VulnerabilityPublished 10 Oct 2025Updated 3 Nov 2025

CVE-2025-62706: Authlib · JWE zip=DEF decompression bomb enables DoS

Affects authlibSource GCVE Database

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Not known to be exploited · CVSS 6.5 Medium : upgrade to 1.6.5 when convenient.

Data as of 29 Oct 2026, 23:56 UTC

CVSS 3.1 base score

6.5Medium

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

Patch: Available – fixed in 1.6.5

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Attack Vector: Network
Attack Complexity: Low
Privileges Required: Low
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

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

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, 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: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; 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 impacted component (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 v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 10 Oct 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: 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 29 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.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), 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

Upgrade authlib to 1.6.5.

Affected software

Product	Affected	Fixed in
PyPIauthlib	Affected <1.6.5	Fixed in 1.6.5

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AI-suggested

A fix is available in Authlib version 1.6.5 that adds a maximum output size limit to DEFLATE decompression, preventing unbounded expansion. Until upgrading, applications should reject or strip JWE tokens with the 'zip=DEF' header to avoid triggering the vulnerability. Additional mitigations include enforcing strict maximum token sizes, failing fast on oversized inputs, and applying rate limiting to reduce the risk of denial of service. Forking the library to add bounded decompression is also a possible workaround.

Description

Authlib versions prior to 1.6.5 contain a vulnerability in the JWE 'zip=DEF' decompression handling that allows unbounded DEFLATE decompression. This can be exploited by an attacker who can supply decryptable tokens to cause excessive memory and CPU usage, resulting in denial of service. The issue arises because the decompression function does not enforce a maximum output size, allowing a small compressed ciphertext to expand into very large plaintext during decryption. No confidentiality or integrity impact is reported, but availability is severely affected. A fix is available in Authlib 1.6.5.

Impact

This vulnerability allows an attacker who can submit decryptable JWE tokens with the 'zip=DEF' compression header to cause denial of service by exhausting memory and CPU resources during decompression. There is no direct impact on confidentiality or integrity. The availability impact is high due to potential process or host resource exhaustion. The attack requires the ability to supply tokens that the server will decrypt, which typically requires some privilege or shared key access. If arbitrary unprivileged parties can submit such tokens, the severity increases.

Summary

Authlib's JWE implementation supports compression via the 'zip=DEF' header parameter, which triggers DEFLATE decompression of the ciphertext. The decompression is performed using zlib.decompress without any limit on the output size, enabling a decompression bomb attack. An attacker able to submit decryptable JWE tokens with crafted compressed payloads can cause the server process to consume excessive memory and CPU resources during decryption. This vulnerability affects Authlib versions before 1.6.5. The issue is confirmed by proof-of-concept code demonstrating that a few kilobytes of compressed data can decompress to tens of megabytes, causing denial of service. The vulnerability is tracked as CVE-2025-62706 with a CVSS 3.1 base score of 6.5 (medium) under typical conditions where the attacker requires some privilege to produce decryptable tokens, or 7.5 (high) if no privilege is required. The vulnerability is fixed in Authlib 1.6.5 by adding a maximum decompression output size limit.

CWE-400Uncontrolled Resource Consumption (opens in a new tab)
Class weaknessLikelihood of exploit: High
The product does not properly control the allocation and maintenance of a limited resource.
MITRE discourages mapping vulnerabilities to this broad entry; the root cause is usually a more specific weakness.
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. A strong authentication and access control model will help prevent such attacks from occurring in the first place.
Architecture and Design
Mitigation of resource exhaustion attacks requires that the target system either: recognizes the attack and denies that user further access for a given amount of time, or; uniformly throttles all requests in order to make it more difficult to consume resources more quickly than they can again be freed.
Architecture and Design
Ensure that protocols have specific limits of scale placed on them.
OWASP Cheat Sheets
File Upload (opens in a new tab)
OWASP ASVS 4.0.3
V12.1.1 (opens in a new tab)
Attack patterns
XML Ping of the Death (opens in a new tab)
Regular Expression Exponential Blowup (opens in a new tab)
Sustained Client Engagement (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).
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

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 29 Oct 2026, 23:56 UTC

WeaknessesCWE-400, CWE-770

Technical details

References

Community reviews

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