Summary
Workarounds
No workaround available.
For more information
If there are any questions or comments about this advisory, please open an issue.
Original DescriptionTraefik's TCP router uses a connection-level read deadline to bound protocol sniffing (peeking a TLS client hello), but then clears the deadline via conn.SetDeadline(time.Time{}) before delegating the connection to TLS forwarding.
A remote unauthenticated client can send an incomplete TLS record header and stop sending data. After the initial peek times out, the router clears the deadline and the subsequent TLS handshake reads can stall indefinitely, holding connections open and consuming resources.
Expected vs Actual
Expected: if an entrypoint-level read deadline is used to bound initial protocol sniffing, TLS handshake reads should remain bounded by a deadline (either the same deadline is preserved, or a dedicated handshake timeout is enforced).
Actual: after protocol sniffing the router clears the connection deadline and delegates to TLS handling; an attacker can keep the TLS handshake stalled beyond the configured read timeout.
Severity
HIGH
CWE: CWE-400 (Uncontrolled Resource Consumption)
Affected Code
- pkg/server/router/tcp/router.go: (*Router).ServeTCP clears the deadline before TLS forwarding
- conn.SetDeadline(time.Time{}) removes the entrypoint-level deadline that previously bounded reads
Root Cause
In (*Router).ServeTCP, after sniffing a TLS client hello, the router removes the connection read deadline:
// Remove read/write deadline and delegate this to underlying TCP server
// (for now only handled by HTTP Server)
if err := conn.SetDeadline(time.Time{}); err != nil {
...
}
TLS handshake reads that happen after this point are not guaranteed to have any deadline, so a client that stops sending bytes can keep the connection open indefinitely.
Attacker Control
Attacker-controlled input is the raw TCP byte stream on an entrypoint that routes to a TLS forwarder. The attacker controls:
- Sending a partial TLS record header (enough to trigger the TLS sniffing path)
- Stopping further sends so the subsequent handshake read blocks
Impact
Each stalled connection occupies file descriptors and goroutines (and may consume additional memory depending on buffering). By opening many such connections in parallel, an attacker can cause resource exhaustion and degrade availability.
Reproduction
Attachments include poc.zip with a self-contained integration harness. It pins the repository commit, applies fix.patch as the control variant, and runs a regression-style test that demonstrates the stall in canonical mode and the timeout in control mode.
Run canonical (vulnerable):
unzip poc.zip -d poc
cd poc
make test
Canonical output excerpt: PROOF_MARKER
Run control (deadline preserved / no stall):
unzip poc.zip -d poc
cd poc
make control
Control output excerpt: NC_MARKER
Recommended Fix
Do not clear the entrypoint-level deadline prior to completing TLS handshake, or enforce a dedicated handshake timeout for the TLS forwarder path.
Fix accepted when: an incomplete TLS record cannot stall past the configured entrypoint-level read deadline (or an explicit handshake timeout), and a regression test covers the canonical/control behavior.
Impact
There is a potential vulnerability in Traefik managing TLS handshake on TCP routers.
When Traefik processes a TLS connection on a TCP router, the read deadline used to bound protocol sniffing is cleared before the TLS handshake is completed. When a TLS handshake read error occurs, the code attempts a second handshake with different connection parameters, silently ignoring the initial error. A remote unauthenticated client can exploit this by sending an incomplete TLS record and stopping further data transmission, causing the TLS handshake to stall indefinitely and holding connections open.
By opening many such stalled connections in parallel, an attacker can exhaust file descriptors and goroutines, degrading availability of all services on the affected entrypoint.
Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.
CVE-2026-26999 has a CVSS score of 7.5 (High). 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 (2.11.38, 3.6.9); 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.
Remediation advice
Frequently Asked Questions
- What is CVE-2026-26999? CVE-2026-26999 is a high-severity uncontrolled resource consumption vulnerability in github.com/traefik/traefik/v2 (go), affecting versions <= 2.11.37. It is fixed in 2.11.38, 3.6.9. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
- How severe is CVE-2026-26999? CVE-2026-26999 has a CVSS score of 7.5 (High). 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 packages are affected by CVE-2026-26999?
github.com/traefik/traefik/v2(go) (versions <= 2.11.37)github.com/traefik/traefik/v3(go) (versions <= 3.6.8)
- Is there a fix for CVE-2026-26999? Yes. CVE-2026-26999 is fixed in 2.11.38, 3.6.9. Upgrade to this version or later.
- Is CVE-2026-26999 exploitable, and should I be worried? Whether CVE-2026-26999 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-2026-26999 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-2026-26999?
- Upgrade
github.com/traefik/traefik/v2to 2.11.38 or later - Upgrade
github.com/traefik/traefik/v3to 3.6.9 or later
- Upgrade