CriticalCVE-2023-43770

Roundcube Webmail Persistent Cross-Site Scripting (XSS) Vulnerability (CVE-2023-43770)

Published 22.09.2023In CISA KEV since 12.02.2024Sources: pwn.watch, CVE.org, OSV

Actively exploited

What is affected, what to do

Roundcube before 1.4.14, 1.5.x before 1.5.4, and 1.6.x before 1.6.3 allows XSS via text/plain e-mail messages with crafted links because of program/lib/Roundcube/rcube_string_replacer.php behavior.

The vulnerability is being actively exploited; treat it as a priority. Update Roundcube Webmail to version 1.4.14, Roundcube Webmail to version 1.5.4, Roundcube Webmail to version 1.6.3. Immediate actions, mitigation and detection rules are in the runbook below.

Affected: Roundcube Webmail lt *..1.4.14
Roundcube Webmail lt 1.5.0..1.5.4
Roundcube Webmail lt 1.6.0..1.6.3
Debian Linux eq 10.0..10.0
Fixed in: Roundcube Webmail 1.4.14
Roundcube Webmail 1.5.4
Roundcube Webmail 1.6.3

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Metrics

6.1

Priority

XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

Source: NVD, CVSS v3

6.1

CVSS v3

Severity Medium on the scale from 0 to 10.

Source: pwn.watch, CVE.org, OSV

63.7 %

EPSS score

High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

Source: FIRST EPSS, percentile 99.22

Show all metrics

Severity: Critical
Priority (0–325) The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first? Read in glossary: 212.71 Methodology
Exploit maturity An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack. Read in glossary: actively exploited
XIEM Effective CVSS XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command. Read in glossary: 6.1 Methodology
CVSS v3 CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions. Read in glossary: 6.1
EPSS score The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“. Read in glossary: 63.7 %; High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
EPSS percentile The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking. Read in glossary: 99.22; Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
KEV status The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching. Read in glossary: Actively exploited
Published: 22.09.2023
In CISA KEV since The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching. Read in glossary: 12.02.2024
CISA assessment (SSVC): Exploitation: active · not automatable · Technical impact: partial; Source: CISA Vulnrichment, as of 1 May 2024
CWE CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect. Read in glossary: CWE-79

Response and mitigation

Why act now?

Prioritisation rationale

This flaw is typically used against government, public-sector, military and defence-adjacent organisations that self-host Roundcube. It can be triggered over the network without authentication; the victim only has to open a crafted plain-text email. The attacker then gains access to mailbox content, can set up forwarding and can take over sessions. CISA does not flag ransomware use, so the risk profile is espionage and data theft rather than encryption. Despite its moderate base severity, exploitation likelihood is very high, so treat it as a 24–72 hour remediation item. Give it top priority if you are a KRITIS operator, a public authority, or run internet-facing Roundcube; purely internal instances can follow an expedited regular patch cycle with gateway filtering in place.

01
Runbook step 1
Immediate response (0-24 h)
- Inventory every Roundcube instance: Cover distro packages, containers, hosting control panels and appliances that bundle Roundcube. Check the version with grep RCMAIL_VERSION program/include/iniset.php or dpkg -l roundcube*.
- Patch: Upgrade to Roundcube 1.4.14, 1.5.4 or 1.6.3 (or later). Affected are all versions before 1.4.14, 1.5.x before 1.5.4 and 1.6.x before 1.6.3.
  
  Debian 10: apply the roundcube security update from the Debian security repository. Look up the package version in the Debian advisory.
  
  Bitnami images (1.5.0/1.6.0): redeploy from a current image.
- Restrict internet-facing instances first: Until patched, allow access only via VPN or ZTNA, or from known source IP ranges.
- Kill sessions after patching: Invalidate all sessions, for example by truncating the Roundcube session table. Reset passwords for any account suspected of compromise.
- Audit mailbox settings: Prioritise executives, IT admins and finance, and review:
  
  new forwarding and Sieve rules (managesieve),
  
  added identities,
  
  changed signatures.
- Hunt the mail store: Search for text/plain messages containing crafted links and quarantine them. A stored message re-triggers the flaw every time it is opened on any instance that is still unpatched.
02
Runbook step 2
Mitigation layers
- Network segmentation: Do not expose the webmail UI directly to the internet. Front it with a reverse proxy that enforces pre-authentication (SSO, mTLS or VPN).
- Mail gateway (primary control): The payload arrives over SMTP, not HTTP, so a WAF in front of Roundcube will not see it.
  
  Add an inbound rule (Rspamd, SpamAssassin or your SEG) for Content-Type: text/plain bodies with URLs containing ", ', <, > or event handlers (onerror=, onmouseover=, onload=, style=).
  
