{"id":65605,"date":"2026-08-03T10:08:57","date_gmt":"2026-08-03T17:08:57","guid":{"rendered":"https:\/\/sn1persecurity.com\/wordpress\/cve-2026-42533-nginx-map-heap-overflow-detection-with-sn1per\/"},"modified":"2026-08-03T10:08:57","modified_gmt":"2026-08-03T17:08:57","slug":"cve-2026-42533-nginx-map-heap-overflow-detection-with-sn1per","status":"publish","type":"post","link":"https:\/\/sn1persecurity.com\/wordpress\/cve-2026-42533-nginx-map-heap-overflow-detection-with-sn1per\/","title":{"rendered":"CVE-2026-42533: NGINX map Directive Heap Buffer Overflow &#8211; Advisory + Nuclei Detection"},"content":{"rendered":"<p>On July 15, 2026, F5 patched <strong>CVE-2026-42533<\/strong>, a heap buffer overflow in NGINX reachable through the <code>map<\/code> directive. F5 scored it 9.2 Critical under CVSS 4.0. Twelve days later, on July 27, fully weaponized exploit code appeared on GitHub claiming ten-out-of-ten reliability against a current Ubuntu build. The vulnerable code path has existed since the <code>map<\/code> directive gained regular expression support in <strong>March 2011<\/strong>.<\/p>\n<p>This advisory is three things at once. First, a plain-English explanation of the bug and, more importantly, of the version arithmetic &#8211; which has a wrinkle that will cause a lot of scanners to report false negatives across an entire release branch. Second, a ready-to-run <strong>Nuclei detection template<\/strong> with the version logic done correctly, plus a configuration audit template that answers the question version detection cannot. Third, how to find every affected NGINX instance across an attack surface with <a href=\"\/wordpress\/sn1per-professional-2026\/\">Sn1per<\/a>.<\/p>\n<h2 id=\"at-a-glance\">CVE-2026-42533 at a glance<\/h2>\n<ul>\n<li><strong>CVE:<\/strong> CVE-2026-42533 &#8211; heap-based buffer overflow (CWE-122) in the NGINX script engine, reached via the <code>map<\/code> directive.<\/li>\n<li><strong>Impact:<\/strong> out-of-bounds write plus an uninitialised-heap read. Chained, the read supplies the ASLR bypass the write needs, which is why public tooling claims pre-authentication RCE rather than the denial of service the vendor advisory leads with.<\/li>\n<li><strong>CVSS:<\/strong> 9.2 Critical under CVSS 4.0 (F5), 8.1 High under CVSS 3.1.<\/li>\n<li><strong>Affected:<\/strong> <strong>0.9.6 through 1.31.2<\/strong> &#8211; but see the version-window section, because that range is <em>not<\/em> contiguous in practice.<\/li>\n<li><strong>Fixed:<\/strong> <strong>1.30.4<\/strong> (stable) and <strong>1.31.3<\/strong> (mainline). NGINX Plus 37.0.3.1 or R36 P7.<\/li>\n<li><strong>Status:<\/strong> weaponized public proof-of-concept since July 27, 2026, with a documented ASLR bypass.<\/li>\n<li><strong>Age:<\/strong> introduced March 2011, disclosed 2026. Fifteen years of exposure.<\/li>\n<li><strong>Shipped alongside:<\/strong> CVE-2026-60005 (slice module memory disclosure) and CVE-2026-56434 (SSI use-after-free). One upgrade closes all three.<\/li>\n<\/ul>\n<h2 id=\"what-is-cve-2026-42533\">What is CVE-2026-42533?<\/h2>\n<p>NGINX evaluates complex string expressions in two passes. The LEN pass measures how many bytes the result needs so the right buffer can be allocated; the VALUE pass writes the bytes into it. Both passes read capture groups from the same mutable array, <code>r-&gt;captures<\/code>, and the whole scheme is correct only if that array holds the same contents during both walks.<\/p>\n<p>A <code>map<\/code> directive whose pattern is a regular expression breaks that guarantee. When such a map executes <em>between<\/em> two references to a capture group, it silently rewrites the shared capture state. The LEN pass measured against one value; the VALUE pass writes a different one. The two disagree, and the copy runs off the end of the allocation.