CVE-2019-1010083: Denial of service in Flask via large mu...

Flask, one of the most widely deployed Python web frameworks, shipped versions before 1.0 with a denial-of-service weakness in how it parsed multipart form data. CVE-2019-1010083 describes a flaw in Flask's (and the underlying Werkzeug form-parsing machinery it depends on) handling of multipart/form-data request bodies: an unauthenticated remote attacker could send a request crafted to make the parser do disproportionately expensive work, driving up CPU and memory consumption on the server. In a worst case, a single malicious request could tie up a worker process long enough to degrade or take down the application for legitimate users — a classic resource-exhaustion (CWE-400) issue that requires no authentication, no user interaction, and no complex setup on the attacker's part.

Unlike memory-corruption or injection bugs, this class of vulnerability doesn't hand an attacker code execution or data access. Its impact is availability: a small, cheap request can force the server to burn resources wildly out of proportion to the request size. For internet-facing Flask applications accepting file or form uploads — which is a large share of production Flask deployments — that asymmetry is exactly what makes DoS bugs in request parsers worth taking seriously, especially when the vulnerable code sits on the unauthenticated request path, before any application-level checks run.

Affected versions and components

CVE-2019-1010083 affects Flask releases prior to 1.0. The vulnerable logic lives in the multipart/form-data parsing path that Flask exposes through request.form and request.files, backed by Werkzeug's form parser. Because Flask relies on Werkzeug to do the actual heavy lifting of decoding multipart bodies, any application built on an old, unpatched Flask (and by extension an old Werkzeug) inherited the exposure automatically, without the application developer writing any vulnerable code themselves — the risk was baked into the framework's default request-handling pipeline.

The fix landed in Flask 1.0, released in 2018. Any project still pinned to a pre-1.0 Flask release — something that was not unusual for long-lived internal tools, legacy services, or vendored dependencies that were never revisited — remains exposed today. Because Flask is frequently pulled in transitively (through internal frameworks, admin panels, or microservice scaffolding), it's common for this dependency to be several layers removed from the code a team actively maintains, which is part of why old parser-level DoS bugs like this one tend to persist quietly in production long after a fix exists upstream.

CVSS, EPSS, and KEV context

NVD scores CVE-2019-1010083 as a CVSS v3 base score of 7.5 (High), reflecting a network-exploitable, low-complexity, no-privilege, no-user-interaction attack whose impact is confined to availability — consistent with the vector AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. That's a meaningful severity rating for a bug that requires nothing more than an HTTP request to trigger, even though it doesn't compromise confidentiality or integrity.

This CVE does not appear on CISA's Known Exploited Vulnerabilities (KEV) catalog, and its EPSS (Exploit Prediction Scoring System) probability sits at the low end typical of older, framework-level DoS advisories with no documented history of widespread active exploitation. That combination — real, uncontested severity on paper, but no evidence of in-the-wild campaigns — is a common profile for parser-level DoS issues in mature open-source frameworks: the risk is legitimate and the fix is cheap, but it rarely becomes the centerpiece of an active exploitation wave the way an RCE would. That doesn't make it safe to ignore; it makes it a textbook case for routine patch hygiene rather than emergency incident response.

Timeline

• Flask 1.0 (2018): The underlying fix to the multipart form-parsing behavior shipped as part of the Flask 1.0 release, well before this issue carried a CVE identifier.
• 2019: CVE-2019-1010083 was assigned and published as part of a broader wave of retroactive CVE identifiers issued that year for known issues in popular open-source projects — including several other Python web-ecosystem libraries — that had been fixed upstream but never formally tracked with a CVE. This is a common pattern for mature open-source projects: the security fix ships quietly in a routine release, and the CVE record catches up later, sometimes over a year afterward.
• Ongoing: Because the fix predates the CVE by a significant margin, most actively maintained Flask deployments were already remediated by the time the identifier was published. The residual risk sits almost entirely with legacy or unmaintained services still running Flask pre-1.0.

The gap between "fix released" and "CVE published" is worth internalizing on its own: vulnerability scanners and SBOM-based tooling that only cross-reference known CVE identifiers would have shown this dependency as "clean" for roughly a year after the fix was available, simply because no CVE existed yet to flag it against. Teams that upgrade purely reactively, waiting for CVE alerts rather than tracking upstream releases, are structurally exposed to this lag.

Remediation steps

1. Upgrade Flask to 1.0 or later (and keep Werkzeug in sync with a compatible, currently maintained release). This is the definitive fix — there is no supported configuration of a pre-1.0 Flask install that closes this gap without upgrading.
2. Set MAX_CONTENT_LENGTH on your Flask application to cap the size of incoming request bodies, including multipart uploads. This is good defense-in-depth regardless of framework version, since it bounds the blast radius of any future parsing-related resource issue.
3. Enforce upload limits at the edge. Reverse proxies, API gateways, and WAFs can cap request body size, multipart part counts, and per-connection concurrency before requests ever reach application workers — a valuable second layer even after patching.
4. Rate-limit and monitor upload-heavy endpoints. Any route accepting file or form uploads is a natural target for resource-exhaustion attempts; alerting on anomalous CPU/memory spikes correlated with upload traffic helps catch both this class of issue and any similar ones that haven't been named yet.
5. Audit for indirect dependence. Confirm that internal frameworks, admin tooling, and vendored code don't pin an old Flask version transitively — a direct requirements.txt check is not sufficient if Flask is bundled inside another internal package.
6. Re-run this check across your fleet, not just your primary services. Legacy internal tools and low-traffic services are exactly where pre-1.0 Flask tends to survive undetected.

How Safeguard Helps

CVE-2019-1010083 is a good illustration of why point-in-time vulnerability scanning isn't enough: the fix existed a full year before the CVE record did, and the vulnerable component (Flask) is often pulled in indirectly rather than declared as a direct, top-level dependency. Safeguard is built to close exactly that gap in software supply chain visibility.

Safeguard continuously inventories every dependency across your services — direct and transitive — building accurate, living SBOMs rather than relying on manifests that go stale the moment someone forgets to update requirements.txt. That means Flask and Werkzeug versions are tracked wherever they appear in your stack, including inside internal frameworks or vendored code paths where they're easy to lose track of.

Because Safeguard correlates dependency inventories against vulnerability intelligence continuously — not just when a new CVE is published — it can flag outdated, risk-bearing versions of libraries like Flask even during the window before a formal CVE identifier exists, using upstream release and advisory signals rather than waiting on NVD alone. When a CVE like this one does get published, Safeguard maps it directly to the affected services in your environment, prioritizes remediation using real context (CVSS severity, EPSS likelihood, KEV status, and whether the vulnerable code path is actually reachable in your deployment), and tracks the fix through to a verified, patched state.

For teams managing large, long-lived codebases where old Flask instances or forgotten internal services are exactly the kind of thing that slips through manual review, that continuous, dependency-graph-aware visibility is what turns a CVE like CVE-2019-1010083 from a quiet, lingering risk into a fast, verifiable fix.