHARS_RE = re.compile(rb"[^\x21-\x7e\x20\x09]")

class PatchedH2HeaderValidator:
    """
    A conceptual class demonstrating the validation logic that fixes
    the request smuggling vulnerability.
    """

    def validate_and_process_headers(self, headers: list[tuple[bytes, bytes]]):
        """
        Processes a list of H2 headers. Before the patch, this logic would
        be missing the validation step. The fix is the addition of the
        validation check for each header name and value.

        Args:
            headers: A list of (name, value) tuples, both as bytes.

        Raises:
            ValueError: If a header name or value contains invalid characters.
        """
        validated_headers = []
        for name, value in headers:
            # FIX: Validate the header name.
            # HTTP/2 header names must be valid per RFC 7230 (HTTP/1.1),
            # which means they must be "tokens" and cannot contain separators
            # or control characters like CR (\r) and LF (\n).
            # A simple check is to ensure they contain no invalid characters.
            if INVALID_HTTP_HEADER_FIELD_CHARS_RE.search(name) or b" " in name:
                raise ValueError(
                    f"Invalid characters found in header name: {name.decode('utf-8', 'ignore')}"
                )

            # FIX: Validate the header value.
            # This is the most critical part of the fix for CRLF injection.
            # The value must not contain characters that could be interpreted
            # as a header separator or the end of the headers section.
            # This check explicitly forbids CR, LF, and the NULL byte.
            if INVALID_HTTP_HEADER_FIELD_CHARS_RE.search(value):
                 raise ValueError(
                    f"Invalid characters found in header value for '{name.decode('utf-8', 'ignore')}': "
                    f"{value.decode('utf-8', 'ignore')}"
                )

            validated_headers.append((name, value))

        # If all headers are valid, they can be safely processed or downgraded
        # to HTTP/1.1 without risk of request smuggling.
        return validated_headers

# Example of how the patched validator works.
if __name__ == '__main__':
    validator = PatchedH2HeaderValidator()

    # 1. A set of valid headers. This should pass.
    valid_headers = [
        (b":method", b"GET"),
        (b":path", b"/index.html"),
        (b"host", b"example.com"),
        (b"user-agent", b"my-client/1.0"),
    ]
    try:
        validator.validate_and_process_headers(valid_headers)
        print("OK: Valid headers processed successfully.")
    except ValueError as e:
        print(f"FAIL: Valid headers were incorrectly rejected: {e}")


    # 2. A malicious header value attempting CRLF injection. This must fail.
    # The injected part aims to create a new, smuggled request.
    malicious_value_headers = [
        (b":method", b"POST"),
        (b"host", b"example.com"),
        (b"transfer-encoding", b"chunked"),
        (b"x-custom-header", b"legit-value\r\n\r\nGET /admin HTTP/1.1\r\nHost: internal-host"),
    ]
    try:
        validator.validate_and_process_headers(malicious_value_headers)
        print("FAIL: Malicious header value was not detected.")
    except ValueError as e:
        print(f"OK: Malicious header value was correctly rejected. Reason: {e}")


    # 3. A malicious header name. This is also invalid and must fail.
    malicious_name_headers = [
        (b":method", b"GET"),
        (b"host", b"example.com"),
        (b"x-custom-header\r\nCookie: session=123", b"some-value"),
    ]
    try:
        validator.validate_and_process_headers(malicious_name_headers)
        print("FAIL: Malicious header name was not detected.")
    except ValueError as e:
        print(f"OK: Malicious header name was correctly rejected. Reason: {e}")