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CVE-2026-48788

Remark42: Cross-Site Scripting (XSS) on /api/v1/img via content-type spoofing

8.2 HighFix available

Remark42 is a self-hosted comment engine for blogs, articles, or any other place where readers can add comments. Versions 1.6.0 through 1.15.0 contain a Cross-Site Scripting (XSS) vulnerability exploitable through content-type spoofing. The Remark42 image proxy fetches an arbitrary remote URL and re-serves the response from Remark42's own origin. During the download phase, the proxy determines whether the resource is an image by inspecting only the Content-Type header advertised by the remote server, never examining the actual bytes; during the serving phase, it instead derives the response Content-Type by sniffing those bytes with http.DetectContentType. An attacker can exploit this inconsistency by hosting a URL that advertises Content-Type: image/png while returning an HTML/JavaScript body: the download check accepts it as an image, the serving path sniffs the body and emits Content-Type: text/html, and the browser renders the attacker-controlled HTML/JavaScript as a document within Remark42's origin. Exploitation requires no Remark42 account on the target instance; the attacker only needs to host the malicious upstream URL and deliver the proxy link to a victim by any means, such as email, direct message, or a link on another website. This issue has been fixed in version 1.16.0.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
None
User interaction
Required

What an attacker can affect

Confidentiality
High
Integrity
Low
Availability
None

Exploit likelihood

0.31%Unchanged since Jun 17, 2026
90%50%10%1%

What each source reports

CVE List Score used 8.2

Remark42: Cross-Site Scripting (XSS) on /api/v1/img via content-type spoofing

Remark42 is a self-hosted comment engine for blogs, articles, or any other place where readers can add comments. Versions 1.6.0 through 1.15.0 contain a Cross-Site Scripting (XSS) vulnerability exploitable through content-type spoofing. The Remark42 image proxy fetches an arbitrary remote URL and re-serves the response from Remark42's own origin. During the download phase, the proxy determines whether the resource is an image by inspecting only the Content-Type header advertised by the remote server, never examining the actual bytes; during the serving phase, it instead derives the response Content-Type by sniffing those bytes with http.DetectContentType. An attacker can exploit this inconsistency by hosting a URL that advertises Content-Type: image/png while returning an HTML/JavaScript body: the download check accepts it as an image, the serving path sniffs the body and emits Content-Type: text/html, and the browser renders the attacker-controlled HTML/JavaScript as a document within Remark42's origin. Exploitation requires no Remark42 account on the target instance; the attacker only needs to host the malicious upstream URL and deliver the proxy link to a victim by any means, such as email, direct message, or a link on another website. This issue has been fixed in version 1.16.0.

Record
CVE-2026-48788
CVSS 3.0
8.2 HighCVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N
Weakness
CWE-79, CWE-436
Updated
2026-11-02
EUVDEUVD-2026-37511 8.2

Remark42 is a self-hosted comment engine for blogs, articles, or any other place where readers can add comments. Versions 1.6.0 through 1.15.0 contain a Cross-Site Scripting (XSS) vulnerability exploitable through content-type spoofing. The Remark42 image proxy fetches an arbitrary remote URL and re-serves the response from Remark42's own origin. During the download phase, the proxy determines whether the resource is an image by inspecting only the Content-Type header advertised by the remote server, never examining the actual bytes; during the serving phase, it instead derives the response Content-Type by sniffing those bytes with http.DetectContentType. An attacker can exploit this inconsistency by hosting a URL that advertises Content-Type: image/png while returning an HTML/JavaScript body: the download check accepts it as an image, the serving path sniffs the body and emits Content-Type: text/html, and the browser renders the attacker-controlled HTML/JavaScript as a document within Remark42's origin. Exploitation requires no Remark42 account on the target instance; the attacker only needs to host the malicious upstream URL and deliver the proxy link to a victim by any means, such as email, direct message, or a link on another website. This issue has been fixed in version 1.16.0.

