CVE-2026-54740 (GCVE-0-2026-54740)
Vulnerability from cvelistv5 – Published: 2026-08-19 20:14 – Updated: 2026-08-20 19:57
VLAI
EPSS
VEX
Title
Lemmy: Lower-ranked federated moderator can remove higher-ranked moderators
Summary
Lemmy is a link aggregator and forum for the fediverse. Prior to 0.19.19 and 1.0.0-alpha.18, a lower-ranked remote moderator can remove a higher-ranked moderator by sending a signed ActivityPub Remove activity to the target instance. The local API uses LocalUser::is_higher_mod_or_admin_check to enforce moderator rank, but CollectionRemove::verify in crates/apub/activities/src/community/collection_remove.rs only calls verify_mod_action. CollectionRemove::receive dereferences self.object as an ApubPerson, creates a CommunityModeratorForm, and calls CommunityActions::leave without checking that the actor outranks the moderator identified by the object field. In communities with federated moderators, a junior moderator can therefore strip senior moderators from the community moderator list even though the local API rejects the same action. This issue is fixed in versions 0.19.19 and 1.0.0-alpha.18.
Severity
6.5 (Medium)
SSVC
Exploitation: poc
Automatable: no
Technical Impact: partial
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2026-08-20 19:57 UTC
CWE
- CWE-862 - Missing Authorization
Assigner
References
8 references
| URL | Tags |
|---|---|
| https://github.com/LemmyNet/lemmy/security/adviso… | x_refsource_CONFIRM |
| https://github.com/LemmyNet/lemmy/pull/6475 | x_refsource_MISC |
| https://github.com/LemmyNet/lemmy/pull/6478 | x_refsource_MISC |
| https://github.com/LemmyNet/lemmy/commit/d127e714… | x_refsource_MISC |
| https://github.com/LemmyNet/lemmy/commit/ede1d348… | x_refsource_MISC |
| https://github.com/LemmyNet/lemmy/releases/tag/0.19.19 | x_refsource_MISC |
| https://github.com/LemmyNet/lemmy/releases/tag/1.… | x_refsource_MISC |
| https://join-lemmy.org/news/2026-06-09_-_Lemmy_Re… | x_refsource_MISC |
Impacted products
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"id": "CVE-2026-54740",
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"url": "https://github.com/LemmyNet/lemmy/security/advisories/GHSA-xmpx-2j2f-c7g7"
}
],
"title": "CISA ADP Vulnrichment"
}
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"cna": {
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{
"product": "lemmy",
"vendor": "LemmyNet",
"versions": [
{
"status": "affected",
"version": "\u003c 0.19.19"
},
{
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"version": "\u003e= 1.0.0-alpha.0, \u003c 1.0.0-alpha.18"
}
]
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],
"descriptions": [
{
"lang": "en",
"value": "Lemmy is a link aggregator and forum for the fediverse. Prior to 0.19.19 and 1.0.0-alpha.18, a lower-ranked remote moderator can remove a higher-ranked moderator by sending a signed ActivityPub Remove activity to the target instance. The local API uses LocalUser::is_higher_mod_or_admin_check to enforce moderator rank, but CollectionRemove::verify in crates/apub/activities/src/community/collection_remove.rs only calls verify_mod_action. CollectionRemove::receive dereferences self.object as an ApubPerson, creates a CommunityModeratorForm, and calls CommunityActions::leave without checking that the actor outranks the moderator identified by the object field. In communities with federated moderators, a junior moderator can therefore strip senior moderators from the community moderator list even though the local API rejects the same action. This issue is fixed in versions 0.19.19 and 1.0.0-alpha.18."
