CVE-2026-48525: PyJWT Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-w7vc-732c-9m39",
  "modified" : "2026-06-15T19:29:12Z",
  "published" : "2026-06-15T19:29:12Z",
  "aliases" : [ "CVE-2026-48525" ],
  "summary" : "PyJWT: Unauthenticated DoS via unbounded Base64URL decoding of unused payload segment in b64=false detached JWS",
  "details" : "> [!NOTE]\n> Practical impact depends on whether request body-size limits are enforced upstream (proxy/web-server/framework). Deployments with typical body-size caps (≤2 MB) bound the amplifier significantly; deployments accepting larger token inputs are more exposed.\n\nWhen verifying detached JWS tokens using the unencoded-payload option (`\"b64\": false`, RFC 7797), PyJWT performs **Base64URL decoding of the compact-serialization payload segment** *before* enforcing the detached-payload rules.\n\nFor `b64=false`, PyJWT later **discards** that decoded payload and replaces it with the caller-provided `detached_payload`. In practice, this turns the middle segment into an attacker-controlled “work amplifier”: a remote client can supply an arbitrarily large Base64URL payload segment that forces **CPU work + memory allocations** even if the signature is invalid.\n\nThis creates an **unauthenticated DoS** vector against any endpoint that verifies detached JWS using PyJWT.\n\n---\n\n## Affected Component(s)\n\n* `jwt/api_jws.py`\n\n  * `PyJWS.decode()` / `PyJWS.decode_complete()`\n  * `_load()` (parsing and Base64URL decoding)\n\n---\n\n## Root Cause (exact logic flaw)\n\n### What happens in the code\n\nIn `jwt/api_jws.py`, `decode_complete()` does the following (order matters):\n\n* Calls `_load(jwt)` first, which decodes the token segments\n* Only after that, checks `header.get(\"b64\")` and if `False`, it replaces `payload = detached_payload` and rebuilds the signing input\n\nThis behavior is visible in `decode_complete()`:\n\n* `_load(jwt)` happens **before** the `b64=false` handling\n* then `payload = detached_payload` and `signing_input = ... detached_payload` happens afterward ([GitHub][1])\n\nInside `_load()`, PyJWT unconditionally performs:\n\n* `payload = base64url_decode(payload_segment)`\n  This is the expensive step the attacker can amplify ([GitHub][1])\n\n### Why this becomes a vulnerability\n\nFor `b64=false` detached JWS, the payload segment in compact form is effectively **not needed** for verification in PyJWT’s own logic (since the library uses `detached_payload` as the real payload). Yet PyJWT still decodes it first, meaning:\n\n* cost is paid **even when signature is invalid**\n* the decoded bytes are **discarded**\n* attacker controls the size of this cost via token length\n\n---\n\n## Impact (evidence-driven)\n\n### Security impact\n\n* **Unauthenticated remote DoS**: decoding work happens before signature rejection → attacker does not need signing key.\n* **CPU amplification**: Base64URL decode time scales linearly with payload segment size.\n* **Memory amplification**: decoded output allocates large byte buffers (tens of MB per request).\n* **Operational impact**: request queueing / worker starvation under modest concurrency bursts.\n\n### Standards context (RFC 7797)\n\nRFC 7797 explicitly notes this option is used when payload is large and/or detached, and discusses interoperability requirements around marking it critical (“crit” with “b64”). ([IETF Datatracker][2])\n(PyJWT supports `crit` validation, but the issue here is decode order / unbounded decode of an unused segment.)\n\n---\n\n## Affected Versions\n\n* **Confirmed affected:** PyJWT **2.12.1** (tested from your local editable install and repo).\n* **Likely affected:** all versions that include detached payload support for JWS decoding, which was introduced in **2.4.0** (“Add detached payload support for JWS encoding and decoding”). ([pyjwt.readthedocs.io][3])\n\n(For GHSA, this phrasing is strong: “confirmed” + “likely since feature introduction”.)