<\/p>\n<p>What makes this more serious than a typical size-mismatch overflow is that the primitive runs in <strong>both directions<\/strong>, and the two directions compose into a complete exploit:<\/p>\n<ul>\n<li>When the clobbering capture is <strong>larger<\/strong> than the original, the VALUE pass writes past the end of a buffer sized for the smaller one. That is an attacker-influenced out-of-bounds write.<\/li>\n<li>When it is <strong>smaller<\/strong>, the buffer is oversized and its tail is never initialised. The response returns raw heap memory, leaking libc and heap pointers.<\/li>\n<\/ul>\n<p>The leak supplies exactly what the write is missing. F5&#8217;s advisory is conservative, describing code execution as conditional on ASLR being disabled or bypassable. Researchers have pushed back on that framing with a specific argument: the bug <em>provides its own ASLR bypass<\/em>, because on a default Ubuntu build a single unauthenticated GET recovers the addresses a payload needs. That is the difference between a theoretical RCE and a practical one, and it is why the published exploit claims the reliability it does.<\/p>\n<p>This is the same defect class as <a href=\"\/wordpress\/cve-2026-42945-nginx-rift-heap-overflow-detection-with-sn1per\/\">CVE-2026-42945<\/a>, &#8220;NGINX Rift,&#8221; disclosed two months earlier. Same file, <code>src\/http\/ngx_http_script.c<\/code>, same two-pass contract violation, different directive as the entry point.<\/p>\n<h2 id=\"version-windows\">The version window trap: why 1.31.x is not safe<\/h2>\n<p>This is the section we would most like people to take away, because it is where automated checks go wrong.<\/p>\n<p>The affected range is usually written as &#8220;0.9.6 through 1.31.2.&#8221; Read literally as a single interval, that is fine. Implemented literally as a single interval, it is also fine. The problem is what happens when someone reasons from the <em>fix<\/em> version instead, which is the natural thing to do: 1.30.4 is fixed, so surely anything above 1.30.4 is fixed.<\/p>\n<p>It is not. NGINX maintains stable and mainline branches in parallel. The fix landed on stable as <strong>1.30.4<\/strong> and on mainline as <strong>1.31.3<\/strong>. But mainline 1.31.0, 1.31.1 and 1.31.2 were all cut <em>before<\/em> that fix. So the vulnerable set has a hole in the middle:<\/p>\n<table>\n<thead>\n<tr>\n<th>Version range<\/th>\n<th>Status<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>0.9.6 &#8211; 1.30.3<\/td>\n<td><strong>Vulnerable<\/strong><\/td>\n<\/tr>\n<tr>\n<td>1.30.4 and later on the stable branch<\/td>\n<td>Fixed<\/td>\n<\/tr>\n<tr>\n<td><strong>1.31.0 &#8211; 1.31.2<\/strong><\/td>\n<td><strong>Vulnerable again<\/strong><\/td>\n<\/tr>\n<tr>\n<td>1.31.3 and later<\/td>\n<td>Fixed<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Any check written as a simple &#8220;is the version below 1.30.4&#8221; comparison will report <strong>every mainline 1.31.x host as patched<\/strong>, including the three releases that are not. That is a false negative across an entire branch, and it fails silently &#8211; the scan comes back clean and nobody investigates. Our template handles it with two disjoint matchers, which we show below.<\/p>\n<p>Contrast this with NGINX Rift, where stable is fixed from 1.30.1 and mainline from 1.31.0. Everything above 1.30.0 is patched, so a single upper bound genuinely is correct there. Two closely related CVEs in the same file, two different shapes of version logic. Getting them backwards produces silent misses in both directions.