Record
EUVD-2026-37511
CVSS 3.0
8.2 HighCVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N
Updated
2026-11-02
GitHub AdvisoriesGHSA-4c8j-mgm4-qqvp 8.2

Remark42: Cross-Site Scripting (XSS) on /api/v1/img via content-type spoofing

Summary

The remark42 image proxy fetches an arbitrary remote URL and re-serves the response from remark42's own origin. The download path decides whether the fetched resource is an image by looking only at the Content-Type header the remote server claims — it never inspects the actual bytes. The serving path then derives the response Content-Type by sniffing those bytes with http.DetectContentType.

An attacker hosts a URL that sets Content-Type to image/png but returns an HTML/JavaScript body:

  • the download check sees image/png → accepts it;
  • the serve path sniffs the body → emits Content-Type: text/html;
  • the browser renders attacker HTML/JS as a document in remark42's origin.

Details

Downloader

backend/app/rest/proxy/image.go — downloadImage(), lines 189-206:

contentType := resp.Header.Get("Content-Type")
if !strings.HasPrefix(contentType, "image/") {
    return nil, fmt.Errorf("invalid content type %s", contentType)
}

maxSize := 5 * 1024 * 1024 // 5MB default
if p.ImageService != nil && p.ImageService.MaxSize > 0 {
    maxSize = p.ImageService.MaxSize
}
lr := io.LimitReader(resp.Body, int64(maxSize)+1)
imgData, err := io.ReadAll(lr)
if err != nil {
    return nil, fmt.Errorf("unable to read image body: %w", err)
}
if len(imgData) > maxSize {
    return nil, fmt.Errorf("image is too large")
}
return imgData, nil          // <-- bytes never validated, returned as-is

Send Content-Type: image/png and the check passes regardless of what the body actually contains.

Server

backend/app/rest/proxy/image.go — Handler(), line 131:

w.Header().Add("Content-Type", p.ImageService.ImgContentType(img))
_, err = io.Copy(w, bytes.NewReader(img))

backend/app/store/image/image.go — ImgContentType(), lines 242-249:

func (s *Service) ImgContentType(img []byte) string {
    contentType := http.DetectContentType(img)
    if contentType == "application/octet-stream" {
        return "image/*"
    }
    return contentType                 // <-- returns text/html for an HTML body
}

PoC

self.send_response(200)
self.send_header("Content-Type", "image/png")
self.send_header("Content-Length", str(len(body)))
self.end_headers()
self.wfile.write(body)            # body = <!DOCTYPE html><script>...</script>

Then have the victim open https://<remark42-host>/api/v1/img?src=<base64(attacker-host)> top-level.

Impact

  • The script can issue authenticated, same-origin API calls with credentials: 'include' — the JWT cookie is sent automatically.
  • The script can read the XSRF-TOKEN cookie and re-send it as the X-XSRF-TOKEN header, defeating CSRF protection. The attacker acts as the victim: delete/edit their comments, change their settings, and — if the victim is admin — perform admin actions.

Triggering requires no remark42 account on the target instance; the attacker only needs to host the malicious upstream URL and deliver the proxy link to a victim by any means (email, DM, link on another site, etc.).

Fix

v1.16.0 adds layered defense to /api/v1/img and /api/v1/picture/{user}/{id}:

  • rest.SafeImgContentType validates sniffed body bytes against a strict allowlist (image/png, image/jpeg, image/gif, image/webp, image/bmp, image/x-icon). Non-image content returns 415 with no body echo. SVG is implicitly excluded.
  • Every response carries Content-Security-Policy: default-src 'none'; sandbox; frame-ancestors 'none', X-Content-Type Options: nosniff, and Content-Disposition: inline; filename="image".
  • The ETag is bumped to "v2:<base64(src)>". Browsers that revalidate cached pre-fix responses get a fresh validated 200 instead of a 304 against the poisoned cached entry.
  • The strict default-src 'none'; sandbox CSP also applies to all /api/v1/* routes as defense-in-depth.

