{"id":"CVE-2026-33057","kind":"vulnerability","aliases":["CVE-2026-33057"],"related":[],"title":"Mesop: unauthenticated Base64 payloads in request body run arbitrary code","description":"WAF Release - 2026-04-15 · Changelog\n\n# Changelog\n\nNew updates and improvements at Cloudflare.\n\nApril 15, 2026\n\n## WAF Release - 2026-04-15\n\nThis week's release introduces a new detection for a critical Remote Code Execution (RCE) vulnerability in Mesop (CVE-2026-33057), alongside protections for high-impact vulnerabilities in Cisco Secure Firewall Management Center (CVE-2026-20079) and FortiClient EMS (CVE-2026-21643). Additionally, this release includes an update to our existing React Server DoS coverage to address recently identified resource exhaustion vectors (CVE-2026-23869).\n\nKey Findings\n\n- Cisco Secure FMC (CVE-2026-20079): A vulnerability in the web-based management interface of Cisco Secure Firewall Management Center (FMC) that allows an unauthenticated, remote attacker to execute arbitrary commands or bypass security filters.\n- FortiClient EMS (CVE-2026-21643): A critical vulnerability in the FortiClient EMS permitting unauthorized access or administrative configuration manipulation via crafted HTTP requests.\n- Mesop (CVE-2026-33057): A vulnerability in the Mesop Python-based UI framework where unauthenticated attackers can execute arbitrary code by sending specially crafted, Base64-encoded payloads in the request body.\n\nImpact\n\nSuccessful exploitation of these vulnerabilities could allow unauthenticated attackers to execute arbitrary code, gain administrative control over network management infrastructure, or trigger server-side resource exhaustion. Administrators are strongly encouraged to apply official vendor updates.\n\n| Ruleset | Rule ID | Legacy Rule ID | Description | Previous Action | New Action | Comments |\n| --- | --- | --- | --- | --- | --- | --- |\n| Cloudflare Managed Ruleset | ...aef9415b | N/A | Cisco Secure FMC - RCE via upgradeReadinessCall - CVE:CVE-2026-20079 | Log | Block | This is a new detection. |\n| Cloudflare Managed Ruleset | ...ee7be621 | N/A | FortiClient EMS - Pre-Auth SQL Injection - CVE:CVE-2026-21643 | Log | Block | This is a new detection. |\n| Cloudflare Managed Ruleset | ...c953a72b | N/A | Mesop - Remote Code Execution - Base64 Payload - CVE:CVE-2026-33057 | Log | Block | This is a new detection. |\n| Cloudflare Managed Ruleset | ...50c08f6f | N/A | React Server - DOS - CVE:CVE-2026-23864 - 1 - Beta | Log | Block | This rule has been merged into the original rule \"React Server - DOS - CVE:CVE-2026-23864 - 1\" (ID: ...61680354) |\n| Cloudflare Managed Ruleset | ...ebd81645 | N/A | XSS, HTML Injection - Link Tag - URI (beta) | N/A | Disabled | This is a new detection. |\n| Cloudflare Managed Ruleset | ...0af34bba | N/A | XSS, HTML Injection - Embed Tag - URI (beta) | N/A | Disabled | This is a new detection. |","summary":"Mesop, a Python UI framework, has a flaw where an unauthenticated attacker sends a specially crafted Base64-encoded request body and executes arbitrary code on the server. Cloudflare's WAF now blocks this pattern, but the framework itself must be upgraded to remove the flaw.","severity":"critical","cvss":9.8,"epss":0.04111,"exploited":false,"exploited_at":null,"products":[{"name":"mesop-dev mesop","fixed":["1.2.3"],"affected":"< 1.2.3","ecosystem":"vendor"}],"ecosystems":["vendor"],"tags":["cloudflare","lab-disclosure","mesop","package","python","self-hosted","web-apps"],"not_affected_if":["mesop-1.2.3-or-later","mesop-not-used"],"how_to_check":"- Run `pip show mesop` or inspect the lockfile to find the installed Mesop version; anything below `1.2.3` is affected.