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EEF-CVE-2026-112880
Summary
Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting') vulnerability in hackney_http_connect in benoitc hackney allows HTTP Request Splitting via the value a proxy_auth_fun callback returns. After a 407 response from an HTTP CONNECT proxy, hackney_http_connect:handle_proxy_auth/11 calls the caller's proxy_auth_fun with the status code and the response headers and writes the returned binary into the Proxy-Authorization line of the CONNECT retry without checking it. A CR, LF or NUL byte in that value adds header lines to the request sent to the proxy.
The value is attacker-influenced when the callback derives it from untrusted data, for example from the proxy's challenge headers. Other caller-supplied values on the CONNECT path, such as the target host and the request headers, were already checked for control bytes.
This issue affects hackney: from 2.0.0 before 4.9.0.
Details
1. Challenge. hackney_http_connect:do_handshake_with_auth/9 in src/hackney_http_connect.erl sends the CONNECT request to the proxy. When the proxy answers 407 and the proxy_auth_fun option is set, it hands the status code and the parsed response headers to hackney_http_connect:handle_proxy_auth/11.
2. Callback result. handle_proxy_auth/11 calls the caller's proxy_auth_fun with those two values. On {ok, AuthHeader} it removes any existing Proxy-Authorization header and appends the binary <<"Proxy-Authorization: ", AuthHeader/binary>> to the header list, then retries the handshake with it.
3. Splitting. The header list is joined with CRLF into the CONNECT request. A CR, LF or NUL byte inside AuthHeader therefore ends the Proxy-Authorization line early and the rest is sent as further header lines. The target host and the request headers go through control-byte checks on this path; the callback result did not. The fix applies the same has_ctl_bytes/1 check to it and returns {error, {proxy_auth_error, invalid_header}}.
Impact
A party who controls what the application's proxy_auth_fun returns, for instance through the challenge headers of a proxy it can influence, can add header lines to the CONNECT request the client sends to its proxy.
Workarounds
Check the value your proxy_auth_fun is about to return for CR, LF and NUL bytes and return {error, Reason} instead when one is present. The callback runs in the application, so the check can be made there.
Configurations
The client connects through an HTTP CONNECT proxy with the proxy_auth_fun option set, and the callback builds its result from data an untrusted party can influence, such as the proxy's 407 challenge headers. Deployments without proxy_auth_fun, or whose callback returns a fixed credential, are not affected.
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The vulnerability requires local access to the device to be exploited. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability.
Limited exploitation activity has been observed. Close monitoring and planned remediation are recommended.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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