<\/p>\n<h2 id=\"am-i-affected\">Am I affected? Quick manual checks<\/h2>\n<p>Locally, one command, read against the table above:<\/p>\n<pre><code>nginx -v\n\n# nginx version: nginx\/1.29.2   -&gt; vulnerable\n# nginx version: nginx\/1.30.1   -&gt; vulnerable (fixes Rift only)\n# nginx version: nginx\/1.31.1   -&gt; vulnerable (mainline cut before the fix)\n# nginx version: nginx\/1.30.4   -&gt; fixed\n# nginx version: nginx\/1.31.3   -&gt; fixed<\/code><\/pre>\n<p>Remotely and unauthenticated, read the Server header, falling back to the footer of NGINX&#8217;s own default error page:<\/p>\n<pre><code>curl -sI https:\/\/target.example.com | grep -i '^server:'\ncurl -s https:\/\/target.example.com\/does-not-exist-12345 | grep -i 'center&gt;nginx'<\/code><\/pre>\n<p>Then check whether the configuration reaches the bug, since a regex-based <code>map<\/code> is the precondition:<\/p>\n<pre><code>grep -rnE '^\\s*map\\s+.*\\{' \/etc\/nginx\/\ngrep -rnE '^\\s*~\\*?.*\\([^?]' \/etc\/nginx\/<\/code><\/pre>\n<p>As always, <strong>absence of a banner is not evidence of patching<\/strong>. <code>server_tokens off<\/code> suppresses the version, and edges routinely rewrite the header. Treat a silent host as unknown.<\/p>\n<h2 id=\"nuclei-template\">CVE-2026-42533 Nuclei detection template<\/h2>\n<p>Below is the full template. It is <strong>non-destructive<\/strong>: one ordinary GET, version comparison only, no overflow payload, no contact with the vulnerable code path. Note the two separate matchers, which is the whole point of the version-window section above.<\/p>\n<pre><code>id: CVE-2026-42533\n\ninfo:\n  name: NGINX - map Directive Regex Heap Buffer Overflow (CVE-2026-42533)\n  author: xer0dayz\n  severity: high\n  description: |\n    Heap-based buffer overflow (CWE-122) in the NGINX script engine. A map\n    directive with a regex pattern, executing between two references to a capture\n    group, rewrites the shared r-&gt;captures array so the LEN and VALUE passes\n    disagree on buffer size.\n\n    NON-DESTRUCTIVE and PASSIVE. One ordinary GET, version comparison only.\n\n    Treat a hit as a patch-gap finding, NOT a proven exploitable RCE. Exploitation\n    is config-gated on a regex-based map whose output is referenced after an\n    earlier unnamed capture.\n  classification:\n    cvss-metrics: CVSS:3.1\/AV:N\/AC:H\/PR:N\/UI:N\/S:U\/C:H\/I:H\/A:H\n    cvss-score: 8.1\n    cve-id: CVE-2026-42533\n    cwe-id: CWE-122\n  metadata:\n    verified: false\n    max-request: 2\n    vendor: f5\n    product: nginx\n    patched-versions: \"1.30.4, 1.31.3\"\n  tags: cve,cve2026,nginx,f5,overflow,rce,patch-gap,passive\n\nhttp:\n  - method: GET\n    path:\n      - \"{{BaseURL}}\"\n      - \"{{BaseURL}}\/{{randstr}}\"\n\n    stop-at-first-match: true\n    redirects: false\n\n    # Two disjoint vulnerable windows, because the fix landed on the stable and\n    # mainline branches at different points. Stable is fixed from 1.30.4, so\n    # everything from 0.9.6 up to 1.30.3 is affected. Mainline 1.31.0 through\n    # 1.31.2 shipped before the fix and is affected again; 1.31.3 closes it.\n    matchers-condition: or\n    matchers:\n      - type: dsl\n        name: nginx-0.9.6-to-1.30.3\n        dsl:\n          - \"version != '' &amp;&amp; compare_versions(version, '&gt;= 0.9.6', '&lt; 1.30.4')\"\n\n      - type: dsl\n        name: nginx-mainline-1.31.0-to-1.31.2\n        dsl:\n          - \"version != '' &amp;&amp; compare_versions(version, '&gt;= 1.31.0', '&lt; 1.31.3')\"\n\n    extractors:\n      - type: regex\n        name: version\n        part: response\n        group: 1\n        internal: true\n        regex:\n          - '(?:[Ss]erver:\\s*nginx\/|&lt;center&gt;nginx\/)([0-9]+\\.[0-9]+\\.[0-9]+)'\n\n      - type: regex\n        name: nginx_version\n        part: response\n        group: 1\n        regex:\n          - '(?:[Ss]erver:\\s*nginx\/|&lt;center&gt;nginx\/)([0-9]+\\.