\n- Check whether Cloudflare WAF rule \"Mesop - Remote Code Execution - Base64 Payload - CVE:CVE-2026-33057\" is active; it is a block-mode detection, not a fix.","fix":"- Upgrade Mesop to `1.2.3` or later.\n- Until upgraded, keep the Cloudflare Managed Ruleset rule for CVE-2026-33057 in Block mode, which is a mitigation only.","status":"open","published_at":"2026-03-20T08:16:11+00:00","first_seen_at":"2026-10-24T19:12:57+00:00","triaged_at":"2026-10-25T05:28:00+00:00","updated_at":"2026-10-25T12:38:49+00:00","category":"package","confidence":0.7,"triage":{"at":"2026-10-25T05:28:00Z","model":"claude-haiku-5-5","output":{"fix":"- Upgrade Mesop to `1.2.3` or later.\n- Until upgraded, keep the Cloudflare Managed Ruleset rule for CVE-2026-33057 in Block mode, which is a mitigation only.","reach":"internet","title":"Mesop: unauthenticated Base64 payloads in request body run arbitrary code","impact":"high","summary":"Mesop, a Python UI framework, has a flaw where an unauthenticated attacker sends a specially crafted Base64-encoded request body and executes arbitrary code on the server. Cloudflare's WAF now blocks this pattern, but the framework itself must be upgraded to remove the flaw.","category":"package","nicknames":[],"who_cares":["python","mesop","self-hosted","web-apps"],"confidence":0.7,"needs_auth":"no","worst_case":"Unauthenticated attacker executes arbitrary code on the Mesop server.","analyst_note":"Check the NVD and Mesop release notes for the affected version range and any deployment-specific conditions; the sources only give the Cloudflare changelog and the NVD entry.","how_to_check":"- Run `pip show mesop` or inspect the lockfile to find the installed Mesop version; anything below `1.2.3` is affected.\n- Check whether Cloudflare WAF rule \"Mesop - Remote Code Execution - Base64 Payload - CVE:CVE-2026-33057\" is active; it is a block-mode detection, not a fix.","needs_analyst":true,"not_affected_if":["mesop-not-used","mesop-1.2.3-or-later"],"who_is_affected":"Teams running Mesop below `1.2.3` on a server reachable from the internet or an untrusted network are exposed, since the request body is the attack path. Applications that do not use Mesop, or that run it only on an internal machine with no untrusted access, are less exposed; the sources do not say whether specific deployment modes are safe.","user_interaction":"no","needs_special_setup":"unknown","severity_if_unknown":"critical"},"cost_usd":0.000798,"input_tokens":4462,"output_tokens":703,"severity_before":"critical"},"analyst_status":null,"analyst_note":null,"search":"'-04':83C,99C '-15':84C,100C '-20079':137C,174C,324C '-2026':12A,15A,120C,136C,142C,165C,173C,208C,230C,323C,346C,368C,387C,407C '-21643':143C,209C,347C '-23864':388C,408C '-23869':166C '-33057':13A,16A,121C,231C,369C '0af34bba':434C '1':389C,409C '15':94C '2026':82C,95C,98C '50c08f6f':380C '61680354':411C 'access':219C 'action':306C,308C 'addit':144C 'address':158C 'administr':221C,276C,289C 'aef9415b':313C 'allow':192C,268C 'alongsid':122C 'app':33B 'appli':294C 'april':93C 'arbitrari':9A,57B,199C,247C,273C 'attack':45B,196C,244C,270C 'auth':341C 'base':181C,239C 'base64':3A,51B,254C,364C 'base64-encoded':50B,253C 'beta':390C,423C,442C 'block':66B,326C,349C,371C,392C 'bodi':7A,54B,260C 'bypass':202C 'c953a72b':358C 'center':134C,189C 'changelog':85C,86C 