CatalogFortiOS SSL VPN
FortiOS SSL VPN RCE (CVE-2024-21762)
Single product fortios-ssl-vpn
Unauthenticated out-of-bounds write in FortiOS and FortiProxy SSL VPN HTTP parsing, enabling remote code execution. CVSS 9.8. Fortinet said it was potentially exploited at disclosure (8 Feb 2024). CISA KEV 9 Feb 2024; ransomware Known. In April 2025 Fortinet documented symlink persistence that can survive the original patch.
Exposed hosts
No public figure yet
Possibly vulnerable
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Compromised hosts
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Scanning seen
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Live trackers
Shadowserver · Possibly vulnerable
What changed on 24 Oct 2026
Updated 24 Oct 2026
Ask your AI about this
Help me defend against FortiOS SSL VPN RCE (CVE-2024-21762). CVE IDs: CVE-2024-21762. Aliases: FG-IR-24-015, Two Bytes is Plenty.
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Work with me to: determine whether my environment is affected (Am I affected checks, version tables, fixed versions); propose prioritised next steps (patch, then documented mitigations if I cannot patch); map detection rules to the log sources I have; and pick retrohunt queries and look-back windows. Output a short action plan.
Guardrails: read-only checks only. Do not attempt exploitation or send exploit traffic. Do not run commands without my approval. Do not share secrets.
My environment: <describe your stack, versions, log sources>Act-now priority
Act nowNewLabel from the published rule on the Method page. Scores below were fetched from public sources on .
CVSS
EPSS
CISA KEV
Exploitation
This page records confirmed in-the-wild exploitation.
Internet exposure
No public exposure figure on this page.
Summary for management
Unauthenticated out-of-bounds write remote code execution in FortiOS and FortiProxy SSL VPN (CVE-2024-21762). CVSS 9.8. Fortinet said it was potentially exploited on 8 Feb 2024. Listed in CISA KEV; ransomware Known.
Confirm FortiOS or FortiProxy is on a FG-IR-24-015 fixed release, disable SSL VPN if you cannot patch, and hunt the April 2025 language-file symlink.
- Aliases
- Published
- Updated
What the bug is
CVE-2024-21762 is an out-of-bounds write (CWE-787) in Fortinet FortiOS and FortiProxy SSL VPN. A remote unauthenticated attacker can execute code or commands via specially crafted HTTP requests. Fortinet rates CVSS 9.8 and noted at publication (8 Feb 2024) that the issue was potentially being exploited in the wild.
Preconditions: the SSL VPN feature is enabled and reachable. An unauthenticated attacker needs HTTP(S) access to the SSL VPN interface. The only complete vendor workaround is to disable SSL VPN entirely. Disabling webmode alone is not a valid workaround (FG-IR-24-015).
Assetnote's 15 Mar 2024 analysis showed the bug in chunked-transfer trailer handling inside SSL VPN request parsing. On a tested FortiOS 7.2.5 build, a two-byte out-of-bounds write was enough to corrupt a saved pointer and reach code execution. This page does not republish exploit details.
Fixed FortiOS builds begin at 7.4.3, 7.2.7, 7.0.14, 6.4.15, 6.2.16, and 6.0.18. Fixed FortiProxy builds begin at 7.4.3, 7.2.9, 7.0.15, and 2.0.14. FortiProxy 1.x must migrate. FortiGuard virtual patch signature HTTP.Chunk.Length.Invalid. (FMWP db update 24.020) is a temporary control.
On 10 Apr 2025 Fortinet disclosed a separate post-exploitation technique: a symbolic link in SSL-VPN language files used after this and earlier FortiOS RCEs to keep read-only filesystem access even after the original access vector was locked down. Devices that never enabled SSL VPN are not impacted by that persistence issue, per Fortinet. CCCS AL25-005 (14 Apr 2025) points to later cleanup builds 7.6.2, 7.4.7, 7.2.11, 7.0.17, and 6.4.16.
Attack path
- SSL VPN reachabilityUnauthenticated access to an internet-facing FortiOS or FortiProxy SSL VPN interface.
