# CVE-2026-53539: Python-Multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service > An attacker can cause CPU exhaustion and denial of service by submitting crafted querystring bodies with semicolon separators to applications using Python-Multipart. - **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`) - **Exploited in the wild:** Not known - **Login needed:** Not needed - **Attackable:** Over the internet - **Victim action needed:** Not needed - **Fix available:** Yes - **Published:** 22 Jun 2026 - **Updated:** 23 Jun 2026 - **Also known as:** CLEANSTART-2026-EB34508, CLEANSTART-2026-SR16406, DEBIAN-CVE-2026-53539, ECHO-1942-6e43-27d8, GHSA-5rvq-cxj2-64vf, MINI-2r4m-69f3-x4pm, MINI-42rh-hcq3-w938, MINI-5xcf-v2vx-q275, MINI-9f6c-xgfg-g7f7, MINI-9g7m-mvqj-vv25, MINI-9rp5-cv62-7m5g, MINI-cvf4-prjx-w93j, MINI-fqx4-6prh-77hw, MINI-hx7x-xj5j-3xr5, MINI-jpp3-qhg2-hhrq, MINI-m74v-grjf-47q9, MINI-q2r5-mcgf-r6rv, MINI-r2qj-36r2-gj58, MINI-rcmw-x7mj-7cwp, MINI-wp85-p8hh-c55m, MINI-x6w7-9mg7-5xxx, PYSEC-2026-3036, ROOT-APP-PYPI-CVE-2026-53539, UBUNTU-CVE-2026-53539 ## What is CVE-2026-53539? CVE-2026-53539 is a high-severity denial of service vulnerability in Kludex python-multipart, published on 22 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim. ### Official description Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.30, when parsing application/x-www-form-urlencoded bodies, QuerystringParser located the field separator with a two step lookup: it first scanned the entire remaining buffer for &, and only when no & existed anywhere ahead did it fall back to scanning for ;. For a body that uses ; as the separator and contains no &, every field iteration performed a full failed & scan over the entire remaining buffer before locating the nearby ;. With N semicolon separated fields in a chunk of size B, this yields O(B^2) byte comparisons per chunk. An attacker can submit a small crafted body of the form a;a;a;... and cause the parser to spend seconds of CPU per request. A handful of concurrent requests can exhaust worker processes. This vulnerability is fixed in 0.0.30. Weakness: CWE-407 (Inefficient Algorithmic Complexity), CWE-400 (Uncontrolled Resource Consumption) ## Which versions are affected by CVE-2026-53539? 3 open source packages ship an affected version, including python-multipart. It affects 1 product. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `python-multipart` | PyPI | All versions | 0.0.30 | | `python-multipart` | PyPI | All versions | 0.0.30 | | `python-multipart` | PyPI | All versions | 0.0.30 | Products: Fastapiexpert Python-Multipart. ## How do I fix CVE-2026-53539? Upgrade python-multipart to 0.0.30 or later. ## Is CVE-2026-53539 being exploited? There is no known exploitation of CVE-2026-53539 in the wild and no public exploit code. ## Other vulnerabilities in Kludex python-multipart - [CVE-2026-24486](https://insights.sbomify.com/CVE-2026-24486/): Python-Multipart has Arbitrary File Write via Non-Default Configuration (high 8.6) - [CVE-2024-24762](https://insights.sbomify.com/CVE-2024-24762/): python-multipart vulnerable to content-type header Regular expression Denial of Service (high 7.5) - [CVE-2026-40347](https://insights.sbomify.com/CVE-2026-40347/): Python-Multipart affected by Denial of Service via large multipart preamble or epilogue data (medium 5.3) - [CVE-2026-53537](https://insights.sbomify.com/CVE-2026-53537/): Python-Multipart: Content-Disposition parameter smuggling via RFC 2231/5987 extended parameters (low 3.7) - [CVE-2026-53538](https://insights.sbomify.com/CVE-2026-53538/): Python-Multipart: Semicolon treated as querystring field separator enables parameter smuggling (low 3.7) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2026-53539 and fix it if it is affected. What it is: CVE-2026-53539 is a high-severity denial of service vulnerability in Kludex python-multipart, published on 22 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim. Exploitation: There is no known exploitation of CVE-2026-53539 in the wild and no public exploit code. Affected packages and fixed versions: - python-multipart (PyPI): affected all versions; fixed in 0.0.30 - python-multipart (PyPI): affected all versions; fixed in 0.0.30 - python-multipart (PyPI): affected all versions; fixed in 0.0.30 Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2026-53539/ (Markdown: https://insights.sbomify.com/CVE-2026-53539/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2026-53539 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-53539 - OSV: https://osv.dev/vulnerability/CVE-2026-53539 --- From sbomify Insights: https://insights.sbomify.com/CVE-2026-53539/ Summaries are generated and may contain mistakes; the official description above is the record.