VulnerabilityPublished 18 Nov 2025

CVE-2025-65015: authlib joserfc · Unthrottled Resource Allocation

Affects authlib joserfcSource CVE Database V5

Not known to be exploited · CVSS 9.2 Critical · fixed in 1.3.5 : patch authlib joserfc as a priority.

Data as of 26 Oct 2026, 14:05 UTC

CVSS 4.0 base score

9.2Critical

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.4% chance of exploitation in 30 days – higher than 37% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 1.3.5

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: None
Vulnerable system availability: High
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: None: There is no loss of integrity within the Vulnerable System.
Vulnerable system availability: High: There is a total loss of availability, resulting in the attacker being able to fully deny access to resources in the Vulnerable System; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the Vulnerable System (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: High: There is a total loss of availability, resulting in the attacker being able to fully deny access to resources in the Subsequent System; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the Subsequent System (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 18 Nov 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Affected software

Product	Affected	Fixed in
PyPIjoserfc`pkg:pypi/joserfc`Other threats	Affected 1.3.3 – 1.4.2 4 affected builds	Fixed in–

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

Upgrade all instances of the joserfc library to versions 1.3.5 or 1.4.2 or later, which contain patches preventing full payload embedding in exception messages and enforce size limits.
Configure production-grade web servers (e.g., Nginx, Apache) or API gateways to enforce strict limits on HTTP header sizes and overall request sizes to block oversized JWT tokens before they reach the application.
Deploy Web Application Firewalls (WAFs) with rules to detect and block anomalously large bearer tokens or suspicious JWT structures.
Monitor application logs and diagnostic tools for unusually large log entries or spikes in memory/disk usage that may indicate exploitation attempts.
Review and harden logging configurations to limit maximum log message sizes and prevent logging of untrusted input in full.
Conduct security testing and code reviews to ensure no other components embed large untrusted data in exceptions or logs.
Educate development and operations teams about the risks of resource exhaustion via unbounded input processing and the importance of input validation and throttling.

Description

### Summary The `ExceededSizeError` exception messages are embedded with non-decoded JWT token parts and may cause Python logging to record an arbitrarily large, forged JWT payload. ### Details In situations where a misconfigured — or entirely absent — production-grade web server sits in front of a Python web application, an attacker may be able to send arbitrarily large bearer tokens in the HTTP request headers. When this occurs, Python logging or diagnostic tools (e.g., Sentry) may end up processing extremely large log messages containing the full JWT header during the `joserfc.jwt.decode()` operation. The same behavior also appears when validating claims and signature payload sizes, as the library raises `joserfc.errors.ExceededSizeError()` with the full payload embedded in the exception message. Since the payload is already fully loaded into memory at this stage, the library cannot prevent or reject it per se. It is therefore the responsibility of the underlying web server (`uvicorn`/`h11`, `gunicorn`, `Starlette`, `Werkzeug`, `nginx`...etc) to enforce limits on header sizes. For example, a `FastAPI`/`Starlette` application running _without_ `uvicorn` and/or `gunicorn` cannot enforce header size limits on its own. With `uvicorn`/`h11`, the [--h11-max-incomplete-event-size <int>](https://uvicorn.dev/settings/#implementation:~:text=%2D%2Dh11%2Dmax%2Dincomplete%2Devent%2Dsize%20%3Cint%3E) option can restrict the total size of the header _plus_ body, but not the header alone. Similarly, [vLLM serve](https://docs.vllm.ai/en/latest/cli/serve/#-h11-max-incomplete-event-size) —due to its reliance on `uvicorn`/`h11` and the need for heavy data transfer in ML inference workloads, sets a default limit of 4 MB for header _plus_ body and is frequently increased. In practice, a robust reverse proxy (such as `nginx`) is typically required because it can explicitly cap maximum header size. Unfortunately, many web applications do not run behind a proper reverse proxy. Given these constraints, the `joserfc` library cannot safely log or embed payloads of arbitrary size. This issue is particularly subtle, as it occurs only when a maliciously crafted JWT finally reaches the Python application, a scenario that most developers will never encounter during routine development and testing. ### PoC **Environment** Ubuntu 24.04 LTS Python 3.12 Tested on `joserfc` version `1.4.1` ```python import logging from datetime import UTC, datetime, timedelta from joserfc import jwt from joserfc.errors import ExceededSizeError, UnsupportedAlgorithmError from joserfc.jwk import OctKey logger = logging.getLogger(__name__) SECRET_KEY = "8c13bd66babc241b29f8553429bdab7deb6f5b74ddfda7765471e57ecd55641e" LONG_JWT_TOKEN = ( "eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NmRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRSUzI1NmRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRSUzI1NmRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRSUzI1NmRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRSUzI1NmRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRSUzI1NmRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGQifQ" "." "eyJpc3MiOiJhdXRoX3NlcnZlciIsImlhdCI6MTc2MzI0OTEwMSwiZXhwIjoxNzY5MjQ5MTAxfQ" "." "6-k2jmkGXD6wXOgYgjPS8E5lS_GjWpgIuY54gokjAn8" ) HEADER = { "alg": ( "RS256dddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd" "RS256dddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd" "RS256dddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd" "RS256dddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd" "RS256dddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd" "RS256dddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd" ), } CLAIMS = { "iss": "auth_server", "iat": datetime.now(UTC), "exp": datetime.now(UTC) + timedelta(minutes=15), } def main(): # Create OctKey from SECRET_KEY key = OctKey.import_key(SECRET_KEY) # Simulate creating a very large JWT # (this will fail with joserfc.errors.UnsupportedAlgorithmError # due to an invalid 'alg' header content try: token = jwt.encode(HEADER, CLAIMS, key) except UnsupportedAlgorithmError: # Use a forged token that has the same header and claims instead # but an invalid signature token = LONG_JWT_TOKEN logger.warning(f"Created JWT: {token}") # Now try to decode the large JWT try: decoded_token = jwt.decode(token, key) logger.warning("This line will never be reached.") logger.warning(decoded_token.claims) except ExceededSizeError: logger.exception( "The JWT size is too large and may be a security attack attempt." ) # this i

