Vulnerability in python-multipart (CVE-2026-53538)
Summary
Vulnerability in python-multipart (CVE-2026-53538). Risk of unauthorized operations or information disclosure. Exploitable via QuerystringParser. Mitigation: upgrade to 0.0.30 or later.
Original description
The description as published by the source.
Summary
QuerystringParser treated ; as a field separator in application/x-www-form-urlencoded bodies, in addition to &. The WHATWG URL standard, modern browsers, and Python's urllib.parse (since the CVE-2021-23336 fix) treat only & as a separator. This creates a parser differential: the same bytes are tokenized into different fields than a WHATWG compliant intermediary would produce, allowing an attacker to smuggle extra form fields past an upstream body inspecting component.
Details
In python_multipart/multipart.py, the FIELD_NAME and FIELD_DATA states located the next separator by scanning for & and, failing that, for ;:
sep_pos = data.find(b"&", i)
if sep_pos == -1:
sep_pos = data.find(b";", i)
As a result, ; acted as a field boundary. Because the fallback only triggered when no & remained in the current chunk, tokenization also depended on unrelated bytes later in the buffer and on how the body was split across write() calls. This is the same class of issue as CVE-2021-23336 in CPython's urllib.parse.
For example, a body inspecting WAF or gateway that follows the WHATWG rule (only & separates fields) receives:
role=user&x=;role=admin
The upstream parses two fields, role=user and x=";role=admin", sees a benign role=user, and forwards the request. QuerystringParser parsed the same bytes as three fields: role="user", x="", and `role="admin"…
Source: GitHub Security Advisories (Global) NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-53538 has been found in python-multipart.
Attackers can target a specific entry point like QuerystringParser over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 3.7/10.
What to do: upgrade python-multipart to 0.0.30 or later.
If unsure, ask your IT team or search "python-multipart CVE-2026-53538" on the vendor's site.
CVE-2026-53538 (python-multipart) — CWE-436 / CVSS v3 3.7
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: QuerystringParser / urllib.parse / FIELD_NAME / FIELD_DATA
Patched: 0.0.30 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
A vulnerability (CWE-436) exists in python-multipart. Attackers reach the vulnerable code path via QuerystringParser without authentication.
Check the vendor advisory for the affected versions of python-multipart. Attack surface: QuerystringParser / urllib.parse / FIELD_NAME / FIELD_DATA.
Severity: Low (CVSS 3.7/10). Risk of unauthorized operations or information disclosure.
Update to 0.0.30.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'python-multipart' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
-
1Identify exposure identify
grep -r 'python-multipart' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
python-multipartを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply patch patch
Upgrade python-multipart to 0.0.30ステージング環境で 0.0.30 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
-
3Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
Affected packages
Fix status by distribution
Fixed packages published by each distribution for this CVE
- Ubuntu UBUNTU-CVE-2026-53538 python-multipart
References
- patch security-advisories@github.com
- web github.com