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CVE-2026-71488 high CVSS 7.5

Vulnerability in league/commonmark (CVE-2026-71488)

Summary

Vulnerability in league/commonmark (CVE-2026-71488). Risk of unauthorized operations or information disclosure. Exploitable via CommonMarkConverter. Mitigation: upgrade to 2.9.0 or later.

Original description

The description as published by the source.

Impact

Affected versions of league/commonmark can have quadratic time complexity when parsing specially crafted Markdown lines. In practical terms, doubling the length of an affected line can make the parser perform roughly four times as much work. The parser identifies locations using character positions, but regular-expression matches report byte positions. These positions differ when a UTF-8 character uses more than one byte. Several parsing paths repeatedly rescan growing portions of the line to translate between the two positions. The Autolink extension can also copy and validate the remaining line at every URL-like prefix.

In current 2.x releases, a single non-ASCII character anywhere on a line can place that whole line on the slower multibyte path. An attacker can combine it with a long run of leading whitespace or repeated Markdown punctuation, causing increasingly large rescans. When the Autolink extension is enabled, repeated URL-like prefixes provide another trigger, even on ASCII-only lines. Each trigger fits within one long line, so complex Markdown structure is unnecessary.

An attacker who can submit Markdown for conversion can use a comparatively small request to consume disproportionate CPU time and allocation activity. Repeated or concurrent requests can occupy all available PHP workers and prevent legitimate requests from completing. The core paths affect CommonMarkConverter, GithubFlavoredMarkdownConverter, and custom environments. The autolink…

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-2026-71488 has been found in league/commonmark.
Attackers can target a specific entry point like CommonMarkConverter over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 7.5/10.
What to do: upgrade league/commonmark to 2.9.0 or later.
If unsure, ask your IT team or search "league/commonmark CVE-2026-71488" on the vendor's site.

CVE-2026-71488 (league/commonmark) — CWE-407 / CVSS v3 7.5
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: CommonMarkConverter / GithubFlavoredMarkdownConverter / AutolinkExtension / GithubFlavoredMarkdownExtension
Patched: 2.9.0 — 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-407) exists in league/commonmark. Attackers reach the vulnerable code path via CommonMarkConverter without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of league/commonmark. Attack surface: CommonMarkConverter / GithubFlavoredMarkdownConverter / AutolinkExtension / GithubFlavoredMarkdownExtension.

🔥 Severity

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

🔧 How to fix

Update to 2.9.0.

🛡️ 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 'league/commonmark' . 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 'league/commonmark' . | 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 league/commonmark 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 league/commonmark to 2.9.0

    ステージング環境で 2.9.0 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

composer league/commonmark
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.9.0"}]}]
Packagist league/commonmark
[{"type":"ECOSYSTEM","events":[{"introduced":"0.6.0"},{"fixed":"2.9.0"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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