Residual exposure

Browser-local caches that already hold a pre-fix text/html response with `Cache-Control:…

Record
GHSA-4c8j-mgm4-qqvp
CVSS 3.0
8.2 HighCVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N
Weakness
CWE-79, CWE-436
Updated
2026-11-02
NVD 8.2

Remark42 is a self-hosted comment engine for blogs, articles, or any other place where readers can add comments. Versions 1.6.0 through 1.15.0 contain a Cross-Site Scripting (XSS) vulnerability exploitable through content-type spoofing. The Remark42 image proxy fetches an arbitrary remote URL and re-serves the response from Remark42's own origin. During the download phase, the proxy determines whether the resource is an image by inspecting only the Content-Type header advertised by the remote server, never examining the actual bytes; during the serving phase, it instead derives the response Content-Type by sniffing those bytes with http.DetectContentType. An attacker can exploit this inconsistency by hosting a URL that advertises Content-Type: image/png while returning an HTML/JavaScript body: the download check accepts it as an image, the serving path sniffs the body and emits Content-Type: text/html, and the browser renders the attacker-controlled HTML/JavaScript as a document within Remark42's origin. Exploitation requires no Remark42 account on the target instance; the attacker only needs to host the malicious upstream URL and deliver the proxy link to a victim by any means, such as email, direct message, or a link on another website. This issue has been fixed in version 1.16.0.

Record
CVE-2026-48788
CVSS 3.0
8.2 HighCVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N
Weakness
CWE-79, CWE-436
Updated
2026-11-02
OSV 8.2

Remark42: Cross-Site Scripting (XSS) on /api/v1/img via content-type spoofing

Remark42 is a self-hosted comment engine for blogs, articles, or any other place where readers can add comments. Versions 1.6.0 through 1.15.0 contain a Cross-Site Scripting (XSS) vulnerability exploitable through content-type spoofing. The Remark42 image proxy fetches an arbitrary remote URL and re-serves the response from Remark42's own origin. During the download phase, the proxy determines whether the resource is an image by inspecting only the Content-Type header advertised by the remote server, never examining the actual bytes; during the serving phase, it instead derives the response Content-Type by sniffing those bytes with http.DetectContentType. An attacker can exploit this inconsistency by hosting a URL that advertises Content-Type: image/png while returning an HTML/JavaScript body: the download check accepts it as an image, the serving path sniffs the body and emits Content-Type: text/html, and the browser renders the attacker-controlled HTML/JavaScript as a document within Remark42's origin. Exploitation requires no Remark42 account on the target instance; the attacker only needs to host the malicious upstream URL and deliver the proxy link to a victim by any means, such as email, direct message, or a link on another website. This issue has been fixed in version 1.16.0.

Record
CVE-2026-48788
CVSS 3.0
8.2 HighCVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N
Weakness
CWE-436, CWE-79
Updated
2026-11-02
OSVGHSA-4c8j-mgm4-qqvp 8.2

Remark42: Cross-Site Scripting (XSS) on /api/v1/img via content-type spoofing

Summary

The remark42 image proxy fetches an arbitrary remote URL and re-serves the response from remark42's own origin. The download path decides whether the fetched resource is an image by looking only at the Content-Type header the remote server claims — it never inspects the actual bytes. The serving path then derives the response Content-Type by sniffing those bytes with http.DetectContentType.

An attacker hosts a URL that sets Content-Type to image/png but returns an HTML/JavaScript body:

  • the download check sees image/png → accepts it;
  • the serve path sniffs the body → emits Content-Type: text/html;
  • the browser renders attacker HTML/JS as a document in remark42's origin.