}
],
"metrics": [
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"attackComplexity": "LOW",
"attackVector": "NETWORK",
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"baseScore": 6.5,
"baseSeverity": "MEDIUM",
"confidentialityImpact": "NONE",
"integrityImpact": "HIGH",
"privilegesRequired": "LOW",
"scope": "UNCHANGED",
"userInteraction": "NONE",
"vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N",
"version": "3.1"
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"description": "CWE-862: Missing Authorization",
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{
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},
{
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"x_refsource_MISC"
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},
{
"name": "https://github.com/LemmyNet/lemmy/commit/d127e714a66f34f6b9c8ea15f20097ac907cdf28",
"tags": [
"x_refsource_MISC"
],
"url": "https://github.com/LemmyNet/lemmy/commit/d127e714a66f34f6b9c8ea15f20097ac907cdf28"
},
{
"name": "https://github.com/LemmyNet/lemmy/commit/ede1d348344ca2027fd1bc6a0cab25575c0c3a4c",
"tags": [
"x_refsource_MISC"
],
"url": "https://github.com/LemmyNet/lemmy/commit/ede1d348344ca2027fd1bc6a0cab25575c0c3a4c"
},
{
"name": "https://github.com/LemmyNet/lemmy/releases/tag/0.19.19",
"tags": [
"x_refsource_MISC"
],
"url": "https://github.com/LemmyNet/lemmy/releases/tag/0.19.19"
},
{
"name": "https://github.com/LemmyNet/lemmy/releases/tag/1.0.0-alpha.18",
"tags": [
"x_refsource_MISC"
],
"url": "https://github.com/LemmyNet/lemmy/releases/tag/1.0.0-alpha.18"
},
{
"name": "https://join-lemmy.org/news/2026-06-09_-_Lemmy_Release_v0.19.19",
"tags": [
"x_refsource_MISC"
],
"url": "https://join-lemmy.org/news/2026-06-09_-_Lemmy_Release_v0.19.19"
}
],
"source": {
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"discovery": "UNKNOWN"
},
"title": "Lemmy: Lower-ranked federated moderator can remove higher-ranked moderators"
}
},
"cveMetadata": {
"assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
"assignerShortName": "GitHub_M",
"cveId": "CVE-2026-54740",
"datePublished": "2026-08-19T20:14:29.391Z",
"dateReserved": "2026-06-15T23:07:33.233Z",
"dateUpdated": "2026-08-20T19:57:12.849Z",
"state": "PUBLISHED"
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"date": "2026-10-02",
"epss": "0.00427",
"percentile": "0.34733"
},
"nvd": {
"cve": {
"affected": [
{
"affectedData": [
{
"product": "lemmy",
"vendor": "LemmyNet",
"versions": [
{
"status": "affected",
"version": "\u003c 0.19.19"
},
{
"status": "affected",
"version": "\u003e= 1.0.0-alpha.0, \u003c 1.0.0-alpha.18"
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"cveTags": [],
"descriptions": [
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"lang": "en",
"value": "Lemmy is a link aggregator and forum for the fediverse. Prior to 0.19.19 and 1.0.0-alpha.18, a lower-ranked remote moderator can remove a higher-ranked moderator by sending a signed ActivityPub Remove activity to the target instance. The local API uses LocalUser::is_higher_mod_or_admin_check to enforce moderator rank, but CollectionRemove::verify in crates/apub/activities/src/community/collection_remove.rs only calls verify_mod_action. CollectionRemove::receive dereferences self.object as an ApubPerson, creates a CommunityModeratorForm, and calls CommunityActions::leave without checking that the actor outranks the moderator identified by the object field. In communities with federated moderators, a junior moderator can therefore strip senior moderators from the community moderator list even though the local API rejects the same action. This issue is fixed in versions 0.19.19 and 1.0.0-alpha.18."
}
],
"id": "CVE-2026-54740",
"lastModified": "2026-09-09T21:15:59.613",
"metrics": {
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"attackVector": "NETWORK",
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"baseScore": 6.5,
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"confidentialityImpact": "NONE",
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"scope": "UNCHANGED",
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},
"published": "2026-08-19T21:16:58.363",
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"url": "https://github.com/LemmyNet/lemmy/commit/d127e714a66f34f6b9c8ea15f20097ac907cdf28"
},
{
"source": "security-advisories@github.com",
"url": "https://github.com/LemmyNet/lemmy/commit/ede1d348344ca2027fd1bc6a0cab25575c0c3a4c"
},
{
"source": "security-advisories@github.com",
"url": "https://github.com/LemmyNet/lemmy/pull/6475"
},
{
"source": "security-advisories@github.com",
"url": "https://github.com/LemmyNet/lemmy/pull/6478"
},
{
"source": "security-advisories@github.com",
"url": "https://github.com/LemmyNet/lemmy/releases/tag/0.19.19"
},
{
"source": "security-advisories@github.com",
"url": "https://github.com/LemmyNet/lemmy/releases/tag/1.0.0-alpha.18"
},
{
"source": "security-advisories@github.com",
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"vulnStatus": "Deferred",
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},
"vulnrichment": {
"containers": {
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"options": [
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"url": "https://github.com/LemmyNet/lemmy/security/advisories/GHSA-xmpx-2j2f-c7g7"
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"title": "CISA ADP Vulnrichment"
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],
"cna": {
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"vendor": "LemmyNet",
"versions": [
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"version": "\u003c 0.19.19"
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{
"status": "affected",
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],
"descriptions": [
{
"lang": "en",
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}
],
"metrics": [
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"attackComplexity": "LOW",
"attackVector": "NETWORK",
"availabilityImpact": "NONE",
"baseScore": 6.5,
"baseSeverity": "MEDIUM",
"confidentialityImpact": "NONE",
"integrityImpact": "HIGH",
"privilegesRequired": "LOW",
"scope": "UNCHANGED",
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},
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],
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},
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"tags": [
"x_refsource_MISC"
],
"url": "https://join-lemmy.org/news/2026-06-09_-_Lemmy_Release_v0.19.19"
}
],
"source": {
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"discovery": "UNKNOWN"
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"title": "Lemmy: Lower-ranked federated moderator can remove higher-ranked moderators"
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Experimental. This forecast is provided for visualization only and may change without notice. Do not use it for operational decisions.
Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the
vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base
model, served locally by ML-Gateway.
They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
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Related by attack behaviour
Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.
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