CVE-2026-86049

highpriority · track · 0EPSS 0.004 · 36 %ileUS
grimoire priorityP2 · Scheduled
Exploitable?
—
no known exploitation
Likely / severe?
0.42% (36th) EPSS
CVSS 7.1 · high
Is it mine?
1 products
Check your exposure
Who says so?
1 sources
osv

EPSS · scored 2026-10-27

Jupyter Server is the backend for Jupyter web applications. Prior to version 2.21.0, the 5xx request logging path in jupyter_server/log.py copies the Referer header into a JSON header block without applying the token scrubbing used for the request URI. A request that returns HTTP 500 while the Referer contains a token-bearing URL can therefore write that token to server logs in plaintext. An attacker who can read those logs can recover the token and use the affected user's Jupyter Server permissions. This issue is fixed in version 2.21.0.

01

cvss

Technical severity, 0–10. One row per scoring source.

7.1
010

base score 0–10 · higher = more severe · hover a vector to decode⚠ sources disagree · medium–high · 6.5–7.1

v3.17.1highCVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:Hsecondary · security-advisories@github.comexploitability 1.2 · impact 5.9
v3.17.1highCVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:Hsecondary · OSV
v3.16.5mediumCVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H/E:Usecondary · CVSS-BT (t0sche)
02

ssvc

A stakeholder decision — act, attend, or track — from exploitation and impact.

Exploitation: none · Automatable: no · Technical Impact: totalcoordinatorcisa_ssvcUS
03

weaknesses

The CWE weakness classes this flaw belongs to.

CWE-532USCWE-532US
04

affected products (1)

Products and version ranges known to be affected.

05

advisories (2)

Vendor and ecosystem advisories describing this.

  • GHSA-c3mw-737p-c7g2Jupyter Server: 5xx request logging leaks token-bearing Referer header values2026-09-17ghsa · osvUS

    ### Summary When a request returns a 500, `jupyter_server/log.py` logs a small JSON block of request headers. The Referer header was copied into it as-is, so a token in the Referer URL ended up in the logs in plain text. ### Impact Anyone who can read the server logs can pick tokens out of these 500 entries. Tokens end up in the Referer during normal token-based login and launch flows. Affected: all versions before 2.21.0. ### PoC Any malformed request that returns a 500 works: ```bash curl -i -X POST \ -H 'Content-Type: application/json' \ -H 'Referer: http://127.0.0.1:8899/tree?token=REFERTOKEN' \ --data '{"name":123}' \ 'http://127.0.0.1:8899/api/kernels?token=VALIDTOKEN' ``` The log shows the token twice, once raw and once redacted: ``` "Referer": "http://127.0.0.1:8899/tree?token=REFERTOKEN", [E ... ServerApp] 500 POST /api/kernels?token=[secret] (...) referer=http://127.0.0.1:8899/tree?token=[secret] ``` ### Patches Fixed in 2.21.0 by 5251352. Header values are now scrubbed before the block is logged. Upgrade to 2.21.0 or later. ### Workarounds Limit who can read the server logs. If you can, avoid flows that put the token in the URL.

  • PYSEC-2026-4054Jupyter Server: 5xx request logging leaks token-bearing Referer header values2026-10-01vendor · osvUS

    ### Summary When a request returns a 500, `jupyter_server/log.py` logs a small JSON block of request headers. The Referer header was copied into it as-is, so a token in the Referer URL ended up in the logs in plain text. ### Impact Anyone who can read the server logs can pick tokens out of these 500 entries. Tokens end up in the Referer during normal token-based login and launch flows. Affected: all versions before 2.21.0. ### PoC Any malformed request that returns a 500 works: ```bash curl -i -X POST \ -H 'Content-Type: application/json' \ -H 'Referer: http://127.0.0.1:8899/tree?token=REFERTOKEN' \ --data '{"name":123}' \ 'http://127.0.0.1:8899/api/kernels?token=VALIDTOKEN' ``` The log shows the token twice, once raw and once redacted: ``` "Referer": "http://127.0.0.1:8899/tree?token=REFERTOKEN", [E ... ServerApp] 500 POST /api/kernels?token=[secret] (...) referer=http://127.0.0.1:8899/tree?token=[secret] ``` ### Patches Fixed in 2.21.0 by 5251352. Header values are now scrubbed before the block is logged. Upgrade to 2.21.0 or later. ### Workarounds Limit who can read the server logs. If you can, avoid flows that put the token in the URL.

06

also known as (2)

Other names and identifiers for this entity.

07

sources

Which sources assert this record, and when.

osvNVDCVE.orgMITRE
published 2026-09-17modified 2026-09-24
08

references (4) nvd

External citations — patches, advisories, analysis.

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