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CVE-2026-49476 high CVSS 7.5

Vulnerability in soupsieve (CVE-2026-49476)

Summary

Vulnerability in soupsieve (CVE-2026-49476). Risk of unauthorized operations or information disclosure. Exploitable via CSSSelector. Mitigation: upgrade to 2.8.4 or later.

Original description

The description as published by the source.

Summary

The CSS selector parser in soupsieve (the CSS selector engine for Beautiful Soup 4) allocates unbounded memory when compiling large comma-separated selector lists. An attacker who can supply a crafted CSS selector string to soupsieve.compile() or Beautiful Soup's .select() / .select_one() can cause the application to allocate hundreds of megabytes of heap memory from a relatively small input, leading to memory exhaustion and denial of service.

To be completely transparent, AI tools helped surface this issue. However, it was independently reproduced and carefully validated. Researchers follow responsible disclosure practices and originally shared this report privately.

A 500 KB selector string triggers allocation of approximately 244 MB of heap memory - a 488x— amplification ratio**.

Details

Affected code: soupsieve/css_parser.py, lines ~204, 925, 1106

The soupsieve CSS parser splits comma-separated selector lists and creates one CSSSelector object per list item. Each CSSSelector object contains parsed selector data structures including SelectorList, Selector, and associated tag/attribute/pseudo-class metadata.

When a selector string such as a,a,a,... (with 250,000 comma-separated items) is passed to sv.compile(), the parser:

  1. Tokenises the entire string and identifies each comma-delimited segment (line ~1106)
  2. Parses each segment into a full Selector object with all associated metadata (line ~925)
  3. Stores all par…

Source: GitHub Security Advisories (Global) NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-49476 has been found in soupsieve.
Attackers can target a specific entry point like CSSSelector over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 7.5/10.
What to do: upgrade soupsieve to 2.8.4 or later.
If unsure, ask your IT team or search "soupsieve CVE-2026-49476" on the vendor's site.

CVE-2026-49476 (soupsieve) — CWE-400 / CVSS v3 7.5
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: CSSSelector / SelectorList / Selector / MemoryError
Patched: 2.8.4 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-400) exists in soupsieve. Attackers reach the vulnerable code path via CSSSelector without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of soupsieve. Attack surface: CSSSelector / SelectorList / Selector / MemoryError.

🔥 Severity

Severity: High (CVSS 7.5/10). Risk of unauthorized operations or information disclosure.

🔧 How to fix

Update to 2.8.4.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'soupsieve' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'soupsieve' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で soupsieve を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Apply patch patch 
    Upgrade soupsieve to 2.8.4

    ステージング環境で 2.8.4 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  3. 3
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

pip soupsieve
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.8.4"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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