Evidence used
- No CISA KEV confirmation is currently recorded.
- A structured source references public exploit or proof-of-concept material.
- EPSS is 40.74% for the current model date.
BlackTreeCVE IntelligenceFull vulnerability report · 2021
CVE-2021-23336High confidence
Web Cache Poisoning
Unknown · python/cpython
Official source article: Siemens SSA-398330 ↗. Check the applicable product and release in the original source.
5.9MediumCVSS 3.1
Patch only the product branches with a verified fix
Fix availability varies by productMedium technical severity with public exploit material referenced by a structured source; prioritise exposed affected systems while verifying vendor guidance. Verified remediation exists for at least one product or source, but 5 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.
R
Open reassessment dashboard →Operational reassessment
Published severity in operational context
Published severityMedium→Operational priority:High, raised one band.upgradedsince 17 Dec 2025
Compensating controls
- Validate the affected product branch and deploy the verified fixed release.
- Restrict the affected network interface to trusted sources where business-safe.
- Increase monitoring for the attack path and post-exploitation behaviour described in the report.
Verification
- Confirm that the asset runs Unknown python/cpython and falls inside the recorded affected range.
- Verify the installed build against the product-specific fixed version after deployment.
- Validate exposure, authentication requirements and compensating controls in the actual environment.
- Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Remediation availability differs by product scope
Verified remediation exists for at least one product or source, but 5 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.
- Siemens ProductCERT: 5 affected or under-investigation product states without a fixed product state in the same current advisory
Distribution package intelligence
14 package statesRelease-specific package status
Alpine, Debian findings are scoped to the named distribution, release and source package. An absent finding does not mean a package is unaffected.
Repository candidate not checked
A published vendor fix does not prove that a matching update is enabled and installable on a particular asset. Confirm the local package candidate before scheduling remediation.
| Distribution release | Source package | Vendor state | Fixed version | Evidence |
|---|---|---|---|---|
| Alpine v3.23v3.23 · community | py3-django | Vendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected. | 3.1.7-r0 | Alpine Security Database ↗Source updated 11 Sep 2026 |
| Alpine v3.23v3.23 · main | python3 | Vendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected. | 3.8.8-r0 | Alpine Security Database ↗Source updated 3 Oct 2026 |
| Alpine v3.22v3.22 · community | py3-django | Vendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected. | 3.1.7-r0 | Alpine Security Database ↗Source updated 11 Sep 2026 |
| Alpine v3.22v3.22 · main | python3 | Vendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected. | 3.8.8-r0 | Alpine Security Database ↗Source updated 3 Oct 2026 |
| Alpine v3.21v3.21 · community | py3-django | Vendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected. | 3.1.7-r0 | Alpine Security Database ↗Source updated 19 Aug 2026 |
| Alpine v3.21v3.21 · main | python3 | Vendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected. | 3.8.8-r0 | Alpine Security Database ↗Source updated 3 Oct 2026 |
| Debian trixietrixie · source | pypy3 | Vendor fix publishedDebian records a fixed source-package version for this release. | 7.3.3+dfsg-3 | Debian Security Tracker ↗Source updated 5 Oct 2026 |
| Debian trixietrixie · source | python-django | Vendor fix publishedDebian records a fixed source-package version for this release. | 2:2.2.19-1 | Debian Security Tracker ↗Source updated 5 Oct 2026 |
| Debian bookwormbookworm · source | pypy3 | Vendor fix publishedDebian records a fixed source-package version for this release. | 7.3.3+dfsg-3 | Debian Security Tracker ↗Source updated 5 Oct 2026 |
| Debian bookwormbookworm · source | python-django | Vendor fix publishedDebian records a fixed source-package version for this release. | 2:2.2.19-1 | Debian Security Tracker ↗Source updated 5 Oct 2026 |
| Debian forkyforky · source | pypy3 | Vendor fix publishedDebian records a fixed source-package version for this release. | 7.3.3+dfsg-3 | Debian Security Tracker ↗Source updated 5 Oct 2026 |
| Debian forkyforky · source | python-django | Vendor fix publishedDebian records a fixed source-package version for this release. | 2:2.2.19-1 | Debian Security Tracker ↗Source updated 5 Oct 2026 |
| Debian sidsid · source | pypy3 | Vendor fix publishedDebian records a fixed source-package version for this release. | 7.3.3+dfsg-3 | Debian Security Tracker ↗Source updated 5 Oct 2026 |
| Debian sidsid · source | python-django | Vendor fix publishedDebian records a fixed source-package version for this release. | 2:2.2.19-1 | Debian Security Tracker ↗Source updated 5 Oct 2026 |
Direct vendor intelligence
1 currentAuthoritative vendor CSAF and VEX advisories
Structured product status and remediation from the issuing vendor. Product-state explanations are always visible; large lists can be searched or downloaded.