\n\n---\n\n# Threat Model \n\n### Typical real deployment\n\nA service verifies signed HTTP requests or webhooks using detached JWS:\n\n* token is provided in JSON body / query / header\n* actual payload is the HTTP request body passed as `detached_payload`\n\n### Attacker\n\n* remote unauthenticated client\n* can send requests to verify endpoint\n* does **not** need a valid signature (invalid signature still triggers the expensive decode path)\n\n### Attack chain\n\n1. Attacker crafts a JWS compact token with header containing `\"b64\": false` and `crit:[\"b64\"]`.\n2. Attacker inflates the **payload segment** (middle segment) to millions of Base64URL characters.\n3. Server calls `PyJWS.decode(...detached_payload=...)`.\n4. PyJWT decodes the inflated segment (CPU + memory).\n5. Signature is rejected afterward (401) — but resources already consumed.\n6. Repeated requests or bursts cause queueing/worker starvation → DoS.\n\n---\n\n# Proof of Concept - file names + results\n\n## PoC placement \n\n* [server_localhost.py](https://github.com/user-attachments/files/26132755/server_localhost.py)\n\n* [client_localhost.py](https://github.com/user-attachments/files/26132757/client_localhost.py)\n\n* [flood_localhost.py](https://github.com/user-attachments/files/26132760/flood_localhost.py)\n\n\n---\n\n## PoC # 1 - Localhost verification server\n\n**File:** [server_localhost.py](https://github.com/user-attachments/files/26132755/server_localhost.py)\n\n**Purpose:** real HTTP endpoint (`POST /verify`) that calls PyJWT detached verification and prints:\n`ok / time_ms / peak_bytes / token_len / error`.\n\n### Results (server console output)\n\n```text\n[+] Listening on http://127.0.0.1:8000\n[+] POST /verify  JSON: {\"token\": \"...\"}\n\n[127.0.0.1] ok=True  time_ms=0.102 peak_bytes=2624     token_len=117      err=None\n[127.0.0.1] ok=False time_ms=2.012 peak_bytes=2000983  token_len=500078   err=InvalidSignatureError\n[127.0.0.1] ok=True  time_ms=1.591 peak_bytes=2001061  token_len=500117   err=None\n\n[127.0.0.1] ok=True  time_ms=0.065 peak_bytes=2304     token_len=117      err=None\n[127.0.0.1] ok=False time_ms=7.534 peak_bytes=8000983  token_len=2000078  err=InvalidSignatureError\n[127.0.0.1] ok=True  time_ms=6.347 peak_bytes=8001061  token_len=2000117  err=None\n\n[127.0.0.1] ok=True  time_ms=0.066 peak_bytes=2304     token_len=117      err=None\n[127.0.0.1] ok=False time_ms=23.034 peak_bytes=32000983 token_len=8000078 err=InvalidSignatureError\n[127.0.0.1] ok=True  time_ms=22.097 peak_bytes=32001061 token_len=8000117 err=None\n```\n\n**Key takeaways from these results**\n\n* At **8,000,000 chars**, a single invalid-signature request still causes:\n\n  * **~23 ms** server work\n  * **~32 MB** peak allocations\n  * returns **401** (invalid signature) → attacker does not need key.\n\n---\n\n## PoC # 2 - Localhost network client\n\n**File:** [client_localhost.py](https://github.com/user-attachments/files/26132757/client_localhost.py)\n**Purpose:** generates baseline + (invalid signature) + (valid signature) tokens and sends them over HTTP to localhost server.\n\n### Results (client output)\n\n#### payload-chars = 500,000\n\n```text\n=== BASELINE (valid b64=false token) ===\nHTTP: 200\nclient_wall_ms: 6.3499...\nserver_time_ms: 0.10197...\nserver_peak_bytes: 2624\n\n=== ATTACK (INVALID signature - attacker needs no key) ===\nHTTP: 401\nclient_wall_ms: 4.1010...\nserver_time_ms: 2.01217...\nserver_peak_bytes: 2000983\nerror: InvalidSignatureError\n\n=== ATTACK (VALID signature - accepted path still wastes) ===\nHTTP: 200\nclient_wall_ms: 3.6586...\nserver_time_ms: 1.59092...\nserver_peak_bytes: 2001061\n```\n\n#### payload-chars = 2,000,000\n\n```text\n=== BASELINE ===\nHTTP: 200\nserver_time_ms: 0.06527...\nserver_peak_bytes: 2304\n\n=== ATTACK (INVALID signature) ===\nHTTP: 401\nserver_time_ms: 7.53430...\nserver_peak_bytes: 8000983\n\n=== ATTACK (VALID signature) ===\nHTTP: 200\nserver_time_ms: 6.34682...\nserver_peak_bytes: 8001061\n```\n\n#### payload-chars = 8,000,000\n\n```text\n=== BASELINE ===\nHTTP: 200\nserver_time_ms: 0.06573...\nserver_peak_bytes: 2304\n\n=== ATTACK (INVALID signature) ===\nHTTP: 401\nserver_time_ms: 23.03403...