- Crafted chunked HTTPA malformed chunked request triggers an out-of-bounds write in SSL VPN HTTP parsing.
- Code executionAttacker code runs on the firewall or proxy OS. Conceptual impact class only.
- Optional persistenceObserved follow-on includes a language-file symlink that can outlive the original RCE patch (Fortinet, Apr 2025).
High-level only. Not a reproduction guide.
Exploitation status
CISA KEV
Listed in CISA KEV as of .
CISA KEV CVE-2024-21762 Fortinet FortiOS Out-of-Bound Write Vulnerability. dateAdded 2024-02-09, due 2024-02-16. knownRansomwareCampaignUse Known.
First seen
Seen . Source: Fortinet FG-IR-24-015
Vendor note at initial publication: this is potentially being exploited in the wild. Exact first compromise day is not stated.
Am I affected?New
FortiOS 7.4.0 through 7.4.2, 7.2.0 through 7.2.6, 7.0.0 through 7.0.13, 6.4.0 through 6.4.14, 6.2.0 through 6.2.15, and 6.0.0 through 6.0.17, plus FortiProxy trains listed in FG-IR-24-015, are in scope when SSL VPN is enabled and reachable. FortiProxy 1.x must migrate. Disabling webmode alone is not a valid workaround.
Read the FortiOS or FortiProxy firmware against first-fixed builds
VersionFG-IR-24-015: FortiOS upgrade to 7.4.3, 7.2.7, 7.0.14, 6.4.15, 6.2.16, or 6.0.18 or above on those trains. FortiProxy: 7.4.3, 7.2.9, 7.0.15, or 2.0.14 or above; 1.x migrate. CCCS AL25-005 later names persistence-cleanup builds 7.6.2, 7.4.7, 7.2.11, 7.0.17, and 6.4.16. Record the firmware from the GUI System Information widget. No public CLI version string was copied from FG-IR-24-015 in this pass.
In the FortiOS or FortiProxy GUI System Information widget, record the firmware version and compare it to FG-IR-24-015 first-fixed builds.- Affected
- FortiOS 7.4.0-7.4.2, 7.2.0-7.2.6, 7.0.0-7.0.13, 6.4.0-6.4.14, 6.2.0-6.2.15, 6.0.0-6.0.17, or a FortiProxy train below 7.4.3 / 7.2.9 / 7.0.15 / 2.0.14, or any FortiProxy 1.x, if SSL VPN is enabled (next check).
- Fixed or out of scope
- FortiOS 7.4.3+, 7.2.7+, 7.0.14+, 6.4.15+, 6.2.16+, or 6.0.18+, or FortiProxy 7.4.3+ / 7.2.9+ / 7.0.15+ / 2.0.14+, after SSL VPN inventory. Prefer also a CCCS cleanup build if SSL VPN was ever enabled.
Confirm SSL VPN is enabled and reachable
ConfigFG-IR-24-015 workaround is to disable SSL VPN entirely. Disabling webmode is not a valid workaround. Fortinet (10 Apr 2025): devices that never enabled SSL VPN are not impacted by the later language-file persistence issue.
In VPN > SSL-VPN Settings (or the FortiProxy equivalent), record whether SSL VPN is enabled and which interface it listens on.- Affected
- SSL VPN is enabled on a firmware below the first-fixed release for that train, or webmode was disabled while SSL VPN itself stayed on.
- Fixed or out of scope
- SSL VPN is disabled, or the device is on a first-fixed release and, if SSL VPN was ever enabled, reviewed for the April 2025 language-file symlink.
Inventory the FortiGuard virtual-patch signature if licensed
PackageFG-IR-24-015 names FMWP signature HTTP.Chunk.Length.Invalid. (db update 24.020) as a temporary control. This is signature-pack inventory, not a substitute for the version check.
In FortiGuard IPS / AV signature lists, confirm HTTP.Chunk.Length.Invalid. is present after FMWP db update 24.020 or later.- Affected
- The signature is absent or older than db update 24.020 while SSL VPN is still enabled on a vulnerable build.
