Summary
AnyCable: Pusher REST API Does Not Verify Request Body MD5 Enabling Signed-Request Replay with Arbitrary Body
The Pusher-compatible REST API includes body_md5 in the HMAC signature string but never computes or verifies the MD5 of the received HTTP body, allowing anyone who observes a signed request to replay it with an entirely different body.
Details
In pusher/http.go, the Handler function extracts body_md5 from the URL query string (line 169) and includes it verbatim in stringToSign (line 175). It then verifies HMAC(stringToSign, secret) == auth_signature. After verification succeeds, handleEvents reads and parses r.Body (lines 201-212) without ever computing md5(body) and comparing it against the body_md5 that was signed. The Pusher protocol specification explicitly requires the server to verify this digest to prevent body-substitution attacks. There is also no auth_timestamp staleness check, so replays are valid indefinitely.
PoC
- Capture a legitimate signed POST to
/apps/<app_id>/events?auth_key=K&auth_timestamp=T&auth_version=1.0&body_md5=LEGIT_MD5&auth_signature=SIGcarrying body{"name":"safe-event","channel":"ch","data":"..."}(e.g., from TLS-terminating load-balancer logs). - Send a new request with the same query string parameters but a different body:
{"name":"injected-event","channel":"admin","data":"malicious-payload"} - The server accepts the request (HMAC over
stringToSignmatches the original) and broadcasts the injected event to all subscribers ofadmin.
Impact
An attacker who can read any single signed Pusher API request (from logs, a shared proxy, or a network tap) can broadcast arbitrary events to any channel indefinitely, potentially forging server-side events, corrupting application state, or delivering phishing messages to WebSocket clients.
CVE-2026-63405 has a CVSS score of 5.9 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.6.15); upgrading removes the vulnerable code path.
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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See it in your environmentNew to Kodem? Get a demo →Remediation advice
After reading r.Body, compute hex(md5(body)) and compare it to the body_md5 query parameter using a constant-time comparison before proceeding. Additionally, reject requests whose auth_timestamp is more than 600 seconds from the current time.
Frequently Asked Questions
- What is CVE-2026-63405? CVE-2026-63405 is a medium-severity security vulnerability in github.com/anycable/anycable (go), affecting versions <= 1.6.14. It is fixed in 1.6.15.
- How severe is CVE-2026-63405? CVE-2026-63405 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of github.com/anycable/anycable are affected by CVE-2026-63405? github.com/anycable/anycable (go) versions <= 1.6.14 is affected.
- Is there a fix for CVE-2026-63405? Yes. CVE-2026-63405 is fixed in 1.6.15. Upgrade to this version or later.
- Is CVE-2026-63405 exploitable, and should I be worried? Whether CVE-2026-63405 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-2026-63405 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-2026-63405? Upgrade
github.com/anycable/anycableto 1.6.15 or later.