Details

Downloader

backend/app/rest/proxy/image.go — downloadImage(), lines 189-206:

contentType := resp.Header.Get("Content-Type")
if !strings.HasPrefix(contentType, "image/") {
    return nil, fmt.Errorf("invalid content type %s", contentType)
}

maxSize := 5 * 1024 * 1024 // 5MB default
if p.ImageService != nil && p.ImageService.MaxSize > 0 {
    maxSize = p.ImageService.MaxSize
}
lr := io.LimitReader(resp.Body, int64(maxSize)+1)
imgData, err := io.ReadAll(lr)
if err != nil {
    return nil, fmt.Errorf("unable to read image body: %w", err)
}
if len(imgData) > maxSize {
    return nil, fmt.Errorf("image is too large")
}
return imgData, nil          // <-- bytes never validated, returned as-is

Send Content-Type: image/png and the check passes regardless of what the body actually contains.

Server

backend/app/rest/proxy/image.go — Handler(), line 131:

w.Header().Add("Content-Type", p.ImageService.ImgContentType(img))
_, err = io.Copy(w, bytes.NewReader(img))

backend/app/store/image/image.go — ImgContentType(), lines 242-249:

func (s *Service) ImgContentType(img []byte) string {
    contentType := http.DetectContentType(img)
    if contentType == "application/octet-stream" {
        return "image/*"
    }
    return contentType                 // <-- returns text/html for an HTML body
}

PoC

self.send_response(200)
self.send_header("Content-Type", "image/png")
self.send_header("Content-Length", str(len(body)))
self.end_headers()
self.wfile.write(body)            # body = <!DOCTYPE html><script>...</script>

Then have the victim open https://<remark42-host>/api/v1/img?src=<base64(attacker-host)> top-level.

Impact

  • The script can issue authenticated, same-origin API calls with credentials: 'include' — the JWT cookie is sent automatically.
  • The script can read the XSRF-TOKEN cookie and re-send it as the X-XSRF-TOKEN header, defeating CSRF protection. The attacker acts as the victim: delete/edit their comments, change their settings, and — if the victim is admin — perform admin actions.

Triggering requires no remark42 account on the target instance; the attacker only needs to host the malicious upstream URL and deliver the proxy link to a victim by any means (email, DM, link on another site, etc.).

Fix

v1.16.0 adds layered defense to /api/v1/img and /api/v1/picture/{user}/{id}:

  • rest.SafeImgContentType validates sniffed body bytes against a strict allowlist (image/png, image/jpeg, image/gif, image/webp, image/bmp, image/x-icon). Non-image content returns 415 with no body echo. SVG is implicitly excluded.
  • Every response carries Content-Security-Policy: default-src 'none'; sandbox; frame-ancestors 'none', X-Content-Type Options: nosniff, and Content-Disposition: inline; filename="image".
  • The ETag is bumped to "v2:<base64(src)>". Browsers that revalidate cached pre-fix responses get a fresh validated 200 instead of a 304 against the poisoned cached entry.
  • The strict default-src 'none'; sandbox CSP also applies to all /api/v1/* routes as defense-in-depth.

Residual exposure

Browser-local caches that already hold a pre-fix text/html response with `Cache-Control:…

Record
GHSA-4c8j-mgm4-qqvp
CVSS 3.0
8.2 HighCVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N
Weakness
CWE-436, CWE-79
Updated
2026-11-02
OSVGO-2026-5804 No score

Remark42: Cross-Site Scripting (XSS) on /api/v1/img via content-type spoofing in github.com/umputun/remark42

Remark42: Cross-Site Scripting (XSS) on /api/v1/img via content-type spoofing in github.com/umputun/remark42

Record
GO-2026-5804
Updated
2026-11-02
OSVopenSUSE-SU-2026:21483-1 No score

Security update for govulncheck-vulndb

This update for govulncheck-vulndb fixes the following issues:

  • Update to version 0.0.20260723T184607 2026-07-23T18:46:07Z (jsc#PED-11136).