- Fixed or out of scope
- HTTP.Chunk.Length.Invalid. is present after 24.020. This is an interim control only; FG-IR-24-015 still requires a fixed firmware.
Limits
- A firmware string alone does not prove SSL VPN is off.
- Disabling webmode is not the vendor workaround.
- The original RCE patch does not remove the April 2025 language-file symlink.
- No sourced Shodan product query is published on this page; do not invent one.
Can't patch yet?New
Fortinet documents two interim controls: disable SSL VPN entirely, and apply FortiGuard virtual patch HTTP.Chunk.Length.Invalid. (FMWP 24.020). Neither replaces a fixed firmware. Disabling webmode is not valid.
Disable SSL VPN until upgraded
FG-IR-24-015 workaround: disable SSL VPN. The same advisory says disable webmode is not a valid workaround. This removes the unauthenticated HTTP path to sslvpnd until a first-fixed build is installed.
Limits. Not a software fix. Breaks remote SSL VPN access. Does not remove the April 2025 language-file symlink if the device was already compromised. Still upgrade to 7.4.3 / 7.2.7 / 7.0.14 / 6.4.15 / 6.2.16 / 6.0.18 (FortiOS) or the matching FortiProxy fixed train.
Verify it is in place
In VPN > SSL-VPN Settings, SSL VPN status is disabled. Confirm the SSL VPN interface is no longer accepting HTTPS sessions. A webmode-only change is not this mitigation.
Apply FortiGuard virtual patch HTTP.Chunk.Length.Invalid. if licensed
FG-IR-24-015 names FMWP signature HTTP.Chunk.Length.Invalid. (db update 24.020) as a temporary control on devices that still run SSL VPN.
Limits. Requires a FortiGuard IPS/AV subscription and a current FMWP pack. Interim only. Does not replace a first-fixed firmware and does not remove post-exploitation persistence.
Verify it is in place
FortiGuard IPS or AV signature lists show HTTP.Chunk.Length.Invalid. after FMWP db update 24.020 or later.
What defenders should do today
Do today
- 1
Upgrade FortiOS or FortiProxy to a fixed release
FortiOS: Fixed in 7.4.3+, Fixed in 7.2.7+, Fixed in 7.0.14+, Fixed in 6.4.15+, Fixed in 6.2.16+, Fixed in 6.0.18+. FortiProxy: Fixed in 7.4.3+, Fixed in 7.2.9+, Fixed in 7.0.15+, Fixed in 2.0.14+ (1.x migrate). Prefer also the Apr 2025 persistence-cleanup builds named by CCCS (7.6.2 / 7.4.7 / 7.2.11 / 7.0.17 / 6.4.16).
- 2
If you cannot patch immediately, disable SSL VPN
Vendor workaround is to disable SSL VPN. Disabling webmode alone is not valid.
- 3
Hunt for language-file symlink persistence
Even after the RCE patch, check for the SSL-VPN language-file symlink described 10 Apr 2025. Reset credentials and review config if impact is plausible.
This week
- 1
Apply FortiGuard virtual patch if licensed
FMWP signature HTTP.Chunk.Length.Invalid. (db update 24.020) is a temporary control, not a substitute for upgrading.
Detection pointers
| Signal | Log source | What to look for |
|---|---|---|
| Safe chunked-versus-normal SSL VPN behaviour check | Active check versus SSL-VPN VIP | Bishop Fox and community checkers compare responses to normal versus chunked POSTs on the SSL-VPN interface to infer vulnerability without exploitation. Run this against the SSL-VPN VIP, not management 443. |
| sslvpnd instability | crashlog / SSL VPN logs (diagnose debug crashlog read) | Unexpected sslvpnd crashes or restarts around anomalous chunked requests on the VPN interface. |
| Language-file symlink under SSL-VPN lang paths | Appliance filesystem | Fortinet Apr 2025 persistence: a symlink tying a user language-file folder to the root filesystem for read-only exfil of configs. |
| No public Sigma or Splunk detection file located | Public rule repositories | A 24 Oct 2026 check of SigmaHQ/sigma emerging-threats listings and splunk/security_content found no detection file for CVE-2024-21762. Use the pointers above and the Bishop Fox check. |
Vendor signatures
IPS1
| Signal | Log source | What to look for | Date |
|---|---|---|---|
| IPS hit HTTP.Chunk.Length.Invalid. | FortiGate IPS / AV logs | FortiGuard virtual patch / IPS signature for malformed chunk length or trailers (FMWP 24.020 and later). |
Vendor advisories and signature updates
Look for past exploitationNew
Search FortiGate IPS, crash, and SSL VPN logs from 8 Feb 2024 for HTTP.Chunk.Length.Invalid. hits and sslvpnd crashes, then inspect language-file folders for the Apr 2025 symlink even on boxes that already have the original RCE patch.