SSA-398330 · CSAF 2.0 · revision 18 · interimSiemens ProductCERTSSA-398330: Vulnerabilities in the additional GNU/Linux subsystem of the SIMATIC S7-1500 CPU 1518(F)-4 PN/DP MFP >= V3.1.0 and < V3.1.5
The vendor explicitly identifies these products as affected by this CVE.
- SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AB0)
- SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AC0)
- SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AB0)
- SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AC0)
- SIPLUS S7-1500 CPU 1518-4 PN/DP MFP (6AG1518-4AX00-4AC0)
- Summary
- The package python/cpython from 0 and before 3.6.13, from 3.7.0 and before 3.7.10, from 3.8.0 and before 3.8.8, from 3.9.0 and before 3.9.2 are vulnerable to Web Cache Poisoning via urllib.parse.parse_qsl and urllib.parse.parse_qs by using a vector called parameter cloaking. When the attacker can separate query parameters using a semicolon (;), they can cause a difference in the interpretation of the request between the proxy (running with default configuration) and the server. This can result in malicious requests being cached as completely safe ones, as the proxy would usually not see the semicolon as a separator, and therefore would not include it in a cache key of an unkeyed parameter.
- Remediation
- Update to V3.1.5 or later version
Medium technical severity with public exploit material referenced by a structured source; prioritise exposed affected systems while verifying vendor guidance. Verified remediation exists for at least one product or source, but 5 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.
Fix availability varies by product01
What, why and how
The package python/cpython from 0 and before 3.6.13, from 3.7.0 and before 3.7.10, from 3.8.0 and before 3.8.8, from 3.9.0 and before 3.9.2 are vulnerable to Web Cache Poisoning via urllib.parse.parse_qsl and urllib.parse.parse_qs by using a vector called parameter cloaking. When the attacker can separate query parameters using a semicolon (;), they can cause a difference in the interpretation of the request between the proxy (running with default configuration) and the server. This can result in malicious requests being cached as completely safe ones, as the proxy would usually not see the semicolon as a separator, and therefore would not include it in a cache key of an unkeyed parameter.
What
The package python/cpython from 0 and before 3.6.13, from 3.7.0 and before 3.7.10, from 3.8.0 and before 3.8.8, from 3.9.0 and before 3.9.2 are vulnerable to Web Cache Poisoning via urllib.parse.parse_qsl and urllib.parse.parse_qs by using a vector called parameter cloaking. When the attacker can separate query parameters using a semicolon (;), they can cause a difference in the interpretation of the request between the proxy (running with default configuration) and the server. This can result in malicious requests being cached as completely safe ones, as the proxy would usually not see the semicolon as a separator, and therefore would not include it in a cache key of an unkeyed parameter.
Why
The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.
How
An attacker operating through a network path may attempt exploitation without authentication after a user interaction. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.
What
The package python/cpython from 0 and before 3.6.13, from 3.7.0 and before 3.7.10, from 3.8.0 and before 3.8.8, from 3.9.0 and before 3.9.2 are vulnerable to Web Cache Poisoning via urllib.parse.parse_qsl and urllib.parse.parse_qs by using a vector called parameter cloaking. When the attacker can separate query parameters using a semicolon (;), they can cause a difference in the interpretation of the request between the proxy (running with default configuration) and the server. This can result in malicious requests being cached as completely safe ones, as the proxy would usually not see the semicolon as a separator, and therefore would not include it in a cache key of an unkeyed parameter.
Why
The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.
How
An attacker operating through a network path may attempt exploitation without authentication after a user interaction. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.