[0-9]+\\.[0-9]+)'<\/code><\/pre>\n<pre><code>nuclei -t CVE-2026-42533.yaml -l targets.txt<\/code><\/pre>\n<p>We verified both windows against a twelve-version mock matrix, checking every boundary: 1.30.3 flags, 1.30.4 does not, 1.31.0 flags again, 1.31.3 does not. All twelve boundary cases pass.<\/p>\n<h2 id=\"config-audit\">The second template: auditing the configuration<\/h2>\n<p>A version match is a patch-gap finding, not proof of exploitability, because this bug is <strong>config-gated<\/strong>. It only surfaces when a regex-based <code>map<\/code> directive&#8217;s output variable is referenced in a string expression <em>after<\/em> an unnamed capture from an earlier regex match. An attacker cannot create that arrangement remotely.<\/p>\n<p>So the companion template reads the configuration and looks for the ingredients directly:<\/p>\n<pre><code>file:\n  - extensions:\n      - all\n\n    matchers:\n      # All three ingredients must be present in the same file: a map block, a\n      # regex-keyed entry that uses an UNNAMED capture, and capture usage.\n      - type: regex\n        name: cve-2026-42533-map-regex-with-unnamed-capture\n        condition: and\n        regex:\n          - '(?im)^\\s*map\\s+[^{\\n]*\\{'\n          - '(?m)^\\s*~\\*?[^\\n;]*\\([^?]'\n          - '\\$[1-9]'<\/code><\/pre>\n<p>The middle pattern deserves a note, because it is where the first draft of this template was wrong. Our initial version required a map block, a regex-keyed entry, and <code>$1<\/code> anywhere in the file. That fired on a <em>safe<\/em> configuration whose map used named captures &#8211; <code>(?&lt;name&gt;...)<\/code> &#8211; but which happened to contain an unrelated <code>$1<\/code> in a rewrite elsewhere in the same file. Named captures are not vulnerable, so that was a false positive on exactly the configuration that had done the right thing.<\/p>\n<p>The fix is to require the unnamed capture to appear <em>inside the map entry itself<\/em>. Expressing &#8220;an open parenthesis not followed by a question mark&#8221; is normally a job for negative lookahead, which Go&#8217;s RE2 engine does not support. The workaround is <code>\\([^?]<\/code> &#8211; an open parenthesis followed by any character that is not a question mark. That clears <code>(?&lt;name&gt;<\/code>, <code>(?P&lt;name&gt;<\/code> and <code>(?:<\/code>, all of which are safe, while still catching the vulnerable <code>(.*)<\/code> form.<\/p>\n<p>Run it against the config tree. <strong>The <code>-file<\/code> flag is required<\/strong>, or Nuclei silently skips file-protocol templates:<\/p>\n<pre><code>nuclei -file -t nginx-script-engine-config-audit.yaml -target \/etc\/nginx\/<\/code><\/pre>\n<p>Two honest limitations. The matcher is file-scoped, so a split configuration that puts the map block in <code>conf.d\/maps.conf<\/code> and the capture usage in <code>sites-enabled\/site<\/code> will not match either file on its own. And a clean audit is never a reason to leave an unpatched binary running, because any future config change can introduce the pattern.<\/p>\n<h2 id=\"sn1per\">Finding affected NGINX across your attack surface with Sn1per<\/h2>\n<p>Checking one host is easy. The real problem is the host you forgot: the staging environment behind a stale DNS record, the appliance that embeds NGINX as a component, the ingress controller from a proof of concept nobody decommissioned. Those hosts are exactly the ones that sit on old builds forever, because no patch process knows they exist.<\/p>\n<p>That is attack surface discovery, and it is what <a href=\"\/wordpress\/sn1per-professional-2026\/\">Sn1per<\/a> does. Discovery first, detection second:<\/p>\n<pre><code>sniper -t example.com -m recon\nsniper -t example.com -m web<\/code><\/pre>\n<p>Both templates ship in Sn1per&#8217;s curated set, so affected hosts surface automatically during enumeration. The check is passive, so it is safe to run against production on a schedule.