How far back to search
Search from (first exploitation). This date is a lower bound.
Fortinet said the issue was potentially exploited as of the 8 Feb 2024 advisory. Public analysis landed 15 Mar 2024. Fortinet documented persistence artefacts on 10 Apr 2025. Keep SSL VPN and crash logs for the full retention period of internet-exposed gateways.
Log sources needed
| Source | Why | Needed |
|---|---|---|
| FortiGate IPS / AV logs | HTTP.Chunk.Length.Invalid. is the FortiGuard virtual-patch signature named in FG-IR-24-015. | Required |
| crashlog / sslvpnd logs | Unexpected sslvpnd restarts around crafted chunked requests. | Required |
| Appliance filesystem (SSL-VPN language-file folders) | Fortinet Apr 2025 symlink persistence. | Required |
| SSL VPN authentication logs and config revisions | CCCS recommends watching unauthorized logins and config diffs. | Optional |
Steps
- 1
Search IPS logs for HTTP.Chunk.Length.Invalid.
FG-IR-24-015 names FMWP signature HTTP.Chunk.Length.Invalid. (db update 24.020) as the virtual patch.
- 2
Review sslvpnd crash history
Assetnote's analysis context includes sslvpnd instability around crafted chunked requests. Use diagnose debug crashlog read on the appliance.
- 3
Inspect SSL-VPN language-file folders for a symlink to the root filesystem
Fortinet (10 Apr 2025) and CCCS AL25-005 describe a symlink under language-file folders that retains read-only FS access after the RCE patch. Devices that never enabled SSL VPN are not impacted by that persistence issue.
Queries
Queries marked origin vuln2shell are our own and are dedicated to the public domain under CC0 1.0. Tune before use. Third-party hunts stay credited with a link.
SPL (Splunk)
FortiGate IPS hits for HTTP.Chunk.Length.Invalid. since 8 Feb 2024
Untestedindex=* earliest="02/08/2024:00:00:00" (sourcetype="fortigate:ips" OR sourcetype="fgt_ips" OR sourcetype="fortigate") "HTTP.Chunk.Length.Invalid" | stats count min(_time) as firstTime max(_time) as lastTime values(action) as action by src_ip, dest_ip, dest_port | convert ctime(firstTime) ctime(lastTime)By vuln2shell, . Source. License: CC0 1.0
Written by vuln2shell. Untested, tune before use.
Signature name from FG-IR-24-015. Confirm the sourcetype your add-on uses.
KQL (Microsoft Sentinel / Defender)
FortiGate IPS hits for HTTP.Chunk.Length.Invalid. since 8 Feb 2024
UntestedCommonSecurityLog | where TimeGenerated >= datetime(2024-02-08) | where DeviceVendor has "Fortinet" | where AdditionalExtensions has "HTTP.Chunk.Length.Invalid" or Message has "HTTP.Chunk.Length.Invalid" or Activity has "HTTP.Chunk.Length.Invalid" | summarize Count = count(), FirstSeen = min(TimeGenerated), LastSeen = max(TimeGenerated) by SourceIP, DestinationIP, DestinationPort, DeviceActionBy vuln2shell, . Source. License: CC0 1.0
Written by vuln2shell. Untested, tune before use.
