How should we interpret CVSS severity scores?

CVSS estimates technical severity, but it does not automatically equal business risk. Prioritize using context like internet exposure, asset criticality, known exploitation, and whether compensating controls exist. A Medium CVSS on an exposed production system can be more urgent than a Critical on an isolated non-production host.

Vulnerability Database

Q: What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

Q: What's the difference between a vulnerability, an exploit, and a zero-day?

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. Risk increases sharply when exploitation becomes reliable or widespread.

Q: Why do vulnerabilities keep reappearing in our environment?

Recurring findings usually come from incomplete asset discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host.

Q: How do we prioritize remediation without burning out the team?

Use a repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress so remediation is steady, not reactive.

Q: How can SynScan help reduce vulnerability risk over time?

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.

359,359

Total vulnerabilities in the database

Bleach linkify(parse_email=True) CPU exhaustion via unbounded email regex scanning — bleach

Summary

Bleach 6.3.0 exposes a documented email-linkification path through bleach.linkify(..., parse_email=True). The implementation scans attacker-controlled text with EMAIL_RE.finditer() over the full character token and has no length, timeout, or linear prefilter before applying the dot-atom email regex. A non-email payload around 30 KB causes multi-second CPU consumption per request/call, creating a direct availability risk for applications that enable email linkification on user-submitted text.

Affected Product

Package: bleach
Ecosystem: pip
Affected versions: verified in 6.3.0; exact first affected version not established
Patched versions: none known at finalization time
Tested version: 6.3.0
Audit commit/tag: v6.3.0 / 5546d5dbce60d08ccb99d981778d74044d646d4e
PyPI sdist SHA256: 6f3b91b1c0a02bb9a78b5a454c92506aa0fdf197e1d5e114d2e00c6f64306d22

Vulnerability Details

CWE: CWE-1333: Inefficient Regular Expression Complexity; related availability impact maps to CWE-400
Component: bleach/linkifier.py, build_email_re(), LinkifyFilter.handle_email_addresses()
Root cause: handle_email_addresses() calls self.email_re.finditer(text) on attacker-controlled text. EMAIL_RE includes a repeated dot-atom local-part pattern, so non-email strings such as repeated a. segments with no @ force repeated long failing scans.
Security boundary violated: user-submitted text processed by a documented safe linkification helper should not allow an attacker to impose superlinear CPU cost through non-email text.
Direct impact: per-request CPU exhaustion / denial-of-service risk in applications that enable parse_email=True on attacker-controlled text.
Chain impact, if any: one proof run observed an unrelated /health request delayed during a concurrent attack request, but this was not reliable across reviewer retests. Treat cross-request service degradation as environment-dependent supporting evidence, not the primary impact.
Severity estimate: Medium / availability-only. The feature is opt-in and deployment body limits/timeouts affect practical severity.

Relevant code path:

bleach/__init__.py:85-125: public linkify(text, ..., parse_email=False) constructs Linker(..., parse_email=parse_email) and calls linker.linkify(text).
bleach/linkifier.py:77-88: EMAIL_RE is compiled from the dot-atom email pattern.
bleach/linkifier.py:292-301: handle_email_addresses() applies self.email_re.finditer(text) to each character token.
bleach/linkifier.py:620-623: character tokens are routed into email handling only when parse_email is true.
docs/goals.rst:30-40: Bleach documents user comments, profile bios, and descriptions as target untrusted text use cases.
docs/linkify.rst:300-305: parse_email=True is the documented option for creating mailto: links.

Attack Preconditions

The consuming application enables the documented parse_email=True option, for example bleach.linkify(user_text, parse_email=True) or Linker(parse_email=True).linkify(user_text).
The attacker can submit text that reaches that linkification path. Authentication depends on the host application; a public comment form would make this unauthenticated, while account-only text fields require user privileges.
The application allows roughly 20-30 KB of text to reach Bleach and lacks a strict timeout or input cap before linkification.
No custom bounded email_re is supplied.

Reproduction

Minimal API trigger:

import bleach
payload = (&quot;a.&quot; * 15000) + &quot;a&quot;
bleach.linkify(payload, parse_email=True)

The saved HTTP proof uses a local harness with POST /preview calling bleach.linkify(request_body, parse_email=True) and a control endpoint using parse_email=False on the same payload. The exploit sends baseline/control/attack requests over HTTP to 127.0.0.1.

Proof Evidence

The proof ran against Bleach 6.3.0 installed from the audited local checkout in an isolated temporary venv. It used Python 3.12.3 on Linux.

Measured HTTP proof results:

Payload: ("a." * 15000) + "a" (30001 bytes)
Normal baseline /preview mean: 0.001425 seconds
Same 30 KB payload with parse_email=False: 0.048349 seconds
Attack payload with parse_email=True: 8.719818 seconds
Slowdown versus the larger baseline/control mean: 180.35x
Requests sent by proof: 20

Evidence files: poc.py poc_results.json exploit_proof.py exploit_results.json

Scope and Limitations

This report does not claim XSS, authentication bypass, data disclosure, remote code execution, persistent crash, or persistent service outage.
parse_email=True is not the default. The affected path is a documented opt-in feature.
The exact first affected version is not established.
Practical impact depends on host application input limits, worker model, request timeout policy, and whether untrusted users can submit text to an email-linkification path.
A reviewer reproduced the direct CPU cost but did not reproduce the proof harness’s /health delay. The direct impact claim is therefore limited to per-request CPU exhaustion.
Bleach is marked deprecated in README.rst, and SECURITY.md has stale supported-version text, but the package still has a 2025 PyPI release and published Mozilla security reporting routes.

Published: Jun 16, 2026
Updated: Jun 17, 2026
GHSA: GHSA-g75f-g53v-794x
Severity: Medium
Exploit:
CISA KEV:

CVSS v3:

Severity: Low
Score: 4.3
AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L

CWEs:

CWE-1333

Affected Software
References

Software	From	Fixed in
bleach	6.3.0	6.3.0.x

https://github.com/mozilla/bleach/security/advisories/GHSA-g75f-g53v-794x

Deep Security Visibility Without the Complexity

SynScan provides clear, real-time security insights so you can monitor your attack surface, spot risks early, and act fast—without extra complexity.

No setup fees
5-min deployment
Cancel anytime

Book a Demo

Frequently Asked Questions

What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.

Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.

Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.