High
FreeScout Vulnerable to Unauthenticated Conversation Thread Injection via HMAC Length Bypass in FetchEmails
Type: advisorySources: OSV
FreeScout is a free help desk and shared inbox built with PHP's Laravel framework. Prior to version 1.8.223, an unauthenticated attacker can inject messages into any existing support conversation by sending a single email to the helpdesk's public address with a crafted `In-Reply-To` header. No credentials, tokens, or prior access are required. The injected message is rendered in the agent UI as a legitimate customer reply, the conversation is automatically reopened, and the `last_reply_from` field is set to the attacker's identity. Version 1.8.223 contains a fix.
- Products
- Not specified
- Affected versions
- Not specified
- Fixed versions
- Not specified
- CVEs
- CVE-2026-53591
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 2 Nov 2026, 21:27 UTC · Revision 1
Source evidence
FreeScout Vulnerable to Unauthenticated Conversation Thread Injection via HMAC Length Bypass in FetchEmails
core-osv · Original sourcePublished 20 Jul 2026, 19:47 UTC
FreeScout is a free help desk and shared inbox built with PHP's Laravel framework. Prior to version 1.8.223, an unauthenticated attacker can inject messages into any existing support conversation by sending a single email to the helpdesk's public address with a crafted `In-Reply-To` header. No credentials, tokens, or prior access are required. The injected message is rendered in the agent UI as a legitimate customer reply, the conversation is automatically reopened, and the `last_reply_from` field is set to the attacker's identity. Version 1.8.223 contains a fix.
osv:CVE-2026-53591
Complete source record
{
"id": "9a4614e79caa31de2a286b4d3045d083ca4251cab0f809b74bff3eed2289b053",
"url": "https://osv.dev/vulnerability/CVE-2026-53591",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "FreeScout Vulnerable to Unauthenticated Conversation Thread Injection via HMAC Length Bypass in FetchEmails",
"summary": "FreeScout is a free help desk and shared inbox built with PHP's Laravel framework. Prior to version 1.8.223, an unauthenticated attacker can inject messages into any existing support conversation by sending a single email to the helpdesk's public address with a crafted `In-Reply-To` header. No credentials, tokens, or prior access are required. The injected message is rendered in the agent UI as a legitimate customer reply, the conversation is automatically reopened, and the `last_reply_from` field is set to the attacker's identity. Version 1.8.223 contains a fix.",
"evidence": "osv:CVE-2026-53591",
"products": [],
"severity": "high",
"structured": {
"id": "CVE-2026-53591",
"aliases": [
"CVE-2026-53591",
"GHSA-8vm3-wwq4-ggfx"
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"metrics": [
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"type": "CVSS_V3",
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:L"
}
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"affected": [
{
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],
"modified": "2026-10-28T02:30:23.682565381Z",
"provider": "osv",
"references": [
"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/53xxx/CVE-2026-53591.json",
"https://github.com/freescout-help-desk/freescout/security/advisories/GHSA-8vm3-wwq4-ggfx",
"https://nvd.nist.gov/vuln/detail/CVE-2026-53591"
],
"provider_metadata": {
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"cna_assigner": "GitHub_M",
"osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/53xxx/CVE-2026-53591.json"
}
},
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"evidence": "/id"
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"severity_evidence": "/severity/0/score"
},
"source_id": "core-osv",
"document_id": 490234,
"published_at": "2026-07-20T19:47:18.338+00:00"
}