\nserver_peak_bytes: 32000983\n\n=== ATTACK (VALID signature) ===\nHTTP: 200\nserver_time_ms: 22.09702...\nserver_peak_bytes: 32001061\n```\n\n**Why this is strong evidence**\n\n* The server clearly does heavy work **before** rejecting invalid signatures.\n* The “valid signature” case shows even accepted requests waste resources due to unused payload segment.\n\n---\n\n## PoC # 3 - Localhost flood / burst concurrency\n\n**File:** [flood_localhost.py](https://github.com/user-attachments/files/26132760/flood_localhost.py)\n**Purpose:** sends **N concurrent** invalid-signature requests over HTTP to demonstrate queueing/worker starvation.\n\n### Results (your run: 20 concurrent @ 8,000,000 chars)\n\n```text\ntotal_wall_ms: 1374.5405770000616\n\n(16, 401, 1156.4504789998864, 21.350951999920653, 32000983, 'InvalidSignatureError')\n(19, 401, 1151.2852699997893, 21.208721999755653, 32000983, 'InvalidSignatureError')\n(18, 401, 1102.7211239997996, 21.685218999664357, 32000983, 'InvalidSignatureError')\n(13, 401, 1102.0718189997751, 21.26572200040755, 32000983, 'InvalidSignatureError')\n(11, 401, 1095.9345460000804, 20.586017000368884, 32000983, 'InvalidSignatureError')\n(17, 401, 1085.2552810001725, 22.893039000337012, 32000983, 'InvalidSignatureError')\n(10, 401, 1078.3629560000918, 22.737160999895423, 32000983, 'InvalidSignatureError')\n(7,  401, 1048.2011740000416, 22.476282000297942, 32000983, 'InvalidSignatureError')\n(8,  401, 378.93017700025666, 21.377330999712285, 32000983, 'InvalidSignatureError')\n(1,  401, 281.45106800002395, 21.34223099983501, 32000983, 'InvalidSignatureError')\n```\n\n**Interpretation**\n\n* Each request still costs ~**20–23 ms** server processing and **~32 MB** peak allocations.\n* But client-observed latency rises up to **~1.15 seconds** because requests queue behind each other → clear worker starvation/HoL blocking.\n* All were rejected with **401 InvalidSignatureError** → still unauthenticated.\n\n---\n\n# Fix \n\n### Goal\n\nPrevent unbounded resource consumption from an attacker-controlled payload segment that is unused in `b64=false` detached flow.\n\n### Minimal change strategy\n\nIn `_load()` (or by refactoring parse order), **do not Base64-decode `payload_segment` until after you know whether `b64=false` applies**.\n\nTwo safe options:\n\n1. **Reject non-empty payload segment when `b64=false`**\n\n   * Parse header first\n   * If `b64` is false and `payload_segment` is non-empty → raise `DecodeError` *before* decoding\n   * Then verification uses `detached_payload` only\n\n2. **Skip decoding payload segment entirely when `b64=false`**\n\n   * Keep payload segment as raw bytes or empty\n   * Use detached payload for signing input\n\nThis aligns with the idea that detached payload is the trusted payload input for verification; the compact payload segment should not become a resource amplification vector.\n\n(Implementation context: the current decode order and unconditional `base64url_decode(payload_segment)` are visible in the file and line region around `_load()` and `decode_complete()` ([GitHub][1]).)\n\n---\n\n# Workarounds\n\n* Enforce strict **max token length** at the HTTP boundary (proxy/gateway).\n* Apply rate limiting on verification endpoints.\n* If detached JWS (`b64=false`) is not needed in your app, reject tokens where header includes `\"b64\": false`.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "pyjwt"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.8.0"
      }, {
        "fixed" : "2.13.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.12.1"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/jpadilla/pyjwt/security/advisories/GHSA-w7vc-732c-9m39"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-48525"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/jpadilla/pyjwt"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/pyjwt/PYSEC-2026-178.yaml"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-400" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-15T19:29:12Z",
    "nvd_published_at" : "2026-05-28T16:16:29Z"
  }
}