02
Exploit reality and attack path
CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.
Observed exploitation No confirmed evidence
?
Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.No CISA KEV match was present at the last successful refresh. This means no confirmation from that source, not proof of no exploitation.
Public PoC / exploit material Reference recorded
?
Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.A structured CVE source labels at least one public reference as exploit material. BlackTree has not independently validated that it is safe, reliable or weaponised.
- Likely attack path
- a network path → Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') → cause the confidentiality, integrity or availability impact described by the vendor
- Attack surface
- Network
- Privileges required
- None: unauthenticated exploitation is possible
- User interaction
- Required interaction required
- Attack complexity
- High: exploitation depends on specific conditions
- Security boundary
- Unchanged: impact remains within the vulnerable component's security authority
- Weakness
?
CWE means Common Weakness Enumeration: a standard category for the underlying weakness. - CWE-444 ↗
CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling'). The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.
- CVSS vector
?
CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity. CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:L/A:H/E:P/RL:U/RC:CCommon Vulnerability Scoring System 3.1: the compact vector below is decoded into plain language.
AVNetworkAttack vector: The vulnerable component can be reached over a network.ACHighAttack complexity: Successful exploitation depends on specific conditions outside the attacker's direct control.PRNonePrivileges required: The attacker does not need an account or existing privileges.UIRequiredUser interaction: Another user must perform an action for exploitation to succeed.SUnchangedScope: The security impact remains within the vulnerable component's authority.CNoneConfidentiality impact: No direct loss is represented by this metric.ILowIntegrity impact: A successful attack can cause a limited loss.AHighAvailability impact: A successful attack can cause a major loss.- Post-exploitation / living off the land
- No specific living-off-the-land technique is confirmed in the structured sources. Monitor normal administration tools for activity inconsistent with the affected service's baseline.
03
Patch and workaround
Operational remediation based on structured source evidence.
- Status
?
Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change. - Fix availability varies by product
- Affected
- Action
- Use the product-specific evidence above. Patch only products with a verified fixed release, and keep every affected or under-investigation state without a matching fix in the remediation queue.
- Workaround
- No verified workaround is recorded. If business-safe, reduce exposure to the affected interface and allow only trusted sources until authoritative guidance is available.
04
Evidence and provenance
Published 15 Feb 2021 · Last source change 17 Dec 2025, 21:16 UTC · CWE-444 · Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
CVE recordCVE.org · 5.2
CVSS sourceCNA
EPSS source FIRST · tracked since 2026-08-14
?
The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.European sourceENISA EUVD · EUVD-2021-10423
Product sourceVendor CSAF · Siemens ProductCERT
Remediation sourceVendor CSAF · Siemens ProductCERT
CWE sourceCISA ADP
NVD statusNVD modified after enrichment
Missing structured fields: affected product. Missing data is not evidence of low risk; review the primary advisory.
Material change intelligence
View recent updates ↗What changed after publication
- Affected versionsThe structured affected or fixed version information changed.Siemens ProductCERT ↗
- Before
- < unspecified; unspecified < 3.6.13; 3.7.0 < unspecified; unspecified < 3.7.10; 3.8.0 < unspecified; unspecified < 3.8.8; 3.9.0 < unspecified; unspecified < 3.9.2 · Fixed: An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
- After
- < unspecified; unspecified < 3.6.13; 3.7.0 < unspecified; unspecified < 3.7.10; 3.8.0 < unspecified; unspecified < 3.8.8; 3.9.0 < unspecified; unspecified < 3.9.2 · Fixed: Update to V3.1.5 or later version
CVE published
Fixed release or patch reference recorded
Fixed release recorded from official guidance
Technical terms and abbreviations used in this report
- CVE
- Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
- CVSS
- Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
- EPSS
- Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
- CWE
- Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
- CNA
- CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
- CISA ADP
- Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
- NVD
- National Vulnerability Database: NIST's enrichment service for CVE records.
- CERT / CSIRT
- A computer security incident response team that publishes warnings or coordinates incident response.
- PoC
- Proof of concept: public material that demonstrates or helps reproduce exploitation.
- CSAF
- Common Security Advisory Framework: a machine-readable format for security advisories.
- LoTL
- Living off the land: abuse of legitimate tools or system functions during an attack.