Go CVE Numbering Authority IDs added or updated with aliases:

  • GO-2026-4730 CVE-2026-33022 GHSA-cv4x-93xx-wgfj
  • GO-2026-4761 CVE-2026-33211 GHSA-j5q5-j9gm-2w5c
  • GO-2026-5102 CVE-2026-45021 GHSA-3vcp-chfh-f6r2
  • GO-2026-5272 CVE-2026-40938 GHSA-94jr-7pqp-xhcq
  • GO-2026-5486 CVE-2026-40924 GHSA-m2cx-gpqf-qf74
  • GO-2026-5542 CVE-2026-40090 GHSA-pj97-4p9w-gx3q
  • GO-2026-5630 CVE-2026-25542 GHSA-rmx9-2pp3-xhcr
  • GO-2026-5643 CVE-2026-40923 GHSA-rx35-6rhx-7858
  • GO-2026-5675 CVE-2026-41492 GHSA-vvf7-6rmr-m29q
  • GO-2026-5711 CVE-2026-40161 GHSA-wjxp-xrpv-xpff
  • GO-2026-5746 CVE-2026-41567 GHSA-x86f-5xw2-fm2r
  • GO-2026-5749 CVE-2026-41328 GHSA-x92x-px7w-4gx4
  • GO-2026-5837 CVE-2026-44840 GHSA-q2m9-6jp9-c6mc
  • GO-2026-5929 CVE-2026-44342 GHSA-26v7-h57m-gh9m
  • GO-2026-5930 CVE-2026-33655 GHSA-6qcr-qxgr-m7fv
  • GO-2026-5933 GHSA-3g4q-2f67-2gvh
  • GO-2026-5934 GHSA-59qp-cfj3-rp64
  • GO-2026-5935 GHSA-7856-g3gv-9wq8
  • GO-2026-5936 CVE-2026-53552 GHSA-26rh-24rg-j3vv
  • GO-2026-5937 CVE-2026-53508 GHSA-2jcc-mxv7-p3f9
  • GO-2026-5938 CVE-2026-53553 GHSA-4g5x-hcwm-82jw
  • GO-2026-5939 CVE-2026-10517 GHSA-698x-9w2p-7vvp
  • GO-2026-5940 CVE-2026-53572 GHSA-6w3m-4hhp-775q
  • GO-2026-5941 CVE-2026-53487 GHSA-gvhc-wv3v-7pf8
  • GO-2026-5942 CVE-2026-46600
  • GO-2026-5944 CVE-2026-50197 GHSA-659f-rgp5-w4wf
  • GO-2026-5945 CVE-2026-6815 GHSA-rmxx-v9rj-vpvg
  • GO-2026-5946 CVE-2026-52831 GHSA-v5px-423j-pf7p
  • GO-2026-5947 CVE-2026-53649 GHSA-xqhv-chqm-fhcc
  • GO-2026-5948 CVE-2026-50554 GHSA-588f-fvcv-xhvf
  • GO-2026-5949 CVE-2026-50553 GHSA-rqrh-8wpv-x7hh
  • GO-2026-5950 CVE-2026-53602 GHSA-339v-266x-79xr
  • GO-2026-5951 CVE-2026-10219 GHSA-6jm8-4fhr-5w64
  • GO-2026-5952 CVE-2026-49834 GHSA-9vcr-p3rj-q5q6
  • GO-2026-5953 CVE-2026-9094 GHSA-c9w5-qp6m-m395
  • GO-2026-5956 CVE-2026-55252 GHSA-h5g6-xmh4-hc37
  • GO-2026-5957 CVE-2026-54158 GHSA-5xfx-xj4h-5p7r
  • GO-2026-5958 CVE-2026-54068 GHSA-gcm7-57gf-953c
  • GO-2026-5959 CVE-2026-54072 GHSA-h29v-hj44-q8cv
  • GO-2026-5961 CVE-2026-54069 GHSA-hvr9-72v2-fff3
  • GO-2026-5962 CVE-2026-54088 GHSA-m93h-4hw7-5qcm
  • GO-2026-5963 CVE-2026-54067 GHSA-mvjr-vv3c-w4qv
  • GO-2026-5964 CVE-2026-54066 GHSA-p4m3-mgmm-c664
  • GO-2026-5965 CVE-2026-54070 GHSA-w7cg-whh7-xp28
  • GO-2026-5966 CVE-2026-54089 GHSA-xqp3-jq6g-x3qm