  Quarantine matches.
- Browser hardening via HTTP headers on the web server:
  
  Set a strict Content-Security-Policy (script-src 'self', no external connect-src, form-action 'self'). Test against your installed plugins before enforcing.
  
  Also set X-Content-Type-Options: nosniff.
- Session protection in config.inc.php:
  
  $config['session_samesite'] = 'Strict';
  
  $config['use_https'] = true;
  
  a short session_lifetime
  
  enable MFA through the 2FA plugin.
- Restrict forwarding (IAM, least privilege): Block or require approval for external forwarding via managesieve at the Dovecot or MTA layer. Log outbound redirects to external domains.
- Endpoint: Restrict webmail use by privileged accounts to hardened browser profiles. Mailbox exfiltration runs from the victim's browser context.
03
Runbook step 3
Detection rules
- Web server access logs: Shortly after _task=mail&_action=show or _action=preview, the same session issues unusual actions such as:
  
  _action=plugin.managesieve-save
  
  _task=settings&_action=save-identity
  
  bursts of _action=viewsource or _action=export
  
  SPL: index=web uri_query="*_task=settings*" (uri_query="*managesieve-save*" OR uri_query="*save-identity*") | stats count by user, src_ip
- Proxy and network telemetry (Zeek http.log, proxy logs): Browser requests to unfamiliar external domains with the webmail host as Referer, right after a message is opened. This suggests JavaScript-driven exfiltration (large POST bodies, Base64-looking payloads).
- Mail gateway and MTA logs: Inbound text/plain mail with event-handler strings inside URLs.
  
  Sigma shape: selection: body|contains: ['onerror=', 'onmouseover='] AND content_type: 'text/plain'
  
  Suricata: SMTP file.data with content:"http"; pcre:"/https?:\/\/[^\s]*[\"'][^\s]*on\w+=/i".
- Dovecot and Sieve logs: New redirect or :copy rules pointing to external addresses, especially across several accounts within a short window.
- Roundcube logs (logs/userlogins, logs/errors): The same account logging in from a new IP or ASN after session activity. This indicates hijacked sessions.

Detection rules

No NEOSEC detection rule is available for this vulnerability yet.

Classification under NIS2 and CRA

Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

NIS2 NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management. Read in glossary: Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
CRA The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates. Read in glossary: Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
ISO/IEC 27001 ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security. Read in glossary: Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

Description

Roundcube before 1.4.14, 1.5.x before 1.5.4, and 1.6.x before 1.6.3 allows XSS via text/plain e-mail messages with crafted links because of program/lib/Roundcube/rcube_string_replacer.php behavior.

Source: pwn.watch, NVD (NIST), CISA KEV Catalog, OSV (Open Source Vulnerabilities), CVE.org (MITRE), cvelistv5

Affected products

The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

Affected products
		Affected versions	Fixed in
roundcube	webmail	`1.5.0 – < 1.5.4` `1.6.0 – < 1.6.3` `< 1.4.14`	`1.5.4` `1.6.3` `1.4.14`

Distribution and container packages (2)

Downstream packages (e.g. Bitnami, Debian, Ubuntu, Red Hat). Versions refer to the distribution, not the vendor.

Distribution and container packages
		Affected versions	Fixed in
bitnami	roundcube	`1.6.0` `1.5.0`	— —

Vendors and products

Affected operating systems

Affected operating systems

Linux	debian	debian_linux	`10.0`

Weakness classes (CWE)

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Base
The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.
Entry at MITRE CWE-79

Public exploit references

Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

Public exploit references

CISA KEV Catalog	CISA Known Exploited Vulnerability (opens in a new tab)cisa.gov	actively exploited	12.02.2024

References & sources

github.comroundcube/roundcubemail/commit/e92ec206a886461245e1672d8530cc93…web, Patch
lists.debian.org[debian-lts-announce] 20230922 [SECURITY] [DLA 3577-1] roundcube security updateweb, Mailing list, Mailing list
roundcube.netRoundcube advisory: CVE-2023-43770web, Release notes, pwn-watch
cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2023-43770government-resource, US government resource
pwn.watchpwn.watch: Roundcube Webmail Persistent Cross-Site Scripting (XSS)pwn-watch
NVD (NIST)NVD: CVE-2023-43770pwn-watch
cisa.govCISA Known Exploited Vulnerabilities Catalogpwn-watch

Linked advisories

ta458 roundpress exploitsProofpoint Threat Insight23.07.2026

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What is affected, what to do

Metrics

Response and mitigation

Prioritisation rationale

Immediate response (0-24 h)

Mitigation layers

Detection rules

Detection rules

Classification under NIS2 and CRA

Description

Affected products

Vendors and products

Affected operating systems

Weakness classes (CWE)

Public exploit references

References & sources

Linked advisories