<\/p>\n<p>The output that matters is not a single verdict but a version inventory across the estate &#8211; which is the only way to catch the hosts sitting on 1.31.1 that a naive &#8220;below 1.30.4&#8221; check would have cleared.<\/p>\n<h2 id=\"editions\">Which Sn1per edition fits<\/h2>\n<ul>\n<li><strong>Sn1per Community Edition<\/strong> &#8211; free and open source. Run the templates by hand against hosts you already know about.<\/li>\n<li><strong><a href=\"\/wordpress\/sn1per-professional-2026\/\">Sn1per Professional 2026<\/a><\/strong> &#8211; web UI, scheduled scans and reporting. Turns a one-off check into continuous coverage.<\/li>\n<li><strong>Sn1per Enterprise<\/strong> &#8211; multi-workspace attack surface management with continuous monitoring and alerting, for estates where the NGINX inventory is genuinely unknown.<\/li>\n<\/ul>\n<h2 id=\"remediation\">Remediation and mitigation<\/h2>\n<ol>\n<li><strong>Upgrade to 1.30.4 (stable) or 1.31.3 (mainline).<\/strong> These close CVE-2026-42533, CVE-2026-60005 and CVE-2026-56434 together, and also carry the earlier CVE-2026-42945 fix. NGINX Plus users want 37.0.3.1 or R36 P7.<\/li>\n<li><strong>Do not stop at 1.30.1.<\/strong> That release fixes only NGINX Rift and leaves this bug open. It is the most common partial-patch mistake we expect to see.<\/li>\n<li><strong>Patch downstream products.<\/strong> NGINX Ingress Controller, Gateway Fabric, App Protect WAF and Instance Manager ship the same script engine.<\/li>\n<li><strong>Interim mitigation only:<\/strong> replace unnamed regex captures with named captures in <code>map<\/code> directives, and reference them only inside the block holding the regex match. Note that CVE-2026-56434, shipped in the same release, has <em>no<\/em> documented workaround, so mitigation cannot substitute for the upgrade.<\/li>\n<li><strong>Do not rely on <code>server_tokens off<\/code>.<\/strong> It hides the version from casual fingerprinting. It does not remediate anything.<\/li>\n<\/ol>\n<h2 id=\"faq\">Frequently asked questions<\/h2>\n<p><strong>Is nginx 1.31.1 affected by CVE-2026-42533?<\/strong><br \/>\nYes. This catches people out. The fix landed on stable as 1.30.4 and on mainline as 1.31.3, and mainline 1.31.0 through 1.31.2 were cut before it. A version higher than 1.30.4 is not automatically patched, because it may be on the other branch.<\/p>\n<p><strong>Does upgrading to 1.30.1 fix CVE-2026-42533?<\/strong><br \/>\nNo. 1.30.1 fixes CVE-2026-42945 (NGINX Rift) only. CVE-2026-42533 was fixed in 1.30.4, alongside CVE-2026-60005 and CVE-2026-56434.<\/p>\n<p><strong>Is there a public exploit for CVE-2026-42533?<\/strong><br \/>\nYes. Weaponized proof-of-concept code has been public since July 27, 2026, including a chained heap leak that defeats ASLR. We do not link or reproduce weaponized code here; the templates in this advisory are detection-only.<\/p>\n<p><strong>Can I safely test whether my server is vulnerable?<\/strong><br \/>\nNot by exploiting it. Triggering the bug corrupts a worker heap and crashes the worker. Use passive version detection plus the configuration audit. Never attempt active confirmation against a bug bounty target or an asset you do not own.<\/p>\n<p><strong>Does a vulnerable version mean I am exploitable?<\/strong><br \/>\nNot necessarily. Exploitation requires a regex-based map whose output is referenced after an earlier unnamed capture. Distributions also backport fixes without changing the version string, and the banner you see may belong to a CDN edge rather than the origin. Treat a version hit as a patch gap to investigate, not a confirmed RCE.