LogScale (CrowdStrike)
sslvpnd crash or restart lines
Untested// Set the time picker to start 2024-02-08 /sslvpnd/ | /(crash|restart|abort|died|signal)/ | groupBy([#host], function=[count(), min(@timestamp, as=firstSeen), max(@timestamp, as=lastSeen)])By vuln2shell, . Source. License: CC0 1.0
Written by vuln2shell. Untested, tune before use.
Heuristic only. Confirm crashlog ingest before treating a hit as exploitation.
Third-party hunts
- Bishop Fox safe vulnerability check
By Bishop Fox, . Guide. License: see source. Origin: community. Detection-only comparison of normal versus chunked POST behaviour on the SSL-VPN VIP.
- CCCS AL25-005 persistence cleanup
By Canadian Centre for Cyber Security, . Guide. License: public. Origin: vendor. Cleanup builds and credential-reset advice after the language-file symlink.
Artefacts to check
Published IOCs
No file hashes from Fortinet or CCCS primary text are indexed here.
What a hit means
- Attempt
An IPS hit for HTTP.Chunk.Length.Invalid. with no crash and no filesystem change.
- Likely compromise
sslvpnd crash or restart clustered with anomalous chunked requests, or unauthorized SSL-VPN logins.
- Confirmed compromise
A language-file symlink to the root filesystem, or other persistence named by Fortinet / CCCS.
Next steps
- 1
Upgrade, then apply persistence cleanup
Install a fixed RCE build. If SSL VPN was ever enabled, also move to a cleanup build named by CCCS and reset credentials.
- 2
Review config revisions
CCCS recommends checking unauthorized SSL-VPN logins and config diffs.
Limits
- Lookback is a lower bound. Search only as far as your logs go; many SIEMs keep 30 to 90 days.
- The original RCE patch does not remove the Apr 2025 language-file symlink.
- No primary hash list is quoted here.
- Devices that never sent IPS or crash logs off-box have no history after rotation.
If you find a hitNew
Disable SSL VPN or upgrade to a FG-IR-24-015 fixed release. If SSL VPN was ever enabled, Fortinet (10 Apr 2025) and CCCS AL25-005 say hunt the language-file symlink and reset credentials, then move to a cleanup build.
Containment
2
Upgrade, then apply persistence cleanup if SSL VPN was ever enabled
Install a first-fixed RCE build from FG-IR-24-015. CCCS AL25-005 (14 Apr 2025) points to later cleanup builds 7.6.2, 7.4.7, 7.2.11, 7.0.17, and 6.4.16 after Fortinet documented a language-file symlink that can survive the original patch. CISA (11 Apr 2025) points to the Fortinet post-exploitation advisory. Fortinet: devices that never enabled SSL VPN are not impacted by that persistence issue.
3
Inspect SSL-VPN language-file folders for a symlink to the root filesystem
Fortinet (10 Apr 2025): a threat actor used known vulnerabilities including FG-IR-24-015, then created a symbolic link in SSL-VPN language files to keep read-only filesystem access after the original vector was locked down.
Rotate
Local and remote credentials that the firewall or proxy could reach
CCCS AL25-005 tells operators to reset credentials after the persistent-access finding. Fortinet documented read-only filesystem access via the language-file symlink, which can expose configuration.
Reported persistence
Evidence to keep
1
Keep IPS hits for HTTP.Chunk.Length.Invalid.
FG-IR-24-015 names this FortiGuard virtual-patch signature. Preserve FortiGate IPS/AV logs from 8 Feb 2024.
2
Keep sslvpnd crash history
Assetnote analysis context includes sslvpnd instability around crafted chunked requests. The catalog page already records diagnose debug crashlog read as the appliance command.
3
Keep language-file folder listings and config revisions
Fortinet and CCCS: inspect language-file folders for a symlink. CCCS also recommends watching unauthorized SSL-VPN logins and config diffs.
Limits
- The original RCE patch does not remove the April 2025 language-file symlink.
- No primary hash list is quoted here.
- Devices that never sent IPS or crash logs off-box have no history after rotation.
