CVE-2026-77463VulnerabilityPublished 9 Nov 2026

github.com/umputun/remark42/backend · Remark42: Rate limit bypass via unconditional trust of X-Forwarded-For/X-Real-IP headers in direct-internet deployment

Affects github.com/umputun/remark42/backendSource GCVE Database

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Authoritative record:CVE-2026-77463· Medium 6.5

Not known to be exploited · CVSS 6.5 Medium : upgrade to 1.16.2 when convenient.

Data as of 9 Nov 2026, 23:24 UTC

CVSS 3.1 base score

6.5Medium

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

Thin evidence

One scorer or none

CVSS scores by source
CNA (GitHub_M)	CVSS 3.1	6.5

EPSS: 0.2% chance of exploitation in 30 days – higher than 12% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 1.16.2

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: Low
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: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
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: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on 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 5 Nov 2026
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 9 Nov 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 github.com/umputun/remark42/backend to 1.16.2.

Affected software

Product	Affected	Fixed in
Gogithub.com/umputun/remark42/backend	Affected >=1.0.0 <1.16.2	Fixed in 1.16.2
Gogithub.com/umputun/remark42	Affected >=1.0.0 <=1.2.0	Fixed in–

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

Description

### Summary Remark42 applied chi's `middleware.RealIP` globally, trusting `True-Client-IP` / `X-Real-IP` / `X-Forwarded-For` headers without verifying the request came from a trusted proxy. The per-IP rate limiters key on the overwritten `r.RemoteAddr`, so any client can bypass every per-IP rate limiter by rotating spoofed IP headers. The same spoofed IP also drives other per-IP controls (stored comment IP, anonymous identity derivation, and `--votes-ip` vote de-duplication). This affects deployments reachable directly from the internet (Remark42's built-in HTTP, static TLS, and ACME modes are all documented and supported), as well as any proxied deployment where the proxy forwards an attacker-controlled recognised header. ### Details The router applies `middleware.RealIP` globally, ahead of the per-route limiters, without any check that the server is behind a trusted proxy (in v1.16.1, `backend/app/rest/api/rest.go:207`). The `rateLimiter` function (v1.16.1, `backend/app/rest/api/rest.go:769-779`) builds a tollbooth limiter with `IPLookup{Name: "RemoteAddr"}`, which reads `r.RemoteAddr`. Since chi's `middleware.RealIP` (v1.16.1, `backend/vendor/github.com/go-chi/chi/v5/middleware/realip.go:42-55`) unconditionally overwrites `r.RemoteAddr` from `True-Client-IP > X-Real-IP > first X-Forwarded-For` (accepting any syntactically valid IP), any client can set its apparent IP to an arbitrary value. This bypasses every per-IP rate limiter that runs after `RealIP`: `/auth` (2/s), avatars (100/s), the image proxy (10/s), open / protected / admin route groups (10/s), root routes (50/s), and `/web/*` (20/s). Comment and vote creation go through the update limiter, whose server default is `UPDATE_LIMIT=0.5`/s (burst 1), not 10/s. The global 1,000 concurrent-request throttle still applies, so this is a per-IP-limit bypass rather than truly unlimited throughput. chi's own documentation warns that `middleware.RealIP` should be used only behind a trusted proxy. ### PoC ```bash # Deploy Remark42 reachable directly from the internet (built-in HTTP / TLS / ACME, no proxy that overwrites the headers). # Comment creation is limited to UPDATE_LIMIT (default 0.5/s, burst 1) per IP. # Rotate X-Real-IP across many requests; each distinct value gets its own token bucket. for i in $(seq 1 100); do curl -s -o /dev/null -w '%{http_code}\n' \ -H "X-Real-IP: 192.0.2.$i" \ -X POST 'https://remark42.example.com/api/v1/comment?site=site1' \ -H 'Authorization: Bearer YOUR_JWT' \ -H 'Content-Type: application/json' \ -d '{"text":"spam","locator":{"site":"site1","url":"https://example.com/post"}}' done | sort | uniq -c # With rotating headers the responses stay 201; without them, requests beyond the burst return 429. ``` ### Impact By rotating the spoofed client IP an attacker bypasses the per-IP token buckets on all rate-limited endpoints and defeats other per-IP controls, enabling: (a) spam comment creation beyond the configured per-IP limit, (b) confirmation-email / notification flooding to supplied addresses, (c) high-volume outbound image fetching and resource consumption via the image proxy (its transport still blocks private/reserved destinations and DNS rebinding, so this is amplification, not new SSRF reach), (d) authentication-endpoint abuse, (e) where `--votes-ip` is enabled, defeating per-IP vote de-duplication (ballot stuffing), and (f) minting fresh anonymous identities and poisoning the stored comment IP and request logs, since the anonymous user ID is derived from `name + RemoteAddr`. The global concurrency ceiling and per-request timeouts remain, so availability impact is limited. ### Resolution Mitigation support was added in v1.16.2 (umputun/remark42#2116, follow-up in #2118). **Upgrading alone does not remediate the issue: `--trusted-proxy` must be configured.** The fix adds `--trusted-proxy` / `TRUSTED_PROXY`. When set, the forwarding headers (`X-Real-IP`, then `CF-Connecting-IP`, then the leftmost public `X-Forwarded-For`) are honoured only when the direct TCP peer falls inside a trusted-proxy network; for a request from any other peer those headers are dropped and the real socket IP is used. When it is **unset**, the previous trust-all behaviour is preserved for backward compatibility and a startup warning is logged, so an un-configured v1.16.2 remains exploitable. `True-Client-IP` is no longer recognised. `TRUSTED_PROXY` takes a comma-separated list; on the command line, repeat `--trusted-proxy` per entry. **Required configuration:** - **Reachable directly from the internet:** set a value that no real client matches so the forwarding headers are always ignored, e.g. `--trusted-proxy=127.0.0.1/32` (only when Remark42 is not actually reached over loopback). - **Behind a reverse proxy:** point `--trusted-proxy` at the proxy's network only, block direct access to Remark42, and make the proxy **overwrite** `X-Real-IP` with the real client and discard client-supplied forwarding headers. A proxy

CWE-348Use of Less Trusted Source (opens in a new tab)
Base weakness
The product has two different sources of the same data or information, but it uses the source that has less support for verification, is less trusted, or is less resistant to attack.
OWASP Cheat Sheets
XML Security (opens in a new tab)
REST Security (opens in a new tab)
JSON Web Token for Java (opens in a new tab)
REST Assessment (opens in a new tab)
Cross-Site Request Forgery Prevention (opens in a new tab)
OWASP ASVS 4.0.3
V13.3.2 (opens in a new tab)
V3.5.3 (opens in a new tab)
V13.2.6 (opens in a new tab)
Attack patterns
Cache Poisoning (opens in a new tab)
DNS Cache Poisoning (opens in a new tab)
User-Controlled Filename (opens in a new tab)
Manipulating Web Input to File System Calls (opens in a new tab)
AJAX Footprinting (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).
CWE-807Reliance on Untrusted Inputs in a Security Decision (opens in a new tab)
Base weaknessLikelihood of exploit: HighOWASP Top 10 A06:2025: Insecure Design (opens in a new tab)
The product uses a protection mechanism that relies on the existence or values of an input, but the input can be modified by an untrusted actor in a way that bypasses the protection mechanism.
Architecture and Design · Attack Surface Reduction
Store state information and sensitive data on the server side only.
Architecture and Design · Libraries or Frameworks
Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
Architecture and Design
For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks…
OWASP Cheat Sheets
Secure Design Principles (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

WeaknessesCWE-348, CWE-807

Technical details

References

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