Vulnerability in python-multipart (CVE-2026-53539)
Summary
Vulnerability in python-multipart (CVE-2026-53539). 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
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.
Details
In python_multipart/multipart.py, both the FIELD_NAME and FIELD_DATA states located the next separator like this:
sep_pos = data.find(b"&", i)
if sep_pos == -1:
sep_pos = data.find(b";", i)
data.find(b"&", i) scans from i to the end of the buffer and returns -1 only when there is no & anywhere in the remainder. For a ; separated body with no &, this failed full buffer scan repeats once per field, making parsing quadratic in the body length.
For example, a 1 MiB url encoded body consisting of a; repeated ~500,000 times, submitted with Content-Type: application/x-www-form-urlencoded, causes the parser to perform on the order of 10^11 byte comparisons, consuming several seconds of CP…
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-2026-53539 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: 7.5/10.
What to do: upgrade python-multipart to 0.0.30 or later.
If unsure, ask your IT team or search "python-multipart CVE-2026-53539" on the vendor's site.
CVE-2026-53539 (python-multipart) — CWE-400 / CVSS v3 7.5
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: QuerystringParser / FIELD_NAME / FIELD_DATA / FormParser
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-400) 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 / FIELD_NAME / FIELD_DATA / FormParser.
Severity: High (CVSS 7.5/10). 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-2026-53539 python-multipart
References
- patch security-advisories@github.com
- web github.com