VulnerabilityPublished 15 Jul 2026

CVE-2026-52869: MCP Python SDK · HTTP transports serve session requests without verifying the authenticated principal

Affects snyk-agent-scanSource GCVE Database

Authoritative record:CVE-2026-52869· High 7.1

Not known to be exploited · CVSS 7.1 High : upgrade to 0.5.17 soon.

Data as of 26 Oct 2026, 11:07 UTC

CVSS 3.1 base score

7.1High

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

EPSS: 0.5% chance of exploitation in 30 days – higher than 44% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 0.5.17

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

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

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: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
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: 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.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 15 Jul 2026
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

Upgrade snyk-agent-scan to 0.5.17.

Affected software

Product	Affected	Fixed in
Homebrewsnyk-agent-scan`pkg:brew/snyk-agent-scan`Other threats	Affected >=0.4.3 <0.5.17	Fixed in 0.5.17

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

AI-suggested

Upgrade the MCP Python SDK to version 1.27.2 or later, which implements verification of the authenticated principal for each session and rejects requests with mismatched principals. For deployments where multiple end users share a single OAuth client, ensure the token verifier populates the AccessToken.subject to isolate sessions per user. Custom authentication backends should implement equivalent principal verification. No other immediate actions are required if the upgrade is applied.

Description

A vulnerability in the MCP Python SDK HTTP transports (SSE and Streamable HTTP in stateful mode) allows session hijacking by bypassing authentication checks. Incoming requests are routed to existing sessions based solely on session IDs without verifying the authenticated principal. This enables an attacker who knows or guesses a session ID to send JSON-RPC messages on that session regardless of the bearer token used. The issue affects versions from 0.4.3 up to but not including 0.5.17. A fix is available in version 1.27.2 or later.

Impact

The vulnerability allows an attacker who obtains or guesses a valid session ID to bypass authentication isolation and send messages on behalf of the session owner. This compromises confidentiality and integrity of session data and operations. The impact is high due to the ability to impersonate authenticated clients within the session context. However, exploitation requires out-of-band access to session IDs, which are UUIDs. Servers not using bearer-token authentication or stateless Streamable HTTP transports are not affected.

Summary

The MCP Python SDK's SSE and Streamable HTTP server transports route incoming requests to existing sessions based only on the session identifier, without verifying that the request is authenticated as the same principal that created the session. This flaw allows an attacker who obtains a session ID to inject JSON-RPC messages into that session, bypassing per-client authentication isolation. The SSE transport has been affected since the first release, and the Streamable HTTP transport since version 1.8.0. The vulnerability requires the server to use bearer-token authentication and stateful HTTP transports. The patch in version 1.27.2 enforces verification of the authenticated principal against the session creator, returning a 404 response if they differ.

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 22 Oct 2026, 17:28 UTC

Technical details

References

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