<\/p>\n<p><script type=\"application\/ld+json\">\n{\"@context\":\"https:\/\/schema.org\",\"@type\":\"FAQPage\",\"mainEntity\":[\n{\"@type\":\"Question\",\"name\":\"Is nginx 1.31.1 affected by CVE-2026-42533?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"Yes. The fix landed on the stable branch as 1.30.4 and on mainline as 1.31.3, and mainline releases 1.31.0 through 1.31.2 were cut before the fix. A version number higher than 1.30.4 is not automatically patched, because it may be on the other branch. Any check written as a simple 'below 1.30.4' comparison will incorrectly clear every 1.31.x host.\"}},\n{\"@type\":\"Question\",\"name\":\"Does upgrading to nginx 1.30.1 fix CVE-2026-42533?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"No. Version 1.30.1 fixes CVE-2026-42945, known as NGINX Rift, only. CVE-2026-42533 was fixed later in 1.30.4 on stable and 1.31.3 on mainline, alongside CVE-2026-60005 and CVE-2026-56434.\"}},\n{\"@type\":\"Question\",\"name\":\"Is there a public exploit for CVE-2026-42533?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"Yes. Weaponized proof-of-concept code has been publicly available since July 27, 2026, including a chained uninitialised heap read that defeats ASLR by leaking libc and heap pointers. Detection-only templates are the appropriate tool for defenders.\"}},\n{\"@type\":\"Question\",\"name\":\"Can I safely test whether my server is vulnerable to CVE-2026-42533?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"Not by exploiting it. Triggering the bug corrupts a worker process heap and crashes the worker, which is itself a denial of service. Use passive version detection combined with a configuration audit instead. Never attempt active confirmation against a bug bounty target or any asset you do not own.\"}},\n{\"@type\":\"Question\",\"name\":\"Does a vulnerable nginx version mean I am exploitable?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"Not necessarily. Exploitation requires a regex-based map directive whose output variable is referenced in a string expression after an earlier unnamed capture. Distributions also backport security fixes without changing the version string, and the banner may belong to a CDN edge rather than the origin. 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A plain-English advisory, a tested non-destructive Nuclei detection template, and the two-window version trap that makes naive scanners clear every mainline 1.31.x host as patched.<\/p>\n","protected":false},"author":1,"featured_media":65606,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_exactmetrics_skip_tracking":false,"_exactmetrics_sitenote_active":false,"_exactmetrics_sitenote_note":"","_exactmetrics_sitenote_category":0,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_post_was_ever_published":false},"categories":[290,80,99,184,289,338,335],"tags":[359,472,471,467,468,473,466,465,437,37,366,469],"class_list":["post-65605","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-attack-surface-management","category-bug-bounties","category-cves","category-news","category-penetration-testing","category-threat-intelligence","category-vulnerability-scanning","tag-359","tag-aslr-bypass","tag-cve-2026-42533","tag-heap-buffer-overflow","tag-memory-corruption","tag-nginx-map-directive","tag-nginx-security","tag-nginx-vulnerability","tag-nuclei-template","tag-sn1per","tag-sn1per-pro","tag-web-server-security"],"aioseo_notices":[],"aioseo_head":"\n\t\t<!-- All in One SEO 5.0.0.1 - aioseo.com -->\n\t<meta name=\"description\" content=\"CVE-2026-42533: critical nginx heap overflow via the map directive, with public ASLR-defeating exploit code. 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