TONESHELL used FakeTLS headers in network packets to impersonate various versions of TLS protocols to blend in with legitimate network traffic.45 TONESHELL variants have utilized FakeTLS headers with the bytes 0x17 0x03 0x03 to represent TLSv1.2 and 0x17 0x03 0x04 for TLSv1.3.5
tool
TONESHELL S1239
- Type
- malware
- Platform
- Windows
- Created
- 15 September 2025
- Last modified
- 12 May 2026
TONESHELL is a custom backdoor that has been used since at least Q1 2021.[1] TONESHELL malware has previously been leveraged by Chinese affiliated actors identified as Mustang Panda.[2][3]
43
TechniquesTop 2%
9
TacticsTop 11%
1
Threat groupunranked
0
CampaignsNone recorded
Enterprise ATT&CK only. Any Mobile or ICS rows on the same ATT&CK page are not carried.
MITRE reference S1239
TONESHELL has used GetForegroundWindow to detect virtualization or sandboxes by calling the API twice and comparing each window handle.7
TONESHELL has used randomized padding to obfuscate payloads.89
TONESHELL has utilized a modified DJB2 algorithm to resolve APIs.10
TONESHELL has been initiated using LNK files that were programmed to display a PDF icon to entice the victim to click on the file to execute an office.exe binary.11
TONESHELL has obtained the username from an infected host.12
Standing S1239
Reach is how much of ATT&CK this tool touches. Coverage is how well defended each thing it does is, as a median per technique rather than a total — a total would just restate the reach. Each figure is ranked against all 825 ATT&CK software entries only where that population actually spreads. Where most of the population shares one value, a percentile would rank the tie instead of the entity, so the raw value is shown and no rank is claimed.
Reach
Techniques43Top 2%
98th percentile · 98% of 825 ATT&CK software entries have this many Enterprise techniques or fewer.
Tactics9Top 11%
89th percentile · 89% of 825 ATT&CK software entries have this many tactics spanned or fewer.
Threat group161% tie · 22 values
61% of the population shares a single value across only 22 distinct values, so a percentile here would rank the tie, not the entity.
Campaigns085% tie · 8 values
85% of the population shares a single value across only 8 distinct values, so a percentile here would rank the tie, not the entity.
Coverage
Rules / technique9medBottom 13%
13th percentile · 87% of 825 ATT&CK software entries have more detection rules per technique.
Tests / technique3med
Detection coverage S1239
916 distinct rules cover the 43 techniques recorded for this tool. The 980 technique-to-rule mappings resolve to 916 distinct rules, because one rule can cover several techniques. 670 Sigma · 246 Splunk.
33 covered by at least one rule10 with no coverage916 distinct rules in total
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