Report · Reddit NetSecAbout CVE-2025-55315Published 10 Nov 2025

CVE-2025-55315: Kestrel · HTTP Request Smuggling

Source Reddit NetSec

Authoritative record:CVE-2025-55315· Critical 9.9

Public exploit available (Exploit-DB) · 66% EPSS · CVSS 9.9 Critical.

Data as of 26 Oct 2026, 11:42 UTC

CVSS 3.1 base score

9.9Critical

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

EPSS: 66.5% chance of exploitation in 30 days – higher than 99% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.

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

Base metrics

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

Temporal metrics

Exploit Code Maturity: Unproven
Remediation Level: Official Fix
Report Confidence: Confirmed

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: Changed: An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.
Exploit Code Maturity: Unproven: No exploit code is available, or an exploit is theoretical.
Remediation Level: Official Fix: A complete vendor solution is available. Either the vendor has issued an official patch, or an upgrade is available.
Report Confidence: Confirmed: Detailed reports exist, or functional reproduction is possible (functional exploits may provide this). Source code is available to independently verify the assertions of the research, or the author or vendor of the affected code has confirmed the presence of the vulnerability.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

t = 0 is the CVE’s publication on cve.org

Disclosed: evidence found.
CVE published 14 Oct 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: evidence found.
1 Exploit-DB entry, 174 days after disclosure
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

CVE ID reserved
by Microsoft Corporation
12 Aug 202520:19 UTC, 63 days before disclosure
CVE published
on cve.org
14 Oct 202517:00 UTC, t = 0
In Threat Radar
record ingested
14 Oct 202517:16 UTC, 17 minutes after disclosure
First Exploit-DB entry (opens in a new tab)
ASP.net 8.0.10 - Bypass
6 Apr 2026, 174 days after disclosure

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

AI-suggested

Organizations should prepare to apply patches from Microsoft or the ASP.NET Core team as soon as they are released to address this vulnerability in Kestrel. In the interim, network defenders should implement strict input validation and HTTP request normalization at the perimeter to detect and block suspicious chunked transfer encoding patterns. Deploying or tuning Web Application Firewalls (WAFs) to identify anomalies consistent with HTTP Request Smuggling attempts can reduce risk. Monitoring HTTP traffic logs for irregularities in chunk extensions or unexpected request sequences is critical. Organizations should also review their application architecture to minimize reliance on Kestrel as a direct internet-facing server, potentially placing it behind reverse proxies that are known to handle HTTP parsing securely. Security teams should update incident response plans to include scenarios involving HTTP Request Smuggling and conduct penetration testing focused on this vulnerability. Finally, educating developers and operations teams about the risks of HTTP Request Smuggling and secure HTTP parsing practices will help mitigate future risks.

Description

CVE-2025-55315 is a medium-severity HTTP Request Smuggling vulnerability in Kestrel, the web server used by ASP. NET Core, exploiting chunk extensions in HTTP/1.1 chunked transfer encoding. This flaw allows an attacker to manipulate how HTTP requests are parsed and forwarded, potentially enabling request splitting, bypassing security controls, or injecting malicious requests. Although no known exploits are currently in the wild, the vulnerability affects web applications relying on Kestrel, which is widely used in enterprise environments. European organizations using ASP. NET Core with Kestrel are at risk of data exposure, session hijacking, or service disruption if exploited. Mitigation requires patching Kestrel once updates are released and implementing strict input validation and web application firewalls to detect abnormal HTTP traffic. Countries with significant Microsoft and . NET adoption, such as Germany, the UK, France, and the Netherlands, are most likely to be impacted.

Impact

For European organizations, the impact of CVE-2025-55315 can be significant due to the widespread use of ASP.NET Core and Kestrel in enterprise web applications and services. Exploitation could lead to unauthorized access to sensitive data, session hijacking, and bypass of security controls, undermining confidentiality and integrity. Attackers could also disrupt service availability by injecting malformed requests or causing application errors. This threat is particularly concerning for industries handling sensitive personal data, such as finance, healthcare, and government sectors, which are heavily regulated under GDPR and other data protection laws. A successful attack could result in data breaches, regulatory fines, reputational damage, and operational downtime. Given the cross-border nature of web applications, the threat could affect multinational corporations and cloud service providers operating in Europe. The absence of known exploits currently provides a window for proactive mitigation, but the potential for rapid weaponization remains high once exploit code becomes available.

Summary

CVE-2025-55315 is a vulnerability classified as HTTP Request Smuggling affecting Kestrel, the cross-platform web server used by ASP.NET Core applications. The issue arises from improper handling of chunk extensions in HTTP/1.1 chunked transfer encoding. HTTP Request Smuggling exploits discrepancies in how front-end and back-end servers parse HTTP requests, allowing attackers to craft specially malformed requests that are interpreted differently by intermediaries and the target server. In this case, Kestrel's parsing of chunk extensions can be manipulated to smuggle requests, enabling attackers to bypass security controls such as firewalls or intrusion detection systems, poison web caches, hijack user sessions, or perform cross-site scripting and other injection attacks. The vulnerability does not require authentication or user interaction, increasing its risk profile. Although the vulnerability is rated medium severity by the source, the lack of a CVSS score and the nature of HTTP Request Smuggling suggest a potentially higher impact. No public exploits have been reported yet, but the vulnerability's presence in a widely used web server component makes it a critical concern for web-facing applications. The vulnerability was publicly disclosed in November 2025, with technical details initially shared on a Reddit NetSec post linking to a Praetorian blog analysis. No official patches or mitigation guidance have been published at the time of disclosure, emphasizing the need for immediate attention from affected organizations.

References

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Exploitation evidence

Timeline

What to do

Description

AI analysisMachine-generated, updated 10 Nov 2025, 16:01 UTC

Public exploits and detection1 Exploit-DB entry

Technical details

References

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