NewsCompaniesFlawsGuides
Get the app

CVE-2023-36851

From Exploited flaws, HARD’s guide to the flaws being used in attacks · Checked
This article is about a flaw in Juniper Junos OS. For every flaw being used in attacks, see Exploited flaws; for Juniper’s, see Juniper flaws being used in attacks.

CVE-2023-36851 is a security flaw in Juniper Junos OS, the operating system on Juniper’s routers and firewalls. Attackers can upload a file to Juniper Junos OS from anywhere, without logging in and make it run. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 13 Nov 2023.[1]

Juniper Junos OS is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. Fixed in version 21.2R3-S8, 21.4R3-S6, 22.1R3-S5 and 22.2R3-S3 and others of Junos OS.

Description

CISA calls it an SRX series missing authentication for critical function in Juniper Junos OS. In its words: “Juniper Junos OS on SRX Series contains a missing authentication for critical function vulnerability that allows an unauthenticated, network-based attacker to cause limited impact to the file system integrity. With a specific request to webauth_operation.php that doesn't require authentication, an attacker is able to upload arbitrary files via J-Web, leading to a loss of integrity for a certain part of the file system, which may allow chaining to other vulnerabilities.”[1]

The CVE record adds: “A Missing Authentication for Critical Function vulnerability in Juniper Networks Junos OS on SRX Series allows an unauthenticated, network-based attacker to cause limited impact to the file system integrity. With a specific request to webauth_operation.php that doesn't require authentication, an attacker is able to upload and download arbitrary files via J-Web, leading to a loss of integrity or confidentiality, which may allow chaining to other vulnerabilities. This issue affects Juniper Networks Junos OS on SRX Series: * 21.2 versions prior to 21.2R3-S8; * 21.4 versions prior to 21.4R3-S6; * 22.1 versions prior to 22.1R3-S5; * 22.2 versions prior to 22.2R3-S3; * 22.3 versions prior to 22.3R3-S2; * 22.4 versions prior to 22,4R2-S2, 22.4R3; * 23.2 versions prior to 23.2R1-S2, 23.2R2.”[2]

The underlying weakness is missing authentication (CWE-306), in MITRE’s list of common software weaknesses.

It scores 5.3 out of 10 on the Common Vulnerability Scoring System, which rates it medium, scored by NVD using CVSS 3.1.[2]

Affected versions

NVD’s record lists these fixed versions: in each line, everything before the version shown is affected.[2]

ProductFixed in
Junos OS21.2R3-S8
Junos OS21.4R3-S6
Junos OS22.1R3-S5
Junos OS22.2R3-S3
Junos OS22.3R3-S2
Junos OS22.4R2-S2, 22.4R3
Junos OS23.2R1-S2, 23.2R2

Most families won’t have Juniper Junos OS; it’s for whoever looks after IT at work.

Exploitation

CISA added it to its catalog of known exploited vulnerabilities on 13 Nov 2023, 47 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 17 Nov 2023 to fix it.[1]

FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 1%. That’s in the top 34% of the flaws it scores.[4]

CISA assesses it as automatable: attackers can run it against many targets at once.

CISA doesn’t say who was attacked, or how.

Fix

Juniper has published a fix or advice.[3] CISA’s required action: “Apply mitigations per vendor instructions or discontinue use of the product if mitigations are unavailable.”[1]

  1. Check whether you use it.Juniper Junos OS is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
  2. Send this page to your IT person.Ask them to confirm it’s fixed (fixed in version 21.2R3-S8, 21.4R3-S6, 22.1R3-S5 and 22.2R3-S3 and others of Junos OS), or not in use. CISA gave US government agencies until 17 Nov 2023 to fix it.
  3. If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.

You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.

Timeline

DateEvent
The flaw is made public in the National Vulnerability Database, with a severity score of 5.3.[2]
CISA adds it to its catalog of flaws known to be used in attacks.[1]
CISA’s deadline for US government agencies to fix it.[1]
FIRST puts the odds of it being used in attacks in the next 30 days at 1%.[4]

See also

Other Juniper Junos OS flaws

  • CVE-2025-21590: improper isolation or compartmentalization ·
  • CVE-2023-36847: EX series missing authentication for critical function ·
  • CVE-2023-36846: SRX series missing authentication for critical function ·
  • CVE-2023-36845: EX series and SRX series PHP external variable modification ·
  • CVE-2023-36844: EX series PHP external variable modification ·
  • CVE-2020-1631: path traversal ·

Other Juniper flaws

  • CVE-2015-7755, Juniper ScreenOS: improper authentication ·

Other missing authentication flaws

  • CVE-2026-111357, Ivanti Connect Secure: missing authentication for critical function ·
  • CVE-2026-67277, MikroTik RouterOS: missing authentication for critical function ·
  • CVE-2026-81578, PaperCut NG/MF: missing authentication for critical function ·
  • CVE-2026-72529, TrueConf Server: missing authentication for critical function ·
  • CVE-2026-56164, Microsoft SharePoint Server: missing authentication for critical function ·

Added the same day

  • CVE-2023-47246, SysAid Server: server path traversal ·
  • CVE-2023-36847, Juniper Junos OS: EX series missing authentication for critical function ·
  • CVE-2023-36846, Juniper Junos OS: SRX series missing authentication for critical function ·
  • CVE-2023-36845, Juniper Junos OS: EX series and SRX series PHP external variable modification ·
  • CVE-2023-36844, Juniper Junos OS: EX series PHP external variable modification ·

Every Juniper flaw on CISA’s list · Everything added in November 2023

References

  1. ^ Cybersecurity and Infrastructure Security Agency. “Juniper Junos OS SRX Series Missing Authentication for Critical Function Vulnerability”. Known Exploited Vulnerabilities Catalog, added 13 Nov 2023.
  2. ^ National Institute of Standards and Technology. “CVE-2023-36851 Detail”. National Vulnerability Database, published 27 Sep 2023.
  3. ^ Juniper. Advisories and fixes: supportportal.juniper.net, supportportal.juniper.net.
  4. ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.011, percentile 66.3, 31 Oct 2026.
  5. CVE Program. “CVE-2023-36851”. The official CVE record.

This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.