Summary
Hackney: ssl:connect/2 post-handshake upgrade has no timeout
The SOCKS5 transport in src/hackney_socks5.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 connect_timeout or recv_timeout options the caller set.
Details
In src/hackney_socks5.erl, line 65, after the SOCKS5 negotiation succeeds, the code calls:
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
- Stand up a SOCKS5 proxy that completes the SOCKS5 greeting and CONNECT reply normally, then goes silent (never sends a TLS ServerHello).
- Issue an HTTPS request through it via hackney with
connect_timeoutandrecv_timeoutset to a short value (e.g. 2000 ms). - Observe the calling process remains blocked well past the configured timeout, consuming a process and socket until killed externally.
Resources
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 connect_timeout and recv_timeout options give a false sense of safety since they are not honored during the TLS upgrade. CVSS v4.0: 8.2 (HIGH).
Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-47071? CVE-2026-47071 is a high-severity uncontrolled resource consumption vulnerability in hackney (erlang), affecting versions >= 0.10.0, < 4.0.1. It is fixed in 4.0.1. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
- Which versions of hackney are affected by CVE-2026-47071? hackney (erlang) versions >= 0.10.0, < 4.0.1 is affected.
- Is there a fix for CVE-2026-47071? Yes. CVE-2026-47071 is fixed in 4.0.1. Upgrade to this version or later.
- Is CVE-2026-47071 exploitable, and should I be worried? Whether CVE-2026-47071 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-47071 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-47071? Upgrade
hackneyto 4.0.1 or later.