CVE-2026-47071: SOCKS5 TLS upgrade ignores caller timeout in hackney
Summary
The SOCKS5 transport in src/hackneysocks5.erl correctly applies the caller-supplied timeout to the SOCKS5 negotiation phase, but then upgrades the tunnel to TLS using ssl:connect/2 (the two-argument form), which defaults to infinity. The Timeout value is in scope at that call site but is never forwarded. A hostile or man-in-the-middled SOCKS5 proxy that completes the SOCKS5 handshake normally and then stalls the TLS exchange will pin the connecting Erlang process and socket indefinitely, regardless of any connecttimeout or recvtimeout options the caller set.
Details
In src/hackneysocks5.erl, line 65, after the SOCKS5 negotiation succeeds, the code calls:
erlang ssl:connect(Socket, SSLOpts)
The three-argument form ssl:connect/3 takes a timeout; the two-argument form used here defaults to infinity. The variable Timeout (already used for SOCKS5 recv calls earlier in the same function) is simply not passed. The bytes that drive the TLS handshake on the upstream side of the tunnel come from whatever endpoint the proxy connects to. A hostile proxy can complete SOCKS5 normally, then either stay silent or send a partial ServerHello and stop, keeping ssl:connect/2 blocked forever. No certificate forgery is needed.
PoC
1. Stand up a SOCKS5 proxy that completes the SOCKS5 greeting and CONNECT reply normally, then goes silent (never sends a TLS ServerHello). 2. Issue an HTTPS request through it via hackney with connecttimeout and recvtimeout set to a short value (e.g. 2000 ms). 3. Observe the calling process remains blocked well past the configured timeout, consuming a process and socket until killed externally.
Impact
Denial of service via unbounded process and socket consumption. Affects hackney 0.10.0 through 4.0.0 for any HTTPS request routed through a SOCKS5 proxy. The connecttimeout and recvtimeout options give a false sense of safety since they are not honored during the TLS upgrade. CVSS v4.0: 8.2 (HIGH).
Resources
Introduction commit: https://github.com/benoitc/hackney/commit/34cdbd1d20a282aacc286a89327465a3925b4c5d Patch commit: https://github.com/benoitc/hackney/commit/5ccdab725c561a6f03d05a51f2d0664f98236dae
Other sources
Uncontrolled Resource Consumption vulnerability in benoitc hackney allows Flooding. The SOCKS5 transport in src/hackneysocks5.erl correctly applies the caller-supplied timeout to the SOCKS5 negotiation phase, but then upgrades the connection to TLS using the two-argument form ssl:connect/2, which defaults to an infinite timeout. The Timeout value is in scope at the call site but is not forwarded. A hostile SOCKS5 proxy that completes the SOCKS5 handshake normally and then goes silent (or sends a partial TLS ServerHello and stalls) will cause the connecting process to block indefinitely, regardless of the connecttimeout or recvtimeout options supplied by the caller.
This issue affects hackney: from 0.10.0 before 4.0.1.
— NVD
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
erlang/hackneyto a version that resolves this vulnerability.Fixed in 4.0.1 - Upgrade
Upgrade
hackneyto a version that resolves this vulnerability.Fixed in 4.0.1Patch 5ccdab725c561a6f03d05a51f2d0664f98236dae
Event History
Frequently Asked Questions
When can this issue be exploited?
It affects connections that use hackney's SOCKS5 transport and then upgrade the tunnel to TLS. A hostile or man-in-the-middle SOCKS5 proxy must complete the SOCKS5 negotiation and then stall or partially respond during the TLS handshake.
Do configured connection or receive timeouts prevent the hang?
No. Although the caller-supplied timeout is used during SOCKS5 negotiation, it is not passed to the TLS upgrade call, which uses an infinite default timeout.
What is the operational impact of exploitation?
Each affected connection can leave the connecting Erlang process and its socket pinned indefinitely. An attacker able to induce enough such connections could exhaust available process or socket resources.