Vulnerability in PyJWT (CVE-2026-102266)
Summary
Vulnerability in PyJWT (CVE-2026-102266). Confidential information can be exposed externally. Exploitable via str. Mitigation: upgrade to 2.14.0 or later.
Original description
The description as published by the source.
Summary
A service that verifies HS256 tokens using an empty oct JWK through PyJWK, including a PyJWK obtained from PyJWKSet, can therefore accept attacker-generated tokens as authenticated.
PyJWT 2.13.0 rejects an empty HMAC key when it is supplied through the raw str/bytes key path, but accepts the same zero-length key when it is supplied as a symmetric PyJWK.
An oct JWK containing an empty Base64URL key value:
{"kty":"oct","k":""}
is decoded to b"". During signature verification, the PyJWK path uses this decoded value directly and does not invoke the empty-key validation in HMACAlgorithm.prepare_key. With the default enforce_minimum_key_length=False, PyJWT emits a warning and proceeds with HMAC verification using the zero-length key.
An attacker can independently calculate HMAC-SHA256(b"", signing_input) and create valid HS256 JWTs with arbitrary claims. A service that verifies HS256 tokens using an empty oct JWK through PyJWK or PyJWKSet can therefore accept attacker-generated tokens as authenticated.
The application must already contain an empty HMAC JWK, for example because a missing Base64URL configuration or secret-manager value was serialized as "k": "". The attacker does not need control of the JWK/JWKS source, a signing oracle, a private key, or a mixed algorithm allow-list.
Details
The vulnerability is caused by inconsistent validation of the same HMAC key material depending on whether it enters PyJWT as a raw…
Source: NIST National Vulnerability Database 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-2026-102266 has been found in PyJWT.
Attackers can target a specific entry point like str over the network to misuse the product.
Confidential information can be exposed externally. CVSS score: 7.4/10.
What to do: upgrade PyJWT to 2.14.0 or later.
If unsure, ask your IT team or search "PyJWT CVE-2026-102266" on the vendor's site.
CVE-2026-102266 (PyJWT) — CWE-347 / CVSS v3 7.4
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: str / bytes / PyJWK / oct
Patched: 2.14.0 — 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-347) exists in PyJWT. Attackers reach the vulnerable code path via str without authentication.
Check the vendor advisory for the affected versions of PyJWT. Attack surface: str / bytes / PyJWK / oct.
Severity: High (CVSS 7.4/10). Confidential information can be exposed externally.
Update to 2.14.0.
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 'PyJWT' . 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 'PyJWT' . | 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
PyJWT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 PyJWT to 2.14.0ステージング環境で 2.14.0 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
- Debian DEBIAN-CVE-2026-102266 pyjwt
- Ubuntu UBUNTU-CVE-2026-102266 pyjwt
References
- advisory nvd.nist.gov
- advisory github.com
- package github.com
- package pypi.org
- web github.com
- web github.com
- web github.com