'cisco':130C,169C,185C,315C 'cloudflar':21B,62B,92C,310C,332C,355C,377C,412C,431C 'code':10A,58B,113C,248C,274C,362C 'command':200C 'comment':309C 'configur':222C 'control':277C 'could':267C 'coverag':156C 'craft':49B,225C,252C 'critic':111C,211C 'cve':11A,14A,119C,135C,141C,164C,172C,207C,229C,321C,322C,344C,345C,366C,367C,385C,386C,405C,406C 'descript':304C 'detect':108C,331C,354C,376C,430C,449C 'dev':19A 'disabl':425C,444C 'disclosur':24B 'dos':155C,384C,404C 'ebd81645':415C 'ee7be621':335C 'em':140C,206C,216C,338C 'emb':439C 'encod':52B,255C 'encourag':292C 'execut':56B,114C,198C,246C,272C,363C 'exhaust':162C,288C 'exist':152C 'exploit':263C 'filter':204C 'find':168C 'firewal':132C,187C 'flaw':41B,79B 'fmc':171C,190C,317C 'forticli':139C,205C,215C,337C 'framework':38B,71B,241C 'gain':275C 'high':126C 'high-impact':125C 'host':30B 'html':418C,437C 'http':226C 'id':300C,303C,410C 'identifi':160C 'impact':127C,261C 'improv':90C 'includ':147C 'infrastructur':281C 'inject':343C,419C,438C 'interfac':183C 'introduc':105C 'key':167C 'lab':23B 'lab-disclosur':22B 'legaci':301C 'link':420C 'log':325C,348C,370C,391C 'manag':133C,182C,188C,280C,311C,333C,356C,378C,413C,432C 'manipul':223C 'merg':397C 'mesop':1A,18A,20A,25B,34B,118C,228C,236C,360C 'mesop-dev':17A 'must':73B 'n/a':314C,336C,359C,381C,416C,424C,435C,443C 'network':279C 'new':87C,107C,307C,330C,353C,375C,429C,448C 'offici':295C 'origin':400C 'packag':26B 'pattern':68B 'payload':4A,256C,365C 'permit':217C 'pre':340C 'pre-auth':339C 'previous':305C 'protect':123C 'python':27B,36B,238C 'python-bas':237C 'rce':115C,318C 'react':153C,382C,402C 'recent':159C 'releas':81C,97C,104C,146C 'remot':112C,195C,361C 'remov':77B 'request':6A,53B,227C,259C 'resourc':161C,287C 'rule':299C,302C,394C,401C 'ruleset':298C,312C,334C,357C,379C,414C,433C 'run':8A 'secur':131C,170C,186C,203C,316C 'self':29B 'self-host':28B 'send':46B,250C 'server':61B,154C,285C,383C,403C 'server-sid':284C 'side':286C 'special':48B,251C 'sql':342C 'strong':291C 'success':262C 'tag':421C,440C 'trigger':283C 'ui':37B,240C 'unauthent':2A,44B,194C,243C,269C 'unauthor':218C 'updat':88C,149C,297C 'upgrad':75B 'upgradereadinesscal':320C 'uri':422C,441C 'vector':163C 'vendor':296C 'via':224C,319C 'vulner':116C,128C,176C,212C,233C,266C 'waf':64B,80C,96C 'web':32B,180C 'web-app':31B 'web-bas':179C 'week':102C 'xss':417C,436C","nicknames":[],"mitigations":[],"who_affected":"Teams running Mesop below `1.2.3` on a server reachable from the internet or an untrusted network are exposed, since the request body is the attack path. Applications that do not use Mesop, or that run it only on an internal machine with no untrusted access, are less exposed; the sources do not say whether specific deployment modes are safe.","tier":"B","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-03-20T08:16:11+00:00","kev":null,"cvss_vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H","cvss_by":"security-advisories@github.com","poc_count":0,"sources":[{"source":"lab","external_id":"https://developers.cloudflare.com/changelog/post/2026-04-15-waf-release/#CVE-2026-33057","url":"https://developers.cloudflare.com/changelog/post/2026-04-15-waf-release/","severity":null,"published_at":"2026-04-15T00:00:00+00:00","modified_at":null,"first_seen_at":"2026-10-24T19:12:57+00:00","fetched_at":"2026-10-24T19:12:57+00:00","note":"Cloudflare: WAF Release - 2026-04-15 · Changelog"},{"source":"nvd","external_id":"CVE-2026-33057","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-33057","severity":null,"published_at":"2026-03-20T08:16:11+00:00","modified_at":"2026-06-17T10:36:52+00:00","first_seen_at":"2026-10-24T19:23:38+00:00","fetched_at":"2026-10-24T19:23:38+00:00","note":null},{"source":"epss","external_id":"CVE-2026-33057","url":"https://api.first.org/data/v1/epss?cve=CVE-2026-33057","severity":null,"published_at":null,"modified_at":null,"first_seen_at":"2026-10-25T12:38:49+00:00","fetched_at":"2026-10-25T12:38:49+00:00","note":null}],"documents":[{"id":2921,"url":"https://developers.cloudflare.com/changelog/post/2026-04-15-waf-release/","title":"Cloudflare: WAF Release - 2026-04-15 · Changelog","kind":"advisory","source":"lab","text":"WAF Release - 2026-04-15 · Changelog\n\n# Changelog\n\nNew updates and improvements at Cloudflare.\n\nApril 15, 2026\n\n## WAF Release - 2026-04-15\n\nThis week's release introduces a new detection for a critical Remote Code Execution (RCE) vulnerability in Mesop (CVE-2026-33057), alongside protections for high-impact vulnerabilities in Cisco Secure Firewall Management Center (CVE-2026-20079) and FortiClient EMS (CVE-2026-21643). Additionally, this release includes an update to our existing React Server DoS coverage to address recently identified resource exhaustion vectors (CVE-2026-23869).\n\nKey Findings\n\n- Cisco Secure FMC (CVE-2026-20079): A vulnerability in the web-based management interface of Cisco Secure Firewall Management Center (FMC) that allows an unauthenticated, remote attacker to execute arbitrary commands or bypass security filters.\n- FortiClient EMS (CVE-2026-21643): A critical vulnerability in the FortiClient EMS permitting unauthorized access or administrative configuration manipulation via crafted HTTP requests.\n- Mesop (CVE-2026-33057): A vulnerability in the Mesop Python-based UI framework where unauthenticated attackers can execute arbitrary code by sending specially crafted, Base64-encoded payloads in the request body.\n\nImpact\n\nSuccessful exploitation of these vulnerabilities could allow unauthenticated attackers to execute arbitrary code, gain administrative control over network management infrastructure, or trigger server-side resource exhaustion. Administrators are strongly encouraged to apply official vendor updates.\n\n| Ruleset | Rule ID | Legacy Rule ID | Description | Previous Action | New Action | Comments |\n| --- | --- | --- | --- | --- | --- | --- |\n| Cloudflare Managed Ruleset | ...aef9415b | N/A | Cisco Secure FMC - RCE via upgradeReadinessCall - CVE:CVE-2026-20079 | Log | Block | This is a new detection. |\n| Cloudflare Managed Ruleset | ...ee7be621 | N/A | FortiClient EMS - Pre-Auth SQL Injection - CVE:CVE-2026-21643 | Log | Block | This is a new detection. |\n| Cloudflare Managed Ruleset | ...c953a72b | N/A | Mesop - Remote Code Execution - Base64 Payload - CVE:CVE-2026-33057 | Log | Block | This is a new detection. |\n| Cloudflare Managed Ruleset | ...50c08f6f | N/A | React Server - DOS - CVE:CVE-2026-23864 - 1 - Beta | Log | Block | This rule has been merged into the original rule \"React Server - DOS - CVE:CVE-2026-23864 - 1\" (ID: ...61680354) |\n| Cloudflare Managed Ruleset | ...ebd81645 | N/A | XSS, HTML Injection - Link Tag - URI (beta) | N/A | Disabled | This is a new detection. |\n| Cloudflare Managed Ruleset | ...0af34bba | N/A | XSS, HTML Injection - Embed Tag - URI (beta) | N/A | Disabled | This is a new detection. |","published_at":"2026-04-15T00:00:00+00:00","relation":"advisory","confidence":null,"note":null}],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}