Summary

The HTTP parser in AIOHTTP has numerous problems with header parsing, which could lead to request smuggling.
This parser is only used when AIOHTTP_NO_EXTENSIONS is enabled (or not using a prebuilt wheel).

Details

Bug 1: Bad parsing of Content-Length values

Description

RFC 9110 says this:
> Content-Length = 1*DIGIT

AIOHTTP does not enforce this rule, presumably because of an incorrect usage of the builtin int constructor. Because the int constructor accepts + and - prefixes, and digit-separating underscores, using int to parse CL values leads AIOHTTP to significant misinterpretation.

Examples

GET / HTTP/1.1\r\n
Content-Length: -0\r\n
\r\n
X

GET / HTTP/1.1\r\n
Content-Length: +0_1\r\n
\r\n
X

Suggested action

Verify that a Content-Length value consists only of ASCII digits before parsing, as the standard requires.

Bug 2: Improper handling of NUL, CR, and LF in header values

Description

RFC 9110 says this:
> Field values containing CR, LF, or NUL characters are invalid and dangerous, due to the varying ways that implementations might parse and interpret those characters; a recipient of CR, LF, or NUL within a field value MUST either reject the message or replace each of those characters with SP before further processing or forwarding of that message.

AIOHTTP’s HTTP parser does not enforce this rule, and will happily process header values containing these three forbidden characters without replacing them with SP.

Examples

GET / HTTP/1.1\r\n
Header: v\x00alue\r\n
\r\n

GET / HTTP/1.1\r\n
Header: v\ralue\r\n
\r\n

GET / HTTP/1.1\r\n
Header: v\nalue\r\n
\r\n

Suggested action

Reject all messages with NUL, CR, or LF in a header value. The translation to space thing, while technically allowed, does not seem like a good idea to me.

Bug 3: Improper stripping of whitespace before colon in HTTP headers

Description

RFC 9112 says this:
> No whitespace is allowed between the field name and colon. In the past, differences in the handling of such whitespace have led to security vulnerabilities in request routing and response handling. A server MUST reject, with a response status code of 400 (Bad Request), any received request message that contains whitespace between a header field name and colon.

AIOHTTP does not enforce this rule, and will simply strip any whitespace before the colon in an HTTP header.

Example

GET / HTTP/1.1\r\n
Content-Length : 1\r\n
\r\n
X

Suggested action

Reject all messages with whitespace before a colon in a header field, as the standard requires.

PoC

Example requests are embedded in the previous section. To reproduce these bugs, start an AIOHTTP server without llhttp (i.e. AIOHTTP_NO_EXTENSIONS=1) and send the requests given in the previous section. (e.g. by printfing into nc)

Impact

Each of these bugs can be used for request smuggling.