Skip to content
Noroxi
CVE-2026-52845· NVD / CVE Program· CNA GitHub_M

Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`

Caddy is an extensible server platform that uses TLS by default. Prior to 2.11.4, forward_auth copy_headers deletes the exact client-supplied identity header before copying the trusted value from the auth gateway. But when the request later goes through php_fastcgi, Caddy normalizes HTTP headers into CGI variables by replacing - with _. This lets a client send an underscore alias that survives the forward_auth delete step but becomes the same PHP/FastCGI variable. Result: a remote client can inject or sometimes override identity/group headers trusted by PHP/FastCGI applications behind Caddy. This vulnerability is fixed in 2.11.4.

HighCVSS 8.1 · v3.1—No exploit Fix available
Published
Jun 23, 2026
Updated
Jul 15, 2026
EPSS
0.4% · 37th percentile
Follow this CVE

Sign in to follow · You’ll be notified if a followed record enters KEV, gets an exploit or is updated.

Report tools

JSON

Action score

32

Monitor

Low priority for now.

CVSS
32 / 40 · 8.1 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.4%

CISA SSVC decision

Exploitation
proof of concept
Automatable
no
Technical impact
total

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · GitHub_M
8.1
NVD has not scored this yet; the score shown is the CNA’s.

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Noroxi analysis

No Noroxi analysis for this record yet

We don't hand-write analysis for the hundreds of thousands of vulnerabilities in the database; that wouldn't be honest. For notable, high-impact vulnerabilities our team writes the mechanism, detection and remediation steps.

We use this product, ask for help

Affected systems

VendorProduct
caddyservercaddy

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • caddyserver caddybefore 2.11.4

Versions reported by the vendor

Affected version ranges reported by the assigning authority (GitHub_M). Independent of NVD's CPE analysis and usually ahead of it.

  • caddyserver caddy

    • < 2.11.4affected

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
Debian:12caddyall versions—
Debian:13caddybefore 2.6.2-12+deb13u12.6.2-12+deb13u1
Debian:14caddybefore 2.11.4-12.11.4-1
Gogithub.com/caddyserver/caddyup to and including 1.0.5—
Gogithub.com/caddyserver/caddy/v2before 2.11.42.11.4

Same product

caddyserver: all records

Other highest-scoring records for the same primary product.

  • CVE-2018-21246Caddy before 0.10.13 mishandles TLS client authentication, as demonstrated by an authentication bypass caused by the lack of the StrictHostM
    40Plan
  • CVE-2026-30851Caddy forward_auth copy_headers Does Not Strip Client-Supplied Headers, Allowing Identity Injection and Privilege Escalation
    35Monitor
  • CVE-2026-27590Caddy: Unicode case-folding length expansion causes incorrect split_path index (SCRIPT_NAME/PATH_INFO confusion) in FastCGI transport
    35Monitor
  • CVE-2026-27586Caddy's mTLS client authentication silently fails open when CA certificate file is missing or malformed
    35Monitor
  • CVE-2026-45135Caddy: Unsafe Unicode Handling in FastCGI splitPos Allows Execution of Non-PHP Files
    32Monitor
  • CVE-2026-52844Caddy: Windows `file_server` path authorization bypass via encoded backslash
    30Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
caddyserver caddy2.11.4 · descriptionVendor (CNA)
debian:caddy2.6.2-12+deb13u1 · Debian:13Package registry (OSV)
Go:github.com/caddyserver/caddy/v22.11.4Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

  • CVE-2026-30851108 days apartCaddy forward_auth copy_headers Does Not Strip Client-Supplied Headers, Allowing Identity Injection and Privilege Escalation
    35Monitor

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • A low-privileged account is enough.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
high · data can be read
Integrity
high · data or configuration can be modified
Availability
none
Attack vector
Network
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Unchanged
Confidentiality impact
High
Integrity impact
High
Availability impact
None

Weakness class (CWE)

CWE-287 · Improper Authentication

CVSS vector

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

nvd-secondary

Root cause

Authentication is added route by route instead of being enforced in one place. When a route added later bypasses the shared middleware, the session check never runs for it.

The bug in code

The whole CWE-287 class

A representative example of this vulnerability class. Not the vendor's source code; it shows the faulty pattern and its fix.

Vulnerable

ts
app.use("/admin", requireSession, adminRouter);
// Route added later skips the middlewareapp.post("/admin-api/config", updateConfig);

Fixed

ts
const admin = express.Router();admin.use(requireSession, requireRole("admin"));
admin.post("/config", updateConfig);app.use("/admin", admin);

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records