  • GO-2026-5967 CVE-2026-50551 GHSA-56mp-4f3v-fgj2
  • GO-2026-5968 CVE-2026-54174 GHSA-fpg8-7664-jc5q
  • GO-2026-5969 GHSA-g936-7jqj-mwv8
  • GO-2026-5970 CVE-2026-56852
  • GO-2026-5973 CVE-2026-54250 GHSA-jxr7-mqhw-9p98
  • GO-2026-5974 CVE-2026-44300 GHSA-wmj8-9953-vff5
  • GO-2026-5975 CVE-2026-50018 GHSA-42j2-w334-qxw7
  • GO-2026-5976 CVE-2026-50141 GHSA-g7mm-9vx7-jm7h
  • GO-2026-5977 CVE-2026-50013 GHSA-qrh4-p6v4-mrfg
  • GO-2026-5978 CVE-2026-50125 GHSA-qw5r-ppcg-f8rj
  • GO-2026-5979 CVE-2026-50006 GHSA-xrcf-6jh3-ggvx
  • GO-2026-5980 CVE-2026-50158 GHSA-2c7f-fxww-6w6c
  • GO-2026-5981 GHSA-mqxv-9rm6-w8qc
  • GO-2026-5982 GHSA-pqg7-v6wh-3pfp
  • GO-2026-5984 CVE-2026-53604 GHSA-2p2f-px33-4vv5
  • GO-2026-5985 GHSA-7rx3-5wx3-5v76
  • GO-2026-5986 CVE-2026-61699 GHSA-cm26-5974-52h8
  • GO-2026-5987 CVE-2026-55513 GHSA-g4x6-jcvr-9m3g
  • GO-2026-5988 CVE-2026-54628 GHSA-hwrq-8wxh-q4xv
  • GO-2026-5989 CVE-2026-55512 GHSA-m3cx-mwpg-32jg
  • GO-2026-5990 CVE-2026-54629 GHSA-mf78-3rpf-r784
  • GO-2026-5991 CVE-2026-53603 GHSA-q4vm-pq3q-8wgq
  • GO-2026-5992 CVE-2026-61549 GHSA-qf34-295c-26v8
  • GO-2026-5996 CVE-2026-54450 GHSA-pph6-vfjv-vpjw
  • GO-2026-5997 CVE-2026-54452 GHSA-xgch-x3mx-cm3c
  • GO-2026-5998 CVE-2026-54495 GHSA-398h-7f66-3h4p
  • GO-2026-5999 CVE-2026-10814 GHSA-jh6h-v6mp-h22v
  • GO-2026-6002 CVE-2026-58196 GHSA-pr64-jmmf-jp54
  • GO-2026-6003 CVE-2026-53714 GHSA-22xc-xg2r-9j7v
  • GO-2026-6004 CVE-2026-52833 GHSA-3v79-m2cg-89ww
  • GO-2026-6005 CVE-2026-53715 GHSA-8fv2-88gg-hm7q
  • GO-2026-6006 CVE-2026-53716 GHSA-cxpq-8v7q-cg56 *…
Record
openSUSE-SU-2026:21483-1
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
github.com/umputun/remark42
Go
 1.6.0 up to 1.16.0
1.16.0
govulncheck-vulndb
openSUSE:Leap 16.0
From an advisory covering 1734 CVEs
 before 0.0.20260723T184607-160000.1.1
0.0.20260723T184607-160000.1.1

History

History
RecordedChangedSources
affected, facts
first indexedCVE List, EUVD, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2026-48788 is a high vulnerability (CVSS 8.2) in Umputun Remark42, published June 16, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.31%, the 23rd percentile. Fixed in github.com/umputun/remark42 1.16.0.

References

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