5.3

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2026-48525

CVE-2026-48525: PyJWT: Unauthenticated DoS via unbounded Base64URL decoding of unused payload segment in b64=false detached JWS

PyJWT is a JSON Web Token implementation in Python. From 2.8.0 to 2.12.1, when 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. For 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. This creates an unauthenticated DoS vector against any endpoint that verifies detached JWS using PyJWT. This vulnerability is fixed in 2.13.0.

(GitHub Advisory)

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CVE-2026-48525

CVE-2026-48525:

5.3

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

SSVC / BOD 26-04 ↗

Exploitation pocAutomatable YesTech Impact Partial

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Package Name	Ecosystem	Vulnerable Versions	First Patched Version
pyjwt	pip	>= 2.8.0, <= 2.12.1	2.13.0

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability is a Denial of Service (DoS) in the PyJWT library when handling detached JWS tokens with the "b64": false header. The root cause lies in the PyJWS._load function, which was called by PyJWS.decode and PyJWS.decode_complete. Before the patch, _load would unconditionally perform a Base64URL decoding of the payload segment from the JWS compact serialization. However, when b64 is false, the RFC specifies that the payload is detached and the segment in the token itself should be ignored. The library correctly discarded this decoded data later, but not before spending significant CPU and memory resources on the decoding process.

An unauthenticated attacker could exploit this by sending a JWS token with "b64": false in its header and a very large string in the payload segment. The server-side PyJWT library would attempt to decode this large, bogus payload, consuming resources and slowing down the application, which happens before the signature is even verified. Repeated requests could lead to worker thread starvation and a full DoS.

The fix, applied in version 2.13.0, modifies the _load function to first check the value of the b64 header. If it is false, the function now skips the expensive decoding step and instead asserts that the payload segment must be empty, effectively closing the resource consumption vulnerability.

Vulnerable functions

PyJWS._load

jwt/api_jws.py

This function unconditionally performed Base64URL decoding on the payload segment of a JWS token, even when the `b64=false` header indicated a detached payload where this segment is unused. An attacker could provide an arbitrarily large payload segment, causing the function to consume excessive CPU and memory before any signature verification, leading to a Denial of Service. The decoded data was subsequently discarded, making the operation a resource-wasting amplifier for the attack.

PyJWS.decode_complete

jwt/api_jws.py

This function is a primary entry point for decoding JWS tokens. It directly calls the vulnerable `_load` function before it processes the `detached_payload`. As a result, it orchestrates the sequence that leads to the unnecessary and expensive decoding of the unused payload segment, making it a key part of the vulnerable execution path.

PyJWS.decode

jwt/api_jws.py

As a public-facing convenience wrapper for `decode_complete`, this function is a common entry point for developers using the library. Any application code calling `PyJWS.decode` to verify a detached JWS token would be vulnerable, as it triggers the same flawed logic path through `decode_complete` and `_load`.

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WAF Rule

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Reasoning

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