Impact

For European organizations, this vulnerability poses a significant risk of denial of service attacks against Python web applications that rely on vulnerable joserfc versions for JWT processing. Attackers can exploit this flaw to overwhelm application memory and logging infrastructure by sending oversized JWT bearer tokens, potentially causing application crashes, degraded performance, or loss of availability. This can disrupt critical services, especially in sectors like finance, healthcare, and government where JWT-based authentication is common. Additionally, logging systems may become unreliable or corrupted due to excessively large log entries, complicating incident response and monitoring. Organizations with inadequate front-end web server configurations that do not enforce request size limits are particularly vulnerable. The lack of authentication or user interaction requirements makes exploitation easier and increases the attack surface. The vulnerability could also be leveraged as part of multi-stage attacks to distract defenders or degrade security monitoring capabilities.

Summary

CVE-2025-65015 is a critical resource exhaustion vulnerability in the joserfc Python library, which implements JSON Object Signing and Encryption (JOSE) standards. The flaw exists in versions 1.3.3 through before 1.3.5 and 1.4.0 through before 1.4.2. When decoding JWT tokens, if a token exceeds expected size limits, the library raises an ExceededSizeError exception embedding the entire JWT payload in the exception message. Since the payload is already fully loaded into memory during the decode operation, the library cannot prevent processing of arbitrarily large tokens. In scenarios where a Python web application uses joserfc behind a misconfigured or absent production-grade web server, an attacker can send oversized bearer tokens in HTTP headers. These large tokens cause Python logging frameworks or diagnostic tools like Sentry to process and store extremely large log messages containing the full JWT header and payload. This can lead to excessive memory consumption, disk usage, and potential denial of service conditions. The vulnerability does not require authentication or user interaction and can be exploited remotely. The issue was patched in joserfc versions 1.3.5 and 1.4.2 by preventing embedding of full payloads in exception messages and adding proper size checks. No known exploits are currently reported in the wild, but the high CVSS 4.0 score of 9.2 reflects the critical risk posed by this vulnerability.

CWE-770Allocation of Resources Without Limits or Throttling (opens in a new tab)
Base weaknessLikelihood of exploit: HighCWE Top 25 (2025): #25
The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.
Requirements
Clearly specify the minimum and maximum expectations for capabilities, and dictate which behaviors are acceptable when resource allocation reaches limits.
Architecture and Design
Limit the amount of resources that are accessible to unprivileged users. Set per-user limits for resources. Allow the system administrator to define these limits. Be careful to avoid CWE-410.
Architecture and Design
Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended.
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
OWASP ASVS 4.0.3
V8.1.4 (opens in a new tab)
V13.4.1 (opens in a new tab)
V11.1.4 (opens in a new tab)
Attack patterns
XML Ping of the Death (opens in a new tab)
Exponential Data Expansion (opens in a new tab)
Serialized Data Parameter Blowup (opens in a new tab)
Serialized Data with Nested Payloads (opens in a new tab)
Oversized Serialized Data Payloads (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 26 Nov 2025, 00:16 UTC

WeaknessesCWE-770

Technical details

References

Community reviews

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