Affected versions
Fortinet FortiOS and FortiProxy (SSL VPN enabled)
| Version | Status | Fixed version / ETA | Advisory |
|---|---|---|---|
| FortiOS 7.4 | Affected | version 7.4.3 or later | 7.4.0 through 7.4.2 Source |
| FortiOS 7.2 | Affected | version 7.2.7 or later | 7.2.0 through 7.2.6 Source |
| FortiOS 7.0 | Affected | version 7.0.14 or later | 7.0.0 through 7.0.13 Source |
| FortiOS 6.4 | Affected | version 6.4.15 or later | 6.4.0 through 6.4.14 Source |
| FortiOS 6.2 | Affected | version 6.2.16 or later | 6.2.0 through 6.2.15 Source |
| FortiOS 6.0 | Affected | version 6.0.18 or later | 6.0.0 through 6.0.17 Source |
| FortiProxy 7.4 | Affected | version 7.4.3 or later | 7.4.0 through 7.4.2 Source |
| FortiProxy 7.2 | Affected | version 7.2.9 or later | 7.2.0 through 7.2.8 Source |
| FortiProxy 7.0 | Affected | version 7.0.15 or later | 7.0.0 through 7.0.14 Source |
| FortiProxy 2.0 | Affected | version 2.0.14 or later | 2.0.0 through 2.0.13 Source |
| FortiProxy 1.2 / 1.1 / 1.0 | EOL / migrate | Migrate to a fixed release | Migrate to a fixed release Source |
Timeline6
Key events
Newest first.
- New
CCCS AL25-005 echoes the persistence finding and lists cleanup builds plus credential reset.
Source - New
Fortinet documents a language-file symlink used after FG-IR-24-015 and earlier FortiOS RCEs to keep read-only filesystem access after the original vector is closed.
Source - New
Assetnote publishes Two Bytes is Plenty, a public technical analysis of the chunked-trailer out-of-bounds write on a tested FortiOS 7.2.5 build.
Source - New
CISA adds CVE-2024-21762 to KEV (due 16 Feb 2024). knownRansomwareCampaignUse Known.
Source - New
Fortinet publishes FG-IR-24-015. The advisory rates the issue CVSS 9.8, says it is potentially being exploited in the wild, and says disabling webmode is not a valid workaround.
Source
Full timeline (6)
Mon 14 Apr 20251
- New
CCCS AL25-005 echoes the persistence finding and lists cleanup builds plus credential reset.
Source
Fri 11 Apr 20251
- New
CISA points to the Fortinet post-exploitation advisory.
Source
Thu 10 Apr 20251
- New
Fortinet documents a language-file symlink used after FG-IR-24-015 and earlier FortiOS RCEs to keep read-only filesystem access after the original vector is closed.
Source
Fri 15 Mar 20241
- New
Assetnote publishes Two Bytes is Plenty, a public technical analysis of the chunked-trailer out-of-bounds write on a tested FortiOS 7.2.5 build.
Source
Fri 9 Feb 20241
- New
CISA adds CVE-2024-21762 to KEV (due 16 Feb 2024). knownRansomwareCampaignUse Known.
Source
Thu 8 Feb 20241
- New
Fortinet publishes FG-IR-24-015. The advisory rates the issue CVSS 9.8, says it is potentially being exploited in the wild, and says disabling webmode is not a valid workaround.
Source
News & write-ups
Research1
- Two Bytes is PlentyNew
Assetnote
Vendor2
- Analysis of Threat Actor ActivityNew
Fortinet
- FG-IR-24-015New
Fortinet
Government2
- CCCS AL25-005New
Canadian Centre for Cyber Security
- CISA alert on Fortinet post-exploitation techniqueNew
CISA
PoC index (for defenders)
Public proof-of-concept links for defensive validation, with a source and a validation status.
Public technical analysis of a working chain against a specific 7.2.5 lab build. Listed for defender awareness. This site does not republish exploit code.
Open questions
First confirmed compromise day is not stated
Fortinet said the issue was potentially being exploited as of 8 Feb 2024. No